March  2024

Updated:   1 March 2024

 

Welcome to the night skies of Autumn, featuring Eridanus, Auriga, Taurus, Gemini, Cancer, Leo, Virgo, Orion, Canis Major, Hydra, Carina, Crux and Jupiter

 

Note:  To read this webpage with mobile phones or tablets, please use them in landscape format, i.e. the long screen axis should be horizontal.

 

The Alluna RC-20 Ritchey Chrétien telescope was installed in March, 2016.

The 20-inch telescope is able to locate and track any sky object (including Earth satellites and the International Space Station) with software called TheSkyX Professional, into which is embedded a unique T-Point model created for our site with the telescope itself.

 

Explanatory Notes:  

 

Times for transient sky phenomena are given using a 24 hour clock, i.e. 20:30 hrs = 8.30 pm. Times are in Australian Eastern Standard Time (AEST), which equals Universal Time (UT) + 10 hours. Daylight saving is not observed in Queensland. Observers in other time zones will need to make their own corrections where appropriate. With conjunctions of the Moon, planets and stars, timings indicate the closest approach. Directions (north or south) are approximate. The Moon’s diameter is given in arcminutes ( ’ ). The Moon is usually about 30’ or half a degree across. The 'limb' of the Moon is its edge as projected against the sky background.

Rise and set times are given for the theoretical horizon, which is a flat horizon all the way round the compass, with no mountains, hills, trees or buildings to obscure the view. Observers will have to make allowance for their own actual horizon. 

Transient phenomena are provided for the current month and the next. Geocentric phenomena are calculated as if the Earth were fixed in space as the ancient Greeks believed. This viewpoint is useful, as otherwise rising and setting times would be meaningless. In the list of geocentric events, the nearer object is given first.

When a planet is referred to as ‘stationary’, it means that its movement across the stellar background appears to have ceased, not that the planet itself has stopped. With inferior planets (those inside the Earth’s orbit, Mercury and Venus), this is caused by the planet heading either directly towards or directly away from the Earth. With superior planets (Mars out to Pluto), this phenomenon is caused by the planet either beginning or ending its retrograde loop due to the Earth’s overtaking it.

Apogee and perigee:   Maximum and minimum distances of the Moon or artificial satellite from the Earth.

Aphelion and perihelion:  Maximum and minimum distances of a planet, asteroid or comet from the Sun.

Eclipses always occur in pairs, a lunar and a solar but not necessarily in that order, two weeks apart.

The zenith is the point in the sky directly overhead from the observer.

The meridian is a semicircle starting from a point on the horizon that is exactly due north from the observer, and arching up into the sky to the zenith and continuing down to a point on the horizon that is exactly due south. On the way down it passes through the South Celestial Pole which is 26.6 degrees above the horizon at Nambour. The elevation of the South Celestial Pole is exactly the same as the observer's latitude, e.g. from Cairns it is 16.9 degrees above the horizon, and from Melbourne it is 37.8 degrees. The Earth's axis points to this point in the sky in the southern hemisphere, and to an equivalent point in the northern hemisphere, near the star Polaris, which from Australia is always below the northern horizon.

All astronomical objects rise until they reach the meridian, then they begin to set. The act of crossing or 'transitting' the meridian is called 'culmination'. Objects closer to the South Celestial Pole than its altitude above the southern horizon do not rise or set, but are always above the horizon, constantly circling once each sidereal day. They are called 'circumpolar'. The brightest circumpolar star from Nambour is Miaplacidus (Beta Carinae, magnitude = 1.67).  

A handspan at arm's length with fingers spread covers an angle of approximately 18 - 20 degrees. Your closed fist at arm's length is 10 degrees across. The tip of your index finger at arm's length is 1 degree across. These figures are constant for most people, whatever their age. The Southern Cross is 6 degrees high and 4 degrees wide, and Orion's Belt is 2.7 degrees long. The Sun and Moon average half-a-degree (30 arcminutes) across.   

mv = visual magnitude or brightness. Magnitude 1 stars are very bright, magnitude 2 less so, and magnitude 6 stars are so faint that the unaided eye can only just detect them under good, dark conditions. Binoculars will allow us to see down to magnitude 8, and the Observatory telescope can reach visual magnitude 17 or 22 photographically. The world's biggest telescopes have detected stars and galaxies as faint as magnitude 30. The sixteen very brightest stars are assigned magnitudes of 0 or even -1. The brightest star, Sirius, has a magnitude of -1.44. Jupiter can reach -2.4, and Venus can be more than 6 times brighter at magnitude -4.7, bright enough to cast shadows. The Full Moon can reach magnitude -12 and the overhead Sun is magnitude -26.5. Each magnitude step is 2.51 times brighter or fainter than the next one, i.e. a magnitude 3.0 star is 2.51 times brighter than a magnitude 4.0. Magnitude 1.0 stars are 100 times brighter than magnitude 6.0 (5 steps each of 2.51 times, 2.51x2.51x2.51x2.51x2.51  =  2.51=  99.625   ..... close enough to 100).

 

The Four Minute Rule

How long does it take the Earth to complete one rotation? No, it's not 24 hours - that is the time taken for the Sun to cross the meridian on successive days. This 24 hours is a little longer than one complete rotation, as the curve in the Earth's orbit means that it needs to turn a fraction more (~1 degree of angle) in order for the Sun to cross the meridian again. It is called a 'solar day'. The stars, clusters, nebulae and galaxies are so distant that most appear to have fixed positions in the night sky on a human time-scale, and for a star to return to the same point in the sky relative to a fixed observer takes 23 hours 56 minutes 4.0916 seconds. This is the time taken for the Earth to complete exactly one rotation, and is called a 'sidereal day'.

As our clocks and lives are organised to run on solar days of 24 hours, and the stars circulate in 23 hours 56 minutes approximately, there is a four minute difference between the movement of the Sun and the movement of the stars. This causes the following phenomena:

    1.    The Sun slowly moves in the sky relative to the stars by four minutes of time or one degree of angle per day. Over the course of a year it moves ~4 minutes X 365 days = 24 hours, and ~1 degree X 365 = 360 degrees or a complete circle. Together, both these facts mean that after the course of a year the Sun returns to exactly the same position relative to the stars, ready for the whole process to begin again.

    2.    For a given clock time, say 8:00 pm, the stars on consecutive evenings are ~4 minutes or ~1 degree further on than they were the previous night. This means that the stars, as well as their nightly movement caused by the Earth's rotation, also drift further west for a given time as the weeks pass. The stars of autumn, such as Orion, are lost below the western horizon by mid-June, and new constellations, such as Sagittarius, have appeared in the east.  The stars change with the seasons, and after a year, they are all back where they started, thanks to the Earth's having completed a revolution of the Sun and returned to its theoretical starting point.

We can therefore say that the star patterns we see in the sky at 11:00 pm tonight will be identical to those we see at 10:32 pm this day next week (4 minutes X 7 = 28 minutes earlier), and will be identical to those of 9:00 pm this date next month or 7:00 pm the month after. All the above also includes the Moon and planets, but their movements are made more complicated, for as well as the Four Minute Drift  with the stars, they also drift at different rates against the starry background, the closest ones drifting the fastest (such as the Moon or Venus), and the most distant ones (such as Saturn or Neptune) moving the slowest.


 

A suggestion for successful sky-watching

Observing astronomical objects depends on whether the sky is free of clouds. Not only that, but there are other factors such as wind, presence of high-altitude jet streams, air temperature, humidity (affecting dew formation on equipment), transparency (clarity of the air), "seeing" (the amount of air turbulence present), and air pressure. Even the finest optical telescope has its performance constrained by these factors. Fortunately, there is an Australian website that predicts the presence and effects of these phenomena for a period up to five days ahead of the current date, which enables amateur and professional astronomers to plan their observing sessions for the week ahead. It is called "SkippySky". The writer has found its predictions to be quite reliable, and recommends the website as a practical resource. The website is at  http://skippysky.com.au  and the detailed Australian data are at  http://skippysky.com.au/Australia/ .

 

 

 

 Solar System

 

Sun:   The Sun begins the month in the zodiacal constellation of Aquarius, the Water-Bearer. It crosses into Pisces, the Fishes on March 12.   Note: the Zodiacal constellations used in astrology have significant differences with the familiar astronomical constellations both in size and the timing of the passage through them of the Sun, Moon and planets.
 

 

Moon: 
 

The Moon is tidally locked to the Earth, i.e. it keeps its near hemisphere facing us at all times, while its far hemisphere is never seen from Earth. This tidal locking is caused by the Earth's gravity. The far side remained unknown until the Russian probe Luna 3 went around the Moon and photographed it on October 7, 1959. Now the whole Moon has been photographed in very fine detail by orbiting satellites. The Moon circles the Earth once in a month (originally 'moonth'), the exact period being 27 days 7 hours 43 minutes 11.5 seconds. Its speed is about 1 kilometre per second or 3679 kilometres per hour. The Moon's average distance from the Earth is 384 400 kilometres, but the orbit is not perfectly circular. It is slightly elliptical, with an eccentricity of 5.5%. This means that each month, the Moon's distance from Earth varies between an apogee (furthest distance) of 406 600 kilometres, and a perigee (closest distance) of 356 400 kilometres. These apogee and perigee distances vary slightly from month to month.  In the early 17th century, the first lunar observers to use telescopes found that the Moon had a monthly side-to-side 'wobble', which enabled them to observe features which were brought into view by the wobble and then taken out of sight again. The wobble, called 'libration', amounted to 7º 54' in longitude and 6º 50' in latitude.  The 'libration zone' on the Moon is the area around the edge of the Moon that comes into and out of view each month, due to libration. This effect means that, instead of only seeing 50% of the Moon from Earth, we can see up to 59%.

The animation loop below shows the appearance of the Moon over one month. The changing phases are obvious, as is the changing size as the Moon comes closer to Earth at perigee, and moves away from the Earth at apogee. The wobble due to libration is the other feature to note, making the Moon appear to sway from side to side and nod up and down.
 


(Credit: Wikipedia)

Solar Eclipse:


TOTAL, APRIL 9
:   The next total eclipse of the Sun will not be visible from Australia, except via the Internet ( click 
here  )  The main eclipse shadow will first strike the Earth over the eastern Pacific, and then pass over Mexico, the eastern half of the USA, Newfoundland and the North Atlantic. Asia, Europe, Africa and South America will miss out this time. The best places to view the eclipse are Mexico, Texas, Arkansas, Kentucky, Ohio, Lake Ontario, Maine, New Brunswick and Newfoundland. The largest city totally in the eclipse track will be Dallas, Texas. This will be the second total eclipse visible from the central United States in just seven years, after the solar eclipse of August 21, 2017. Totality will pass through the town of Wapakoneta, Ohio, the home of the late Neil Armstrong, the first man on the Moon. This will be the last total solar eclipse visible in the contiguous United States until August 23, 2044.

 Australia will experience a total solar eclipse on July 22, 2028. It will strike the Western Australian mainland in the Kimberley District in the middle of the day, and head south-east past Alice Springs, Birdsville and Dubbo to Sydney, then across the Tasman Sea to Dunedin in New Zealand. It will be worth waiting for !


 

Lunar Eclipse:


PENUMBRAL, MARCH 2
5:   The next lunar eclipse visible from Australia will occur on this night, but it is only penumbral. The eclipse will begin at 2:53 pm, when the Moon is below our horizon. Maximum eclipse will be at 5:12 pm. The Full Moon will rise at 5:55 pm, but the Earth's shadow in penumbral eclipses is very faint. Although about two thirds of the Moon will be immersed in the penumbra, the effect will be hardly noticeable. The eclipse will end at 7:32 pm.

 


Lunar Phases:
 



Last Quarter:               March 4    
          01:24 hrs           diameter = 30.9'
New Moon:
                 March 10            19:01 hrs           diameter = 33.5'     Lunation #1252 begins     
First Quarter:
                March 17   
         14:11 hrs           diameter = 30.8'
Full Moon:                 
  March 25            17:01 hrs           diameter = 29.5'     Penumbral lunar eclipse

Last Quarter:               April 2                 13:16 hrs           diameter = 31.5'
New Moon:
                 April 09               04:21 hrs           diameter = 33.2'     Lunation #1253 begins, total solar eclipse     
First Quarter:
              
April 16               05:13 hrs           diameter = 30.2'
Full Moon:                  
April 24               09:50 hrs           diameter = 29.9'



Lunar Orbital Elements:


March 10:                   Moon at perigee (356 898 km) at 17:02 hrs, diameter = 33.5'
March 12:                   Moon at ascending node at 11:16 hrs, diameter = 33.2'
March 24:                   Moon at apogee (406 301 km) at 00:58 hrs, diameter = 29.4'
March 26:                   Moon at descending node at 14:04 hrs, diameter = 29.6'

April 8:                        Moon at perigee (358 842 km) at 04:00 hrs, diameter = 33.3'
April 8:                        Moon at ascending node at 22:18 hrs, diameter = 33.2'
April 20:                      Moon at apogee (405 640 km) at 11:55 hrs, diameter = 29.5'
April 22:                      Moon at descending node at 20:41 hrs, diameter = 29.6'

 

Moon at 8 days after New, as on March 18.

 

The photograph above shows the Moon when approximately eight days after New, just after First Quarter. A rotatable view of the Moon, with ability to zoom in close to the surface (including the far side), and giving detailed information on each feature, may be downloaded  here.  A professional version of this freeware with excellent pictures from the Lunar Reconnaissance Orbiter and the Chang orbiter (giving a resolution of 50 metres on the Moon's surface) and many other useful features is available on a DVD from the same website for 20 Euros (about AU $ 33) plus postage.


Lunar Feature for this Month
     
 

Each month we describe a lunar crater, cluster of craters, valley, mountain range or other object, chosen at random, but one with interesting attributes. A recent photograph from our Alluna RC20 telescope will illustrate the object. As all large lunar objects are named, the origin of the name will be given if it is important. This month we will look at the south-western shore of the flat lava plain known as Mare Crisium (the Sea of Crises).

The craters Picard, Lick, Yerkes, Tebbutt and Shapley. Photograph taken at 5:33 pm on 14 September 2018.


Mare Crisium is a huge oval basin 620 kilometres across (east to west) and 570 kilometres across (north to south). Its position near the Moon's north-eastern limb means that foreshortening makes it appear that its north-south axis is the longer one. Mare Crisium was created about 3.85 billion years ago, so it is remarkably undamaged considering its age. The other maria are aged as follows:  Australe, Fecunditatis, Smythii, Tranquillitatis, Insularum, Marginis and Nubium (4.55 - 3.92 billion years), Frigoris (4.55 -3.85 billion years), Humorum, Nectaris and Humboldtianum (3.92 - 3.85 billion years), Imbrium (3.85 - 3.8 billion years), Oceanus Procellarum (3.85 - 3.2 billion years), Serenitatis (3.2 billion years to present day). The eastern half of Mare Crisium is marked only by craterlets, but there are a few large impact craters in the south-western quadrant shown above.

Picard (23 kilometres diameter) is the most recent of these and looks quite fresh. It obviously occurred after the lava flooded the Mare, filling it up to a level similar to that of the surrounding Maria. The two craters Yerkes (36 kilometres diameter) and Lick (31 kilonmetres diameter) are slightly older (3.82 billion years) and were formed before the extrusion of magma that levelled the whole interior of the Mare. This magma (called 'lava' once it has reached the surface) forced breaches in the walls of Yerkes and Lick, and flooded their interiors to the same level as the Mare outside, turning them into 'ghost craters'.

The craters Tebbutt (31 kilometres diameter) and Shapley (23 kilometres diameter) are found embedded in the south-west wall of the Mare. North-west of Yerkes is a small pyroclastic area with clefts and domes.

Picard

Jean-Felix Picard (1620-1682) and some assistants travelled to the site of the ruined Uraniborg in Denmark in 1671, so that observations and timings of the eclipses of Jovian satellites could be done as close to simultaneously as possible by himself at Uraniborg and by Cassini in Paris. Cassini also wanted to check the exactness of a selection of Tycho’s other observations from the same site, to see if his pre-telescopic star positions were as accurate as claimed. If they were, they could possibly be utilised in dealing with the longitude problem, using the ‘close approach of the Moon to bright stars’ technique of Johannes Werner – the lunar distance method. One of the local Danish astronomers who took part in these projects was Ole Christensen Rømer (1644-1710).

They worked over a period of several months, observing about 140 eclipses of Jupiter’s moon Io, while in Paris Cassini observed the same eclipses. By comparing the times of the eclipses, the difference in longitude between the two observing sites could be calculated. Picard later used these data with a variation on the experiments of Posidonius and al-Farghani, to calculate the diameter of the Earth. He measured the distance of a single degree of latitude, by seeing how far he would need to travel along a line of longitude passing through Amiens, Paris and Le Ferté-Alais, so that the elevation of Polaris at a given sidereal time fell or rose by 1⁰. He used only a primitive theodolite, but achieved a result of 12 658 kilometres (the accepted modern value is 12 714 kilometres – his error was only 56 km). This is his main claim to fame.


Lick

In the USA, James Lick, a land baron who had made a fortune in housing during the San Francisco gold rush and was one of the richest men in California, decided to fund the Lick Observatory as his memorial with a telescope “superior to and more powerful than any yet made.” After his death in 1876, $ 700 000 from his estate was provided to commission a 36 inch (0.9 metre) Alvan Clark & Son refractor and dome. Completed in 1888 on 1283-metre Mount Hamilton, 100 kilometres south-east of the centre of San Francisco and overlooking the southern end of San Francisco Bay, it was the first observatory to be built on a mountaintop, and for a time it boasted the largest telescope of its type in the world. Having no slewing motors, it needed to be “yanked around” (as Lick astronomer Sherburne W. Burnham described it) by hand. James Lick was interred in the base of the pier. In 1889 the first Observatory Director Edward Singleton Holden (1846-1914) founded the Astronomical Society of the Pacific and became its first president.

Yerkes

George Ellery Hale (1868-1938) was born into a wealthy family. His father, who had made a fortune after the great Chicago fire of 1871 by supplying elevators for the new skyscrapers being built, encouraged his interest in astronomy and the Sun by equipping him with bigger and better telescopes and spectrometers. By the time he was 23, Hale had his own private solar astronomical laboratory with a 12 inch Brashear refractor. He was educated at the Massachusetts Institute of Technology (MIT) and Harvard University, and as an undergraduate he invented the spectroheliograph, with which he made his discoveries of solar vortices and the magnetic fields of sunspots. In 1892 he was offered and accepted a position on the faculty of the University of Chicago, and immediately launched into a life-long campaign to fund and build ever bigger and better astronomical observatories with huge telescopes.

He had heard of the giant new Lick refractor near San Francisco, and decided to build one a little larger. He proposed that the University order a 40 inch (1.02 m) Alvan Clark & Son refractor for a new observatory to be built at nearby Williams Bay in Wisconsin, and convinced Charles Tyson Yerkes to donate  $ 300 000 to pay for it, the intention being to "lick the Lick”. Yerkes was a mass-transit railroad magnate who had made a fortune from take-overs of local Chicago streetcar systems. He apparently felt that being known as the benefactor of the world’s largest telescope would greatly enhance his ability to raise credit, for he expected his business to expand rapidly. This was indeed the case, as from 1900 he became heavily involved in building the London Underground.

By May 1897, Yerkes Observatory was completed and the telescope was being tested. On the 29th a lifting cable for the observatory floor parted and the floor crashed down 13 metres, luckily in the early morning hours when no-one was about. There was one casualty: poor 64-year-old Alvan Graham Clark himself, when he heard the news, had a stroke and died eleven days later. Major repairs postponed the official opening for five months. Commissioned on 21 October with George Hale as Director, it is still the world’s largest refractor.

Tebbutt

On 13 May 1861, the Australian astronomer John Tebbutt (1834-1916) discovered the long-period comet that bears his name. It was one of the most brilliant comets ever seen, and became known as the Great Comet of 1861. There was no means then of telegraphing the information to England where it became visible on June 29, but, even so, Tebbutt was acknowledged as the discoverer of the comet, and as the first person to compute its approximate orbit. The Earth passed through the comet’s tail, the first time that this had been observed. In 1908, Tebbutt published his Astronomical Memoirs, giving an account of his 54 years’ work. His observatory is preserved as an historic site at Windsor, Sydney. For a time, Tebbutt and his observatory appeared on the Australian $ 100 banknote.

Shapley

Harlow Shapley (1885-1972), was offered a position working under George Hale at the Mount Wilson Observatory in 1914. At the time, astronomers were concerned with the nature of "spiral nebulae". These were clouds of gas and stars that had been discovered in their hundreds by William Herschel with his home-made reflecting telescopes between 1784 and 1815, and then in their thousands as telescopes improved and astrophotography became possible. Many had a spiral or 'whirlpool' shape, which led people to believe that they were spinning. Astronomers were unsure about what they were. Pierre-Simon de Laplace thought that they were small clouds in the Milky Way, and were possibly new solar systems being born. Kant thought that they were other Milky Ways lying at at such a great distance that they looked small - he called them "welt-inseln (world islands)". Von Humboldt suggested calling them "island-universes", which made the idea popular.

By 1920 Harlow Shapley had decided that the spiral nebulae were small objects inside our own galaxy, and the Milky Way itself comprised the whole observable universe - there was nothing else. He based this claim partly on the work of one of his colleagues, Adriaan van Maanen, who had closely studied photographs of some individual spiral nebulae taken decades apart. Using a blink microscope to switch from an old plate to a new one, he reported that he had seen evidence that the spirals were rotating. Shapley accepted van Maanen's claim uncritically. The Director of Mount Wilson, George Hale, encouraged Shapley’s investigations and assumptions, but sounded a note of caution, warning him, “substitute new hypotheses for old ones as soon as the evidence may demand.”      

On 26 April 1920 a 'Great Debate' was held at the Smithsonian Museum of Natural History in Washington, regarding the nature of the spiral nebulae and the scale of the universe. It was organised by George Hale, to debate whether the spiral nebulae were small objects in the Milky Way, or huge, remote island universes. He asked his employee Shapley to defend the first position, and asked Heber Doust Curtis of the Lick Observatory to put the case that the spirals were other giant Milky Ways in deep space. Oddly enough, Shapley and Curtis both travelled from California across the United States on the same Southern Pacific express. (Curtis was heading east to become Director of the Allegheny Observatory at the University of Pittsburgh.) In their conversations en route, both avoided the topic of spiral nebulae, even during an extended stopover in the middle of nowhere when the locomotive broke down. The thrust of the debate was concerned with the size of our Galaxy, and whether the spiral nebulae were part of it or remote ‘island-universes’ in their own right. Shapley was by then fully convinced that our Galaxy was the total universe, and was of sufficient size to contain these mysterious objects, although some astronomers believed that the spiral nebulae were clouds in our Galaxy which were condensing into stars or solar systems, in the manner suggested by de Laplace over a century before.

It was not a debate in the true sense. Each speaker was allotted forty minutes, and both used lantern slides to illustrate their talks. Shapley spoke first, and spent most of his time speaking at a level suitable for the general public, e.g. showing slides of open and globular clusters and describing the term ‘light-year’. He presented the evidence for his distances to globular clusters, and described how he had measured the immense size of the Milky Way without going into what he called “dreary technicalities”.  A major point was the location of the solar system far from our Galaxy’s centre. Shapley knew that in the audience were two representatives from the Harvard University Board. Edward Pickering had recently died, and they were casting around for a new Director for their Observatory. His talk was aimed to impress them with his maturity and scientific ability, rather than to convince the audience of his point of view. For this reason, he hardly mentioned the spiral nebulae, and his teacher Henry Norris Russell spoke lucidly from the floor in support of Shapley. As it turned out, the person who most impressed the Harvard people was Russell. They offered him the Directorship, but he refused it, not wishing to devote his time to administration. Russell lobbied instead for Shapley, who was offered a one-year temporary Directorship in 1921, which was made permanent before it had expired.

Working at Harvard in the first two decades of the twentieth century was Henrietta Swan Leavitt (1868-1921). Her research interest was photometry – the determination of the brightness or apparent magnitude of a star from its photographic image. She measured the brightnesses of star images on plates of the Magellanic Clouds (two dwarf galaxies in the southern sky) taken at the Boyden Field Station at Arequipa in Peru, and in 1907 discovered 1777 variable stars on them. In 1908 Miss Leavitt reported that 16 of the variables in the Small Magellanic Cloud (SMC) were bright, yellow, short-period pulsating stars known as Cepheids. These stars change brightness as they expand and contract on a regular basis (often in a few days): as they expand they fade, and as they contract they brighten. She noticed that the brightest of these Cepheids were the slowest to complete a cycle of variation – the fainter ones cycled faster, so in this group there was a relationship between each Cepheid’s period and its apparent magnitude. As the Cloud was compact, she could assume that the Cepheids were all at the same distance from us, and brighter ones were so because of their own intrinsic luminosity. This meant that differences in apparent magnitude were the same as differences in absolute magnitude, so each Cepheid’s period was also related to its absolute magnitude or luminosity. Also, as they could be resolved individually in the SMC, they had to be giant stars. By 1912 she had measured the changing brightnesses of 25, and announced her discovery which is now known as the period-luminosity relationship.

In a nutshell, if a person measured the period of a Cepheid's cycle, this would give its absolute magnitude. Simple arithmetic in comparing its absolute magnitude with its apparent magnitude would tell us how far away it was, which would be the same as any cluster or galaxy in which the Cepheid was located. To find the distance of a spiral nebula, you would first locate a Cepheid in it. Then find the Cepheid's apparent brightness and do a simple calculation. This is what Edwin Hubble (1889-1953) determined to do when he started work at Mount Wilson Observatory in 1920 after wartime service as a Major in the Army.

The 100 inch Hooker Telescope at Mount Wilson Observatory near Los Angeles had been commissioned in November 1917, and one of the handlers (called ‘mule skinners’) who used mule teams to drag loads of building materials and telescope parts up to the summit of Mount Wilson was Milton Humason (1891-1972). On arrival at the Observatory site, Humason had asked the Director George Hale for a job there, and had the good fortune to be appointed as a caretaker. In 1919, Harlow Shapley promoted him to a position in the photo lab, then to Night Assistant (operator of the telescopes for the astronomers). When Hubble arrived, he asked Humason to assist him by using the new telescope to photograph nearby spiral nebulae. The canny Humason quickly earned the egocentric Hubble’s support and friendship by always addressing him as ‘Major’. Between them, they found that the superlative Hooker Telescope, then the world’s largest, could resolve the outer arms of some nearby spirals into individual stars, which was strong evidence that they were the ‘island-universes’ outside the Milky Way that Kant and Curtis had believed. To finally prove that the spirals were indeed extragalactic, their distances needed to be known, so Hubble and Humason began to search for Henrietta Leavitt’s Cepheid variable stars in those close spirals.

Harlow Shapley, like Hubble born in Missouri, could not abide Hubble’s pretentious English accent, English mannerisms (saying “What-ho!“ and "Bai Jove!"), and affectations such as pipe-smoking and wearing jodhpurs, a cape and riding boots to work (all acquired while spending nearly a year at Cambridge University after the war), so when offered the Directorship of Harvard Observatory in 1921, Shapley left Mount Wilson permanently. This took him away from the centre of the action, which he had to observe from the other side of the country. Meanwhile, Humason spent many hours guiding the 100 inch telescope for each photograph, some exposures being 70 hours long, spread over weeks. By 1924 Hubble had found Cepheids in a number of nearby spirals, and was using them as ‘standard candles’ to find their approximate distances. He began by measuring six Cepheids in the Andromeda nebula M31. The first of these reached only 18th magnitude at maximum, yet its long period of 31 days indicated that it was a giant star, thousands of times brighter than the Sun, and dim only because of its great remoteness. The application of Leavitt’s period-luminosity relationship gave a distance of 930 000 light-years. For M31 to loom so large at such a distance, then it had to rival the Milky Way in size. (This was notable work, but if a more exact distance-luminosity calibration for Cepheids determined from parallax had been known at the time, then the results might have been closer to the modern value of 2 540 000 light-years, nearly three times the distance.) Hubble wrote to Shapley at Harvard to let him know before the public announcement was made, that the Cepheid distances proved that the spiral nebulae were indeed huge accumulations of millions of stars, far from the Milky Way. Shapley glumly showed the letter to his staff member Cecilia Payne saying, "Here is the letter that has destroyed my universe."

At a joint American Astronomical Society + American Association for the Advancement of Science meeting in Washington DC on 1 January 1925 (99 years ago), the eminent astrophysicist Henry Norris Russell read Hubble's paper for him, announcing that M31 was much too far away to be considered a part of the Milky Way system, but had to be a separate galaxy. Therefore, the Milky Way was not alone, but was merely an ordinary galaxy, surrounded by thousands of other galaxies. The Great Debate was finally settled in Curtis’ favour by Hubble, thanks to Henrietta Leavitt’s Cepheids. Cecilia Payne was at the meeting and reported back to Shapley, who regretted his error for the rest of his life, but went on to do valuable work on what he now fully accepted to be external galaxies in the depths of space, entirely remote from the Milky Way. Hubble’s protégé Allan Sandage wrote, “What are spiral nebulae? No-one knew before 1900. Few people knew in 1920. All astronomers knew after 1924.”

Hubble never gave Vesto Slipher credit for originally discovering the expansion of the universe in any of his published papers, nor to Humason, who did the hard graft. We can now give credit where it’s due. Hubble always felt that the rate of the cosmological expansion was too high to be real, always calling the velocities of spiral nebulae ‘apparent velocities’, hedging his bets in case the redshift was due to some unknown cause. When Shapley suggested that the external star systems be known as ‘galaxies’, Hubble refused to use the term, preferring to call them ‘non-galactic nebulae’, and then ‘extra-galactic nebulae’. The term ‘galaxies’ only came into general use after Hubble had died in 1953, and the new Mount Palomar 200 inch and 48 inch Schmidt telescopes had begun operation.

 

 

The area around the Mare Crisium is located inside the rectangle.
 

Click  here  for the  Lunar Features of the Month Archive

 

 

Geocentric Events:



It should be remembered that close approaches of Moon, planets and stars are only perspective effects as seen from the Earth - that is why they are called 'geocentric or Earth-centred phenomena'. The Moon, planets and stars do not really approach and dance around each other as it appears to us from the vantage point of our speeding planet.


March 3:             Moon 1.8º south of the star Dschubba (Delta Scorpii, mv= 1.86) at 5.45 hrs
March 3:             Moon 1.6º north of the star Pi Scorpii (mv= 2.89) at 6:49 hrs
March 3:             Limb of Moon 18 arcminutes north of the star Alniyat (Sigma Scorpii, mv= 2.9) at 16:05 hrs
March 3:             Moon 1.1º north of the star Antares (Alpha Scorpii, mv= 0.88) at 18:44 hrs
March 3:             Moon 2.5º north of the star Tau Scorpii (mv= 2.82) at 21:03 hrs
March 5:             Moon 2.15º north of the star Alnasl (Gamma2 Sagittarii, mv= 2.98) at 11:35 hrs
March 5:             Moon 2.1º north of the star Kaus Media (Delta Sagittarii, mv= 2.72) at 16:53 hrs
March 5:             Moon 2.2º south of the star Kaus Borealis (Lambda Sagittarii, mv= 2.92) at 19:01 hrs
March 6:             Moon 1.7º south of the star Nunki (Sigma Sagittarii, mv= 2.02) at 4:19 hrs
March 6:             Moon 1.9º north of the star Ascella (Zeta Sagittarii, mv= 2.6) at 7:25 hrs
March 7:             Moon 1.6º south of Pluto ar 14:13 hrs
March 7:             Venus 1.7º north of the star Deneb Algiedi (Delta Capricorni, mv= 2.85) at 8:58 hrs
March 8:             Moon 2.5º south of Mars at 17:58 hrs
March 9:             Mercury 26 arcminutes north of Neptune at 01:02 hrs
March 9:             Limb of Moon 50 arcminutes south of the star Deneb Algiedi (Delta Capricorni, mv= 2.85) at 12:22 hrs
March 9:             Moon 2.9º south of Venus at 3:25 hrs
March 10:           Moon 1.2º south of Saturn at 3:01 hrs
March 11:           Limb of Moon 10 arcminutes south of Neptune at 4:20 hrs
March 11:           Limb of Moon 11 arcminutes south of Mercury at 13:09 hrs
March 14:           Moon 3.3º north of Jupiter at 7:45 hrs
March 14:           Moon 1.2º south of Saturn at 3:01 hrs
March 15:           Mars 1.5º north of the star Deneb Algedi (Delta Capricorni, mv= 2.85) at 13:15 hrs
March 15:           Moon occults the star Alcyone (Eta Tauri, mv= 2.85) between 11:50 and 12:57 hrs
March 17:           Limb of Moon 19 arcminutes south of the star Elnath (Beta Tauri, mv= 1.65) at 6:20 hrs
March 17:           Saturn 1.25º south of the star Hydor (Lambda Aquarii, mv= 3.73) at 18:14 hrs
March 17:           Neptune in conjunction with the Sun at 21:28 hrs  (diameter = 2.2")
March 18:           Mercury at perihelion at 2:34 hrs (diameter = 6.2")
March 19:           Limb of Moon 32 arcminutes south of the star Pollux (Beta Geminorum, mv= 1.15) at 15:51 hrs
March 20:           Venus at aphelion at 3:04 hrs  (diameter = 10.6")
March 22:           Venus 19 arcminutes north of Saturn at 9:05 hrs
March 25:           Mercury at Greatest Elongation East (18º 33') at 3:43 hrs  (diameter = 7.4")
March 27:           Moon 1.36º north of the star Spica (Alpha Virginis, mv=0.98) at 9:05 hrs
March 30:           Moon 1.4º south of the star Dschubba (Delta Scorpii, mv= 1.86) at 12.40 hrs
March 30:           Moon 2.1º north of the star Pi Scorpii (mv= 2.89) at 13:15 hrs
March 30:           Limb of Moon 15 arcminutes north of the star Alniyat (Sigma Scorpii, mv= 2.9) at 20:10 hrs
March 30:           Limb of Moon 19 arcminutes north of the star Antares (Alpha Scorpii, mv= 0.88) at 23:58 hrs
March 31:           Moon 1.7º north of the star Tau Scorpii (mv= 2.82) at 4:46 hrs
 

April 1:               Moon 2.6º north of the star Alnasl (Gamma2 Sagittarii, mv= 2.98) at 17:05 hrs
April 1:               Moon 1.8º north of the star Kaus Media (Delta Sagittarii, mv= 2.72) at 21:39 hrs
April 2:               Moon 2.8º south of the star Kaus Borealis (Lambda Sagittarii, mv= 2.92) at 00:31 hrs
April 2:               Mercury at eastern stationary point at 8:13 hrs
April 2:               Moon 1.1º south of the star Nunki (Sigma Sagittarii, mv= 2.02) at 14:12 hrs
April 2:               Moon 2.65º north of the star Ascella (Zeta Sagittarii, mv= 2.6) at 15:51 hrs
April 3:               Moon 1.6º south of Pluto at 21:29 hrs
April 5:               Moon 1.05º south of the star Deneb Algedi (Delta Capricorni, mv= 2.85) at 12:09 hrs
April 6:               Limb of Moon 41 arcminutes south of Mars at 16:19 hrs
April 6:               Limb of Moon 10 arcminutes south of Saturn at 20:20 hrs
April 7:               Limb of Moon 8 arcminutes north of Neptune at 19:13 hrs
April 8:               Limb of Moon 14 arcminutes north of Venus at 01:16 hrs
April 9:               Moon 1.3º south of Mercury at 12:23 hrs
April 9:               Uranus of the star Botein (Delta Arietis (mv= 4.34) at 16:42 hrs
April 11:             Moon 3.7º north of Jupiter at 4:39 hrs
April 11:             Mars 26 arcminutes north of Saturn at 8:42 hrs
April 11:             Moon 3.4º north of Uranus at 7:08 hrs
April 11:             Moon occults the star Alcyone (Eta Tauri, mv= 2.85) between 23:56 and 00:33 hrs
April 12:             Mercury at inferior conjunction at 8:56 hrs
April 13:             Moon occults the star Elnath (Beta Tauri, mv= 1.65) between 13:44 and 14:34 hrs
April 16:             Moon 1.3º south of the star Pollux (Beta Gemiorum, mv= 1.15) at 01:30 hrs
April 19:             Mercury 1.7º north of Venus at 18:39 hrs
April 21:             Jupiter 30 arcminutes south of Uranus at 13:42 hrs
April 22:             Pluto at western quadrature at 2:48 hrs   (diameter = 0.1")
April 22:             Neptune 1.9º north of the star 27 Piscium (mv= 4.9) at 5:31 hrs
April 23:             Moon 1.9º north of the star Spica (Alpha Virginis, mv=0.98) at 13:28 hrs
April 25:             Mercury at western stationary point at 22:41 hrs
April 26:             Moon 1.1º south of the star Dschubba (Delta Scorpii, mv= 1.86) at 16.18 hrs
April 26:             Moon 2.3º north of the star Pi Scorpii (mv= 2.89) at 16:51 hrs
April 27:             Moon occults the star Alniyat (Sigma Scorpii, mv= 2.9) between 03:16 and 4:24 hrs
April 27:             Limb of Moon 17 arcminutes north of the star Antares (Alpha Scorpii, mv= 0.88) at 8:21 hrs
April 27:             Moon 2.3º north of the star Tau Scorpii (mv= 2.82) at 11:11 hrs
April 28:             Moon 2.3º north of the star Alnasl (Gamma2 Sagittarii, mv= 2.98) at 21:01 hrs
April 29:             Moon 1.3º north of the star Kaus Media (Delta Sagittarii, mv= 2.72) at 5:13 hrs
April 29:             Moon 2.7º south of the star Kaus Borealis (Lambda Sagittarii, mv= 2.92) at 09:15 hrs
April 29:             Mars 15 arcminutes south of Neptune at 14:33 hrs
April 29:             Moon 1.1º south of the star Nunki (Sigma Sagittarii, mv= 2.02) at 17:55 hrs
April 29:             Moon 2.5º north of the star Ascella (Zeta Sagittarii, mv= 2.6) at 19:37 hrs
 


 

The Planets for this month:

 

Mercury:    The innermost planet passed through superior conjunction (on the far side of the Sun) on February 28 and is now in the western twilight sky. It begins the month in the constellation Aquarius, and crosses into Pisces on March 12. On March 11 the thin crescent Moon will be close by Mercury, but both will be swamped by the glare of the nearby Sun.  This will not be a very favourable appearance of Mercury as it will remain close to the horizon each evening until it passes through inferior conjunction on April 12. Its maximum eastern elongation from the Sun will be 18.5 degrees on March 25, but the altitude above the theoretical horizon will be only about 10 degrees (half-a-handspan).


 

Venus:    This, the brightest planet, was an 'evening star' in the western sky for most of 2023. It passed through inferior conjunction (between the Earth and the Sun) on August 13 last, and then moved to the pre-dawn sky as a 'Morning Star', where it still is. Currently in the constellation Capricornus, it rises in the east a little before 4:30 am at mid-month. The waning crescent Moon will be just to the right (south) of Venus on March 9. Venus will cross into Aquarius at dawn on March 10, and at 5 am on that date, Venus will be halfway between Mars and the eastern horizon. At 5 am on March 22, Venus and Saturn will be only 20 arcminutes apart in the pre-dawn sky. Venus will pass on the far side of the Sun (superior conjunction) on June 5, and will then move to the western twilight sky as an 'Evening Star', becoming visible to the casual observer during August.

In March Venus is a beautiful sight in the morning sky, and observers with a small telescope will see it as a little almost-full Moon, becoming a little smaller and a little 'fuller' each night as it leaves the Earth behind.

(The coloured fringes to the second, fourth and fifth images below are due to refractive effects in our own atmosphere, and are not intrinsic to Venus itself. The planet was closer to the horizon when these images were taken than it was for the first and third photographs, which were taken when Venus was at its greatest elongation from the Sun.)  

             October 2023                       May-July 2024                    January 2025                         March 2025                            April 2025               

Click here for a photographic animation showing the Venusian phases. Venus is always far brighter than anything else in the sky except for the Sun and Moon. For most of 2023, Venus appeared as an 'Evening Star' in the western twilight sky, but last August it moved to the pre-dawn eastern sky to be a 'Morning Star'. It is now low to the eastern horizon before sunrise. It will reappear as an 'Evening Star' in the west in August.

Because Venus is visible as the 'Evening Star' and as the 'Morning Star', astronomers of ancient times believed that it was two different objects. They called it Hesperus when it appeared in the evening sky and Phosphorus when it was seen before dawn. They also realised that these objects moved with respect to the so-called 'fixed stars' and so were not really stars themselves, but planets (from the Greek word for 'wanderers'). When it was finally realised that the two objects were one and the same, the two names were dropped and the Greeks applied a new name Aphrodite (Goddess of Love)  to the planet, to counter Ares (God of War). We use the Roman versions of these names, Venus and Mars, for these two planets.



Venus at 6.55 pm on September 7, 2018. The phase is 36 % and the angular diameter is 32 arcseconds.

 

 

Mars:   In the first three weeks of March, the red planet is cruising through the constellation Capricornus. It crosses into Aquarius on March 20. It is in the pre-dawn eastern sky, having been in conjunction with the Sun on November 18. It is on the far side of its orbit, and is very small (4 arcseconds in diameter) and faint (magnitude = 1.2). The waning crescent Moon will make a fine threesome with Mars and Venus on March 8 and 9. On April 6 and 7, the waning crescent Moon will make a fine threesome with Mars and Saturn.

In this image, the south polar cap of Mars is easily seen. Above it is a dark triangular area known as Syrtis Major. Dark Sinus Sabaeus runs off to the left, just south of the equator. Between the south polar cap and the equator is a large desert called Hellas. The desert to upper left is known as Aeria, and that to the north-east of Syrtis Major is called Isidis Regio.  Photograph taken in 1971.



Mars photographed from Starfield Observatory, Nambour on June 29 and July 9, 2016, showing two different sides of the planet.  The north polar cap is prominent.

 

Brilliant Mars at left, shining at magnitude 0.9, passes in front of the dark molecular clouds in Sagittarius on October 15, 2014. At the top margin is the white fourth magnitude star 44 Ophiuchi. Its type is A3 IV:m. Below it and to the left is another star, less bright and orange in colour. This is the sixth magnitude star SAO 185374, and its type is K0 III. To the right (north) of this star is a dark molecular cloud named B74. A line of more dark clouds wends its way down through the image to a small, extremely dense cloud, B68, just right of centre at the bottom margin. In the lower right-hand corner is a long dark cloud shaped like a figure 5. This is the Snake Nebula, B72. Above the Snake is a larger cloud, B77. These dark clouds were discovered by Edward Emerson Barnard at Mount Wilson in 1905. He catalogued 370 of them, hence the initial 'B'. The bright centre of our Galaxy is behind these dark clouds, and is hidden from view. If the clouds were not there, the galactic centre would be so bright that it would turn night into day.


Mars near opposition, July 24, 2018


Mars, called the red planet but usually coloured orange, in mid-2018 took on a yellowish tint and brightened by 0.4 magnitude, making it twice as bright as previous predictions for the July 27 opposition. These phenomena were caused by a great dust storm which completely encircled the planet, obscuring the surface features so that they were only seen faintly through the thick curtain of dust. Although planetary photographers were mostly disappointed, many observers were interested to see that the yellow colour and increased brightness meant that a weather event on a distant planet could actually be detected with the unaided eye - a very unusual thing in itself.

The three pictures above were taken on the evening of July 24, at 9:05, 9:51 and 11:34 pm. Although the fine details that are usually seen on Mars were hidden by the dust storm, some of the larger features can be discerned, revealing how much Mars rotates in two and a half hours. Mars' sidereal rotation period (the time taken for one complete rotation or 'Martian day') is 24 hours 37 minutes 22 seconds - a little longer than an Earth day. The dust storm began in the Hellas Desert on May 31, and after two months it still enshrouded the planet. In September it began to clear, but by then the close approach had passed.
 

Central meridian: 295º.
 

 

The two pictures immediately above were taken on the evening of September 7, at 6:25 and 8:06 pm. The dust storm was finally abating, and some of the surface features were becoming visible once again. This pair of images also demonstrates the rotation of Mars in 1 hour 41 minutes (equal to 24.6 degrees of longitude), but this time the view is of the opposite side of the planet to the set of three above. As we were now leaving Mars behind, the images are appreciably smaller (the angular diameter of the red planet had fallen to 20 arcseconds). Well past opposition, Mars on September 7 exhibited a phase effect of 92.65 %.


 
Central meridian: 180º.

 

 

Jupiter:   Jupiter is now found one-and-a-half handspans above the north-western horizon at 7 pm on March 1. Observations at such a low elevation are always affected by turbulence in our atmosphere. The waxing crescent Moon will be just to the right of Jupiter on March 14. Jupiter will be 30 arcminutes south of Uranus just after sunset on April 21, and will cross into Taurus on April 28. Jupiter will pass though conjunction with the Sun on May 19 and will then leave the evening sky. It will reappear in the eastern pre-dawn sky in mid-June. . 

     

Jupiter as photographed from Nambour on the evening of April 25, 2017. The images were taken, from left to right, at 9:10, 9:23, 9:49, 10:06 and 10:37 pm. The rapid rotation of this giant planet in a little under 10 hours is clearly seen. In the southern hemisphere, the Great Red Spot (bigger than the Earth) is prominent, sitting within a 'bay' in the South Tropical Belt. South of it is one of the numerous White Spots. All of these are features in the cloud tops of Jupiter's atmosphere.



Jupiter as it appeared at 7:29 pm on July 2, 2017. The Great Red Spot was in a similar position near Jupiter's eastern limb (edge) as in the fourth picture in the series above. It will be seen that in the past two months the position of the Spot had drifted when compared with the festoons in the Equatorial Belt, so must rotate around the planet at a slower rate. In fact, the Belt enclosing the Great Red Spot rotates around the planet in 9 hours 55 minutes, and the Equatorial Belt takes five minutes less. This high rate of rotation has made the planet quite oblate. The prominent 'bay' around the Red Spot in the five earlier images appeared to be disappearing, and a darker streak along the northern edge of the South Tropical Belt was moving south. In June this year the Spot began to shrink in size, losing about 20% of its diameter. Two new white spots have developed in the South Temperate Belt, west of the Red Spot. The five upper images were taken near opposition, when the Sun was directly behind the Earth and illuminating all of Jupiter's disc evenly. The July 2 image was taken just four days before Eastern Quadrature, when the angle from the Sun to Jupiter and back to the Earth was at its maximum size. This angle means that we see a tiny amount of Jupiter's dark side, the shadow being visible around the limb of the planet on the left-hand side, whereas the right-hand limb is clear and sharp. Three of Jupiter's Galilean satellites are visible, Ganymede to the left and Europa to the right. The satellite Io can be detected in a transit of Jupiter, sitting in front of the North Tropical Belt, just to the left of its centre.  
 

Jupiter at opposition, May 9, 2018

     

Jupiter reached opposition on May 9, 2018 at 10:21 hrs, and the above photographs were taken that evening, some ten to twelve hours later. The first image above was taken at 9:03 pm, when the Great Red Spot was approaching Jupiter's central meridian and the satellite Europa was preparing to transit Jupiter's disc. Europa's transit began at 9:22 pm, one minute after its shadow had touched Jupiter's cloud tops. The second photograph was taken three minutes later at 9:25 pm, with the Great Red Spot very close to Jupiter's central meridian.

The third photograph was taken at 10:20 pm, when Europa was approaching Jupiter's central meridian. Its dark shadow is behind it, slightly below, on the clouds of the North Temperate Belt. The shadow is partially eclipsed by Europa itself. The fourth photograph at 10:34 pm shows Europa and its shadow well past the central meridian. Europa is the smallest of the Galilean satellites, and has a diameter of 3120 kilometres. It is ice-covered, which accounts for its brightness and whitish colour. Jupiter's elevation above the horizon for the four photographs in order was 50º, 55º, 66º and 71º. As the evening progressed, the air temperature dropped a little and the planet gained altitude. The 'seeing' improved slightly, from Antoniadi IV to Antoniadi III. At the time of the photographs, Europa's angular diameter was 1.57 arcseconds. Part of the final photograph is enlarged below.

 

Jupiter at 11:34 pm on May 18, nine days later. Changes in the rotating cloud patterns are apparent, as some cloud bands rotate faster than others and interact. Compare with the first photograph in the line of four taken on May 9. The Great Red Spot is ploughing a furrow through the clouds of the South Tropical Belt, and is pushing up a turbulent bow wave.
 

Jupiter at opposition, June 11, 2019

     
    

Jupiter reached opposition on June 11, 2019 at 01:20 hrs, and the above photographs were taken that evening, some twenty to twenty-two hours later. The first image above was taken at 10:01 pm, when the Great Red Spot was leaving Jupiter's central meridian and the satellite Europa was preparing to transit Jupiter's disc. Europa's transit began at 10:11 pm, and its shadow touched Jupiter's cloud tops almost simultaneously. Europa was fully in transit by 10:15 pm. The second photograph was taken two minutes later at 10:17 pm, with the Great Red Spot heading towards Jupiter's western limb.

The third photograph was taken at 10:41 pm, when Europa was about a third of its way across Jupiter. Its dark shadow is trailing it, slightly below, on the clouds of the North Temperate Belt. The shadow is partially eclipsed by Europa itself. The fourth photograph at 10:54 pm shows Europa and its shadow about a quarter of the way across. This image is enlarged below. The fifth photograph shows Europa on Jupiter's central meridian at 11:24 pm, with the Great Red Spot on Jupiter's limb. The sixth photograph taken at 11:45 pm shows Europa about two-thirds of the way through its transit, and the Great Red Spot almost out of sight. In this image, the satellite Callisto may be seen to the lower right of its parent planet. Jupiter's elevation above the horizon for the six photographs in order was 66º, 70º, 75º, 78º, 84º and 86º. As the evening progressed, the 'seeing' proved quite variable.

There have been numerous alterations to Jupiter's belts and spots over the thirteen months since the 2018 opposition. In particular, there have been major disturbances affecting the Great Red Spot, which appears to be slowly changing in size or "unravelling".  It was very fortuitous that, during the evenings of the days when the 2018 and 2019 oppositions occurred, there was a transit of one of the satellites as well as the appearance of the Great Red Spot. It was also interesting in that the same satellite, Europa, was involved both times.


Jupiter's moon Europa has an icy crust with very high reflectivity, which accounts for its brightness in the images above. On the other hand, the largest moon Ganymede (seen below) has a surface which is composed of two types of terrain: very old, highly cratered dark regions, and somewhat younger (but still ancient) lighter regions marked with an extensive array of grooves and ridges. Although there is much ice covering the surface, the dark areas contain clays and organic materials and cover about one third of the moon. Beneath the surface of Ganymede is believed to be a saltwater ocean with two separate layers.

Jupiter is seen here on 17 November 2022 at 8:39 pm. To its far right is its largest satellite, Ganymede. This "moon" is smaller than the Earth but is bigger than Earth's Moon. Its diameter is 5268 kilometres, but at Jupiter's distance its angular diameter is only 1.67 arcseconds. Despite its small size, Ganymede is the biggest moon in the Solar System. Jupiter is approaching eastern quadrature, which means that Ganymede's shadow is not behind it as in the shadows of Europa in the two sequences taken at opposition. In the instance above as seen from Earth (which is presently at a large angle from a line joining the Sun to Ganymede), the circular shadow of Ganymede is striking the southern hemisphere cloud tops of Jupiter itself. The shadow is slightly distorted as it strikes the spherical globe of Jupiter. If there were any inhabitants of Jupiter flying across the cloud bands above, and passing through the black shadow, they would experience an eclipse of the distant Sun by the moon Ganymede.


Above is a 7X enlargement of Ganymede, showing markings on its rugged, icy surface. The dark area in its northern hemisphere is called Galileo Regio.
 



Saturn:   The ringed planet is located in the constellation of Aquarius, and will remain there until it crosses into Pisces on April 19, 2025. Saturn reached conjunction (behind the Sun) on February 29, and then moved to the pre-dawn eastern sky. It will not be observable in the morning sky until late in March.

   

Left: Saturn showing the Rings when edge-on.    Right: Over-exposed Saturn surrounded by its satellites Rhea, Enceladus, Dione, Tethys and Titan - February 23/24, 2009.




 
Saturn with its Rings wide open on July 2, 2017. The shadow of its globe can just be seen on the far side of the Ring system. There are three main concentric rings: Ring A is the outermost, and is separated from the brighter Ring B by a dark gap known as the Cassini Division, which is 4800 kilometres wide, enough to drop Australia through. Ring A also has a gap inside it, but it is much thinner. Called the 'Encke Gap', it is only 325 kilometres wide and can be seen in the image above. The innermost parts of Ring B are not as bright as its outermost parts. Inside Ring B is the faint Ring C, almost invisible but noticeable where it passes in front of the bright planet as a dusky band. Spacecraft visiting Saturn have shown that there are at least four more Rings, too faint and tenuous to be observable from Earth, and some Ringlets. Some of these extend from the inner edge of Ring C to Saturn's cloudtops. The Rings are not solid, but are made up of countless small particles, 99.9% water ice with some rocky material, all orbiting Saturn at different distances and speeds. The bulk of the particles range in size from dust grains to car-sized chunks. At bottom centre, the southern hemisphere of the planet can be seen showing through the gap of the Cassini Division. The ring system extends from 7000 to 80 000 kilometres above Saturn's equator, but its thickness varies from only 10 metres to 1 kilometre. The globe of Saturn has a diameter at its equator of 120 536 kilometres. Being made up of 96% hydrogen and 3% helium, it is a gas giant, although it has a small, rocky core. There are numerous cloud bands visible.

The photograph above was taken when Saturn was close to opposition, with the Earth between Saturn and the Sun. At that time, the shadow of Saturn's globe upon the Ring system was directly behind the planet and hardly visible. The photograph below was taken at 7:14 pm on September 09, 2018, when Saturn was near eastern quadrature. At such a time, the angle from the Sun to Saturn and back to the Earth is near its maximum, making the shadow fall at an angle across the Rings as seen from Earth. It may be seen falling across the far side of the Ring to the left side of the globe.

The photograph above was taken at 8:17 pm on November 03, 2022, when Saturn was again near eastern quadrature. The shadow of the planet once again falls across the far side of the rings, but in the intervening four years the angle of the rings as seen from Earth has been greatly reduced. The shadow of Ring B across the globe of Saturn is much darker from this angle.



The photograph above was taken at 7:41 pm on December 07, 2023, 14 days after eastern quadrature. The shadow of the planet once more falls across the far side of the rings, but in the intervening 13 months the angle of the rings as seen from Earth has lessened considerably. The light-coloured equatorial zone on Saturn shows through the gap known as the Cassini Division.


The change in aspect of Saturn's rings is caused by the plane of the ring system being aligned with Saturn's equator, which is itself tilted at an angle of 26.7 degrees to Saturn's orbit. As the Earth's orbit around the Sun is in much the same plane as Saturn's, and the rings are always tilted in the same direction in space, as we both orbit the Sun, observers on Earth see the configuration of the rings change from wide open (top large picture) to half-open (bottom large picture) and finally to edge on (small picture above). This cycle is due to Saturn taking 29.457 years to complete an orbit of the Sun, so the complete cycle from "edge-on (2009) → view of Northern hemisphere, rings half-open (2013) → wide-open (2017) → half-open (2022) → edge-on (2025) → view of Southern hemisphere, rings half-open (2029) → wide-open (2032) → half-open (2036) → edge-on (2039)" takes 29.457 years. The angle of the rings will continue to reduce until they are edge-on again in 2025. They will appear so thin that it will seem that Saturn has no rings at all. The rings will be wide-open again in 2032.


 


Uranus:
This ice giant planet shines at about magnitude 5.8, so a pair of binoculars or a small telescope is required to observe it. Uranus is currently in the constellation of Aries, and it passed through eastern quadrature on February 8. At mid-month, Uranus will be less than a handspan above the western horizon, so observing conditions will not be ideal. The waxing crescent Moon will make a triangle with Jupiter and Uranus on March 14. Uranus will be in conjunction with the Sun on May 13.



 

Neptune:   The icy blue planet will be in conjunction with the Sun on March 17, so cannot be observed this month. It will not return to the evening sky until next July.

Neptune, photographed from Nambour on October 31, 2008


Pluto:
   The erstwhile ninth and most distant planet passed through conjunction with the Sun on January 20, so is now in the night sky after midnight. It will pass through western quadrature (rising at midnight) on April 22. Pluto's angular diameter is 0.13 arcseconds, less than one twentieth that of Neptune. Located in Capricornus, it is close to the border with Sagittarius. It is a 14.1 magnitude object, very small and faint. A telescope with an aperture of 25 cm is capable of locating Pluto when the seeing conditions are right. The waning crescent Moon will be above Pluto on March 7.

 

  

The movement of the dwarf planet Pluto in two days, between 13 and 15 September, 2008. Pluto is the one object that has moved.
Width of field:   200 arcseconds

This is a stack of four images, showing the movement of Pluto over the period October 22 to 25, 2014. Pluto's image for each date appears as a star-like point at the upper right corner of the numerals. The four are equidistant points on an almost-straight line. Four eleventh magnitude field stars are identified.  A is GSC 6292:20, mv = 11.6.  B is GSC 6288:1587, mv = 11.9.  C is GSC 6292:171, mv = 11.2.  D is GSC 6292:36, mv = 11.5.  (GSC = Guide Star Catalogue).   The position of Pluto on October 24 (centre of image) was at Right Ascension = 18 hours 48 minutes 13 seconds,  Declination =  -20º 39' 11".  The planet moved 2' 51" with respect to the stellar background during the three days between the first and last images, or 57 arcseconds per day, or 1 arcsecond every 25¼ minutes.


 


Planetary Alignments:

 

On the morning of March 8, the waning crescent Moon, Mars and Venus will be lined up together at 5 am. Faint Mars will be midway between the Moon and Venus. The Moon will be alongside Venus on March 9. Saturn will be below the Moon, but will be difficult to find in the glare of the Sun. Mars will cross into Aquarius on March 20. By March 22, Venus and Saturn will be close together in the sky, and Mars will be about two-thirds of a handspan above them both. On the morning of April 6, the Moon, Mars and Saturn will make an interesting threesome, but Venus will be close to the horizon as it approaches conjunction with the Sun. On April 11, Mars will pass 27 arcminutes north of Saturn, and on April 20, Mercury will reappear, just to the left of Venus. On the morning of April 24, Mars will cross into Pisces, and on April 29 will pass south-east of the planet Neptune at a distance of only 2 arcminutes. Unfortunately, this rare close approach will occur at 2:38 pm in the afternoon. Venus will leave the morning sky when it passes on the far side of the Sun (superior conjunction) on June 5. It will reappear in the western twilight sky in August. Jupiter and Uranus will come within 30 arcminutes of each other just after sunset on April 21. Mars and Jupiter will have a very close approach at 2:15 am on the morning of August 15, when they will rise above the east-north-eastern horizon together, being only 19 arcminutes apart.  

On the morning of April 11, Mars will pass 27 arcminutes north of Saturn. On the morning of April 24, Mars will cross into Pisces, and on April 29 will pass south-east of the planet Neptune at a distance of only 2 arcminutes. Unfortunately, this rare close approach will occur at 2:38 pm in the afternoon.

 

 

Meteor Showers: 



No meteor showers in March.

Lyrids                              April 23                  Waxing gibbous Moon, 97% sunlit                           ZHR = 15
                                        Radiant:  Near the star Vega. 
      Associated with Comet Thatcher.

Pi Puppids                      April 24                   Almost Full Moon, 99% sunlit                                  ZHR = 10
                                        Radiant:  Between the False Cross and the tail of Canis Major


Use this  
Fluxtimator  to calculate the number of meteors predicted per hour for any meteor swarm on any date, for any place in the world.


ZHR = zenithal hourly rate (number of meteors expected to be observed at the zenith in one hour). The maximum phase of meteor showers usually occurs between 3 am and sunrise. The reason most meteors are observed in the pre-dawn hours is because at that time we are on the front of the Earth as it rushes through space at 107 000 km per hour (30 km per second). We are meeting the meteors head-on, and the speed at which they enter our atmosphere is the sum of their own speed plus ours. In the evenings, we are on the rear side of the Earth, and many meteors we see at that time are actually having to catch us up. This means that the speed at which they enter our atmosphere is less than in the morning hours, and they burn up less brilliantly.

Although most meteors are found in swarms associated with debris from comets, there are numerous 'loners', meteors travelling on solitary paths through space. When these enter our atmosphere, unannounced and at any time, they are known as 'sporadics'. On an average clear and dark evening, an observer can expect to see about ten meteors per hour. They burn up to ash in their passage through our atmosphere. The ash slowly settles to the ground as meteoric dust. The Earth gains about 80 tonnes of such dust every day, so a percentage of the soil we walk on is actually interplanetary in origin. If a meteor survives its passage through the air and reaches the ground, it is called a 'meteorite'.  In the past, large meteorites (possibly comet nuclei or small asteroids) collided with the Earth and produced huge craters which still exist today. These craters are called 'astroblemes'. Two famous ones in Australia are Wolfe Creek Crater and Gosse's Bluff. The Moon and Mercury are covered with such astroblemes, and craters are also found on Venus, Mars, planetary satellites, minor planets, asteroids and even comets.




 

Comets:


Green Comet ZTF (C/2022 E3)

This comet was discovered on 2 March 2022 at the Zwicky Transient Facility (ZTF) at the Hale Observatory on Mount Palomar. It was found on CCD images taken by the famous 48-inch Schmidt Telescope. It was not be very bright, and in the first weeks of February it was only faintly visible to the unaided eye from sites far from the light pollution of cities and towns.

The comet had two tails, the brighter being green in colour, probably due to the presence of diatomic carbon in its coma. Its last visit was 50 000 years ago, when it may have been observed by early aborigines. It made its closest approach to Earth on February 2, when it was only 42 million kilometres away. On February 1 it was be in the vicinity of the star Polaris (the 'North Star') which is never visible from Australia. In the next days it moved south, and was close to the bright star Capella on February 5, but the tails were rapidly fading. Comet ZTF was in the vicinity of the planet Mars on February 10 and 11, when the photograph below was secured. It continues to move south, but is now too faint to be seen.

Comet ZTF (C/2022 E3), photographed from Nambour at 9 pm on February 11, 2023.

The same comet, photographed the following night, showing its rapid movement through the constellation Taurus.

 
Comet SWAN (C/2020 F8) and Comet ATLAS (2019 Y4)

Both of these comets appeared recently in orbits that caused them to dive towards the Sun's surface before swinging around the Sun and heading back towards the far reaches of the Solar System. Such comets are called 'Sun grazers', and their close approach to the Sun takes them through its immensely powerful gravitational field and the hot outer atmosphere called the 'corona'. They brighten considerably during their approach, but most do not survive and disintegrate as the ice which holds them together melts. While expectations were high that these two would emerge from their encounter and put on a display as bright comets with long tails when they left the Sun, as they came close to the Sun they both broke up into small fragments of rock and ice and ceased to exist.
 

Comet 46P/Wirtanen

In December 2018, Comet 46P/Wirtanen swept past Earth, making one of the ten closest approaches of a comet to our planet since 1960. It was faintly visible to the naked eye for two weeks. Although Wirtanen's nucleus is only 1.2 kilometres across, its green atmosphere became larger than the Full Moon, and was an easy target for binoculars and small telescopes. It reached its closest to the Sun (perihelion) on December 12, and then headed in our direction. It passed the Earth at a distance of 11.5 million kilometres (30 times as far away as the Moon) on December 16, 2018.



Comet 46/P Wirtanen was photographed on November 29, 2018 between 9:45 and 9:47 pm.  The comet's position was Right Ascension = 2 hrs 30 min 11 secs, Declination = 21º 43' 13", and it was heading towards the top of the picture. The nearest star to the comet's position, just to its left, is GSC 5862:549, magnitude 14.1. The spiral galaxy near the right margin is NGC 908. The right-hand star in the yellow circle is SAO 167833, magnitude 8.31.

 

Comet 46/P Wirtanen on November 30, 2018. This image is a stack of five exposures between 8:13 and 9:05 pm. The comet's movement over the 52 minute period can be seen, the five images of the comet merging into a short streak. It is heading towards the upper left corner of the image, and is brightening as it approaches the Sun, with perihelion occurring on December 12. The images of the stars in the five exposures overlap each other precisely. The length of the streak indicates that the comet is presently moving against the starry background at 1.6º per day. The comet at 9:05 pm was at Right Ascension = 2 hrs 32 min 56 secs, Declination = 20º 27' 20". The upper star in the yellow circle is SAO 167833, magnitude 8.31, the same one circled in the preceding picture but with higher magnification. It enables the two photographs to be linked.

Comet 46/P Wirtanen at perihelion on December 12, 2018, at 00:55 am. It was faintly visible to the unaided eye, but easily visible through binoculars. The circled star has a magnitude of 15.77, and the brighter one just to its left is GSC 60:1162, magnitude 13.8.
 

Comet Lulin

This comet, (C/2007 N3), discovered  in 2007 at Lulin Observatory by a collaborative team of Taiwanese and Chinese astronomers, is now in the outer Solar System, and has faded below magnitude 15.

Comet Lulin at 11:25 pm on February 28, 2009, in Leo. The brightest star is Nu Leonis, magnitude 5.26.

 

The LINEARrobotic telescope operated by Lincoln Near Earth Asteroid Research is used to photograph the night skies, searching for asteroids which may be on a collision course with Earth. It has also proved very successful in discovering comets, all of which are named ‘Comet LINEAR’ after the centre's initials. This name is followed by further identifying letters and numbers. Generally though, comets are named after their discoverer, or joint discoverers. There are a number of other comet and near-Earth asteroid search programs using robotic telescopes and observatory telescopes, such as:
Catalina Sky Survey, a consortium of three co-operating surveys, one of which is the Australian Siding Springs Survey (below),
Siding Spring Survey, using the 0.5 metre Uppsala Schmidt telescope at Siding Spring Observatory, N.S.W., to search the southern skies,
LONEOS, (Lowell Observatory Near-Earth Object Search), concentrating on finding near-Earth objects which could collide with our planet,
Spacewatch, run by the Lunar and Planetary Laboratory of the University of Arizona,
Ondrejov, run by Ondrejov Observatory of the Academy of Sciences in the Czech Republic, 
Xinglong, run by Beijing Astronomical Observatory 

Nearly all of these programs are based in the northern hemisphere, leaving gaps in the coverage of the southern sky. These gaps are the areas of sky where amateur astronomers look for comets from their backyard observatories.

To find out more about current comets, including finder charts showing exact positions and magnitudes, click here. To see pictures of these comets, click here.

 

The 3.9 metre Anglo-Australian Telescope (AAT) at the Australian Astronomical Observatory near Coonabarabran, NSW.

 

 

 

Deep Space

 

 

Sky Charts and Maps available on-line:


There are some useful representations of the sky available here. The sky charts linked below show the sky as it appears to the unaided eye. Stars rise four minutes earlier each night, so at the end of a week the stars have gained about half an hour. After a month they have gained two hours. In other words, the stars that were positioned in the sky at 8 pm at the beginning of a month will have the same positions at 6 pm by the end of that month. After 12 months the stars have gained 12 x 2 hours = 24 hours = 1 day, so after a year the stars have returned to their original positions for the chosen time. This accounts for the slow changing of the starry sky as the seasons progress.

The following interactive sky charts are courtesy of Sky and Telescope magazine. They can simulate a view of the sky from any location on Earth at any time of day or night between the years 1600 and 2400. You can also print an all-sky map. A Java-enabled web browser is required. You will need to specify the location, date and time before the charts are generated. The accuracy of the charts will depend on your computer’s clock being set to the correct time and date.

To produce a real-time sky chart (i.e. a chart showing the sky at the instant the chart is generated), enter the name of your nearest city and the country. You will also need to enter the approximate latitude and longitude of your observing site. For the Sunshine Coast, these are:

latitude:   26.6o South                      longitude:   153o East

Then enter your time, by scrolling down through the list of cities to "Brisbane: UT + 10 hours". Enter this one if you are located near this city, as Nambour is. The code means that Brisbane is ten hours ahead of Universal Time (UT), which is related to Greenwich Mean Time (GMT), the time observed at longitude 0o, which passes through London, England. Click here to generate these charts.

_____________________________________


Similar real-time charts can also be generated from another source, by following this second link:

Click here for a different real-time sky chart.

The first, circular chart will show the full hemisphere of sky overhead. The zenith is at the centre of the circle, and the cardinal points are shown around the circumference, which marks the horizon. The chart also shows the positions of the Moon and planets at that time. As the chart is rather cluttered, click on a part of it to show that section of the sky in greater detail. Also, click on Update to make the screen concurrent with the ever-moving sky.

The stars and constellations around the horizon to an elevation of about 40o can be examined by clicking on

View horizon at this observing site

The view can be panned around the horizon, 45 degrees at a time. Scrolling down the screen will reveal tables showing setup and customising options, and an Ephemeris showing the positions of the Sun, Moon and planets, and whether they are visible at the time or not. These charts and data are from YourSky, produced by John Walker.

The charts above and the descriptions below assume that the observer has a good observing site with a low, flat horizon that is not too much obscured by buildings or trees. Detection of fainter sky objects is greatly assisted if the observer can avoid bright lights, or, ideally, travel to a dark sky site. On the Sunshine Coast, one merely has to travel a few kilometres west of the coastal strip to enjoy magnificent sky views. On the Blackall Range, simply avoid streetlights. Allow your eyes about 15 minutes to become dark-adapted, a little longer if you have been watching television. Small binoculars can provide some amazing views, and with a small telescope, the sky’s the limit.

This month, the Eta Carinae Nebula is ideally placed for viewing, being high in the south-south-east as darkness falls. It culminates at 11 pm at mid-month, and is visible until dawn.

 



The Stars and Constellations for this
month:  

 

This description of the night sky is for 9 pm on March 1 and 7 pm on March 31. They start at Taurus, which is very low in the west-north-west. Most planets will not be available in the evenings this month - Mercury and Neptune are too close to the Sun, and Venus, Mars, Saturn and Pluto are in the pre-dawn eastern sky.

 

The largest planet, Jupiter, is visible in the evening sky this month. It is in Aries and will be close to the west-north-western horizon. On March 1 it will set at 9:20 pm. At the end of March it will set at 7:40 pm. We will lose Jupiter from our evening sky on May 19, when it will be in conjunction with the Sun. The faint planet Uranus is also in Aries and  low to the horizon. It is between Jupiter and the Pleiades star cluster. At mid-month it will set in the west-north-west at 9:45 pm. Uranus sets at 9:45 pm on March 1, so is not well placed for viewing this month.

This month, Orion (see below) is high in the north-west. By mid-month, Orion will have set by midnight. Canis Major (the Large Dog) is just west of the zenith at this time, with the brilliant white star Sirius (Alpha Canis Majoris) showing the Dog's heart. Sirius, also known as the Dog Star, is the brightest star in the night sky and is about a handspan from the zenith. The brilliant white star Rigel (Beta Orionis) is about two handspans west of the zenith. Nearly overhead are the constellations Puppis, the Stern (of the ship, Argo) and Columba, the Dove. Columba culminates at 8 pm on March 1.

Sirius (Alpha Canis Majoris) is the brightest star in the night sky. It has been known for centuries as the Dog Star. It is a very hot A0 type star, larger than our Sun. It is bright because it is one of our nearest neighbours, being only 8.6 light years away. The four spikes are caused by the secondary mirror supports in the telescope's top end. The faintest stars on this image are of magnitude 15. To reveal the companion Sirius B, which is currently 10.4 arcseconds from its brilliant primary, the photograph below was taken with a magnification of 375x, although the atmospheric seeing conditions in the current heatwave were more turbulent. The exposure was much shorter to reduce the overpowering glare from the primary star.



Sirius is a binary, or double star. Whereas Sirius A is a main sequence star like our Sun, only larger, hotter and brighter, its companion Sirius B is very tiny, a white dwarf star nearing the end of its life. Although small, Sirius B is very dense, having a mass about equal to the Sun's packed into a volume about the size of the Earth. In other words, a cubic centimetre of Sirius B would weigh over a tonne. Sirius B was once as bright as Sirius A, but reached the end of its lifespan on the main sequence much earlier, whereupon it swelled into a red giant. Its outer layers were blown away, revealing the incandescent core as a white dwarf. All thermonuclear reactions ended, and no fusion reactions have been taking place on Sirius B for many millions of years. Over time it will radiate its heat away into space, becoming a black dwarf, dead and cold. Sirius B is 63000 times fainter than Sirius A. Sirius B is seen at position angle 62º from Sirius A (roughly east-north-east, north is at the top), in the photograph above which was taken at Nambour on January 31, 2017.  That date is exactly 155 years after Alvan Graham Clark discovered Sirius B in 1862 with a brand new 18.5 inch (47 cm) telescope made by his father, which was the largest refractor existing at the time.


The constellation Taurus with the clusters Pleiades and Hyades is between Orion and the north-western horizon. The brightest star in Taurus is a star dominating (but not actually a member of) the Hyades cluster. This is Aldebaran, a K5 orange star with a visual magnitude of 0.87. It is only half as far away as the Hyades. The Pleiades is a small group like a question mark, and is often called the Seven Sisters, although excellent eyes are needed to detect the seventh star without optical aid. All the stars in this cluster are hot and blue. They are also the same age, as they formed as a group out of a gas cloud or nebula. There are actually more than 250 stars in the Pleiades.  The Pleiades will have disappeared by 10 pm early in the month, and the rest of Taurus follows them below the horizon soon after.

The Pleiades is the small cluster at centre left, while the Hyades is the much larger grouping at centre right.

Wisps of a nebula through which the Pleiades are passing can be seen around the brighter stars in the cluster.


Between Orion’s head and the north-western horizon is a large constellation shaped roughly like a pentagon. It is north of Taurus. This is Auriga the Charioteer, its brightest star being Capella, at the bottom of the tilted pentagon. Capella is the sixth brightest star in the sky, after Sirius, Canopus, Alpha Centauri, Beta Centauri and Vega. To the left of Capella is a small triangle of stars known as 'The Kids’. The lowest star in this triangle is Epsilon Aurigae, one of the largest stars known. It is also very distant.

To the east of Auriga, Gemini is quite high, the two twin stars at its eastern end, Pollux and Castor being due north. At 9 pm at the beginning of the month they are straddling the meridian (the line that runs from due south to due north and passing through the zenith - directly overhead). When a sky object crosses the meridian, it is said to be culminating. At that point, it ceases rising and begins setting. The Twins will have set by 1 am at mid-month.

Tonight, at a little over two handspans above the northern horizon, and directly above Pollux and Castor, is the first magnitude star Procyon, which is the brightest star in the constellation Canis Minor (the Small Dog).

High in the north-east is another zodiacal constellation, Leo, the Lion. The bright star Regulus (Alpha Leonis) marks the Lion’s heart, and Denebola, the star marking the tip of the lion's tail, is low in the east-north-east. From the southern hemisphere, we always see the Lion upside-down. His head and mane are marked by a curved line of stars shaped like an upside-down question mark. This line is also known as the 'Reaping Hook' or 'Sickle', the star Regulus marking the end of the Sickle's handle.

Between Gemini and Leo is the faint constellation of Cancer the Crab. Though a fairly unremarkable constellation in other ways, Cancer does contain a large star cluster called Praesepe or the Beehive, which presents well in binoculars. Also known as M44* , Praesepe is a little more than halfway along a line between Pollux and Regulus.

Rising above the eastern horizon is the next zodiacal constellation after Leo, Virgo, the Virgin. The brightest star in Virgo is Spica, an ellipsoidal variable star whose brightness averages magnitude 1. This makes it the sixteenth brightest star, and its colour is blue-white. Spica is about 10 degrees above the theoretical eastern horizon at this time. Between Denebola and Spica is a fainter star, Porrima, which has a magnitude of 2.74. 

High in the east above Spica is the constellation Corvus the Crow, shaped like a quadrilateral of magnitude 3 stars. A large but faint constellation, Hydra, the Water Snake, winds its way from near Procyon around the north-eastern part of the sky at an altitude of about 60 degrees above the horizon. It passes over the top of Corvus and Virgo to end near Libra, which will not rise until 10 pm (at the beginning of the month).

Hydra has one bright star, Alphard, mv=2.2, an orange star that was known by Arabs in ancient times as ‘The Solitary One’, as it lies in an area of sky with no other bright stars nearby.

Well up in the south-south-east, Crux (Southern Cross) is almost horizontal. The two Pointers Alpha and Beta Centauri lie below Crux. Crux will have rotated clockwise to a vertical position by 1 am at mid-month. Surrounding Crux on three sides is the large constellation Centaurus, and between Crux and the southern horizon are two brilliant stars, Alpha and Beta Centauri. Beta is the one nearer to Crux. These two stars are also known as the Guardians of the Cross.

Crux is at centre, lying horizontally. Beneath Crux lies the Coalsack. Towards the bottom are the two Pointers, Alpha and Beta Centauri. At top centre, the Eta Carinae nebula, also shown below.

To the right of Crux is a small, fainter quadrilateral of stars, Musca, the Fly. Out of all the 88 constellations, it is the only insect. Below and to the right of Alpha Centauri and underneath Musca is a (roughly) equilateral triangle of 4th magnitude stars. This is the constellation Triangulum Australe, the Southern Triangle. It is about half a handspan above the south-south-eastern horizon.

Between Crux and Sirius is a very large area of sky filled with interesting objects. This was once the constellation Argo Navis, named for Jason’s famous ship used by the Argonauts in their quest for the Golden Fleece. The constellation Argo was found to be too large, so modern star atlases divide it into three sections - Carina (the Keel) , Vela (the Sails) and Puppis (the Stern).

The central part of the Eta Carinae nebula, showing dark lanes, molecular clouds, and glowing clouds of fluorescing hydrogen

The Keyhole, a dark cloud obscuring part of the Eta Carinae Nebula

The Homunculus, a tiny planetary nebula ejected by the eruptive variable star, Eta Carinae

One and a half handspans south of Sirius is the second brightest star in the night sky, Canopus (Alpha Carinae). On the border of Carina and Vela is the False Cross, larger and more lopsided than the Southern Cross. The False Cross is a little more than a handspan above Crux and to the right, and is also lying on its side at this time of year. It is high in the south, and will soon culminate. Both of these Crosses are actually more like kites in shape, for, unlike Cygnus (the Northern Cross) they have no star at the intersection of the two cross arms.

A handspan above the south-south-western horizon is Achernar, Alpha Eridani. It is the brightest star in Eridanus the River, which winds its way with faint stars from Achernar in a northerly direction to Cursa, a mv= 2.9 star close to brilliant Rigel in Orion. At magnitude 0.49, Achernar is the ninth brightest star. It swings down towards the south-south-westerly horizon during the evening, and sets soon after midnight.

High in the south, about 43 degrees above the horizon, the Large Magellanic Cloud (LMC) is faintly visible as a diffuse glowing patch. It is a little less than a handspan below (south of) Canopus. About a handspan below the LMC is the Small Magellanic Cloud (SMC), a smaller glowing patch. The LMC and SMC are described below. At the beginning of March, the planets Venus and Mars will rise in the east-south-east at about 4 am.

The zodiacal constellations visible tonight, starting at the western horizon and heading east (passing about two handspans north of the zenith, are Aries, Taurus, Gemini, Cancer, Leo and Virgo.

 

 

The season of the Hunter and his Dogs

 

Two of the most spectacular constellations in the sky may be seen near the zenith as soon as darkness falls. These are Orion the Hunter, and his large dog, Canis Major. Orion straddles the celestial equator, midway between the south celestial pole and its northern equivalent. This means that the centre of the constellation, the three stars known as Orion's Belt, rise due east and set due west. 

Orion:

This is one of the most easily recognised constellations, as it really does give a very good impression of a human figure. From the northern hemisphere he appears to stand upright when he is high in the sky, but from our location ‘down under’ he appears lying down when rising and setting, and upside down when high in the sky. You can, though, make him appear upright when high in the sky (near the meridian), by observing him from a reclining chair, with your feet pointing to the south and your head tilted back

Orion has two bright stars marking his shoulders, the red supergiant Betelgeuse and Bellatrix. A little north of a line joining these stars is a tiny triangle of stars marking Orion’s head. The three stars forming his Belt are, from west to east, Mintaka, Alnilam and Alnitak. These three stars are related, and all lie at a distance of 1300 light years. They are members of a group of hot blue-white stars called the Orion Association.

The red supergiant star, Betelgeuse, is a variable star. In late 2019, its brightness suddenly dropped from 0.5 to 1.5 over the period of several weeks. Such a dimming was unprecedented, a full magnitude, taking it from being the 9th brightest night-time star to the 21st. Prior to this event, Betelgeuse was slightly brighter than nearby Aldebaran in Taurus (the 14th brightest star), but by the end of January it was noticeably dimmer. It continued to fade, and soon it was fainter than its close neighbour Bellatrix (26th brightest) and approaching the brightness of the stars Alnilam (29th brightest) and Alnitak (30th brightest) in Orion’s Belt, a change easily noticeable to the naked eye. Astronomers also detected that its shape had become 'lop-sided'. Whether Betelgeuse would recover or become the Milky Way’s first supernova since Cassiopeia A in ~1667 AD was unknown. By April of 2020 Betelgeuse had returned to normal brightness.

To the south of the Belt, at a distance of about one Belt-length, we see another faint group of stars in a line, fainter and closer together than those in the Belt. This is Orion’s Sword. Orion’s two feet are marked by brilliant Rigel and fainter Saiph. Both of these stars are also members of the Orion Association.

The Saucepan, with Belt at right, M42 at upper left.

Orion is quite a symmetrical constellation, with the Belt at its centre and the two shoulder stars off to the north and the two knee stars to the south. It is quite a large star group, the Hunter being over twenty degrees (a little more than a handspan) tall. 

The stars forming the Belt and Sword are popularly known in Australia as ‘The Saucepan’, with the Sword forming the Saucepan’s handle. Tonight this asterism appears right-side up, as in the photographs above. The faint, fuzzy star in the centre of the Sword, or the Saucepan's handle, is a great gas cloud or nebula where stars are being created. It is called the ‘Great Nebula in Orion’ or ‘M42’ (number 42 in Messier’s list of nebulae). A photograph of it appears below:

The Sword of Orion, with the Great Nebula, M42, at centre

The central section of the Great Nebula in Orion. At the brightest spot is a famous multiple star system, the Trapezium, illustrated below.

New stars are forming in the nebula. At the brightest spot is a famous multiple star system, the Trapezium, illustrated below.

Canis Major

Above Orion as twilight ends (facing west), a brilliant white star will be seen about one handspan away. This is Sirius, or Alpha Canis Majoris, and it is the brightest star in the night sky with a visual magnitude of -1.43. It marks the heart of the hunter's dog, and has been known for centuries as the Dog Star. As we see him tonight, the dog is on his feet with his tail at upper left. A front leg stretches down from Sirius to Mirzam. It is also known as Beta Canis Majoris, which tells us that it is the second-brightest star in the constellation. Mirzam is about one-third of a handspan below Sirius.

The hindquarters of the Dog are indicated by a large right-angled triangle of stars located above and to the left of Sirius. The end of his tail is the top-left corner of the triangle, about one handspan south (above and to the left) of Sirius. It is marked by a blue-white star, Aludra.

Both Sirius and Rigel are bright white stars and each has a tiny, faint white dwarf companion. Whereas a small telescope can reveal the companion to Rigel quite easily, the companion to Sirius the Dog Star, (called ‘the Pup’), can only be observed by using a powerful telescope with excellent optics during rare periods of a completely still atmosphere, as it is very close to brilliant Sirius and is usually lost in the glare..

Canis Major as it appears almost overhead at 9 pm at mid-month (observer facing west).

Canis Minor    

By 8.00 pm at mid-month, this small constellation is about one and a half handspans due north of the zenith. It contains only two main stars, the brighter of which is Procyon (Alpha Canis Minoris). This yellow-white star of mv= 0.5 forms one corner of a large equilateral triangle, the other two corners being the red Betelgeuse and white Sirius. Beta Canis Minoris is also known as Gomeisa, a blue-white star of mv= 3.1.

 

 

Some fainter constellations

 

Between the two Dogs is the constellation Monoceros the Unicorn, undistinguished except for the presence of the remarkable Rosette Nebula. South of Orion is a small constellation, Lepus the Hare. Between Lepus and the star Canopus is the star group Columba the Dove. Eridanus the River winds its way from near Orion west of the zenith to Achernar, high in the south-west. Between Achernar and the western horizon is the star Fomalhaut, a white star of first magnitude in the small constellation of Piscis Austrinus (the Southern Fish). To the left of Fomalhaut is the triangular constellation of Grus, the Crane. Between the zenith and the south-western horizon are a number of small, faint constellations, Horologium, Pictor, Caelum, Mensa, Tucana, Phoenix, Hydrus and Reticulum. The LMC lies in the constellation Dorado, and the South Celestial Pole is in the very faint constellation Octans.

 

 

Finding the South Celestial Pole

 

The South Celestial Pole is that point in the southern sky around which the stars rotate in a clockwise direction. The Earth's axis is aimed exactly at this point. For an equatorially-mounted telescope, the polar axis of the mounting also needs to be aligned exactly to this point in the sky for accurate tracking to take place.

Project a line from the top of the Cross (the star Gacrux) down through its base (the star Acrux) and continue straight on towards the south for another four Cross lengths. This will locate the approximate spot. There is no bright star to mark the Pole, whereas in the northern hemisphere they have Polaris (the Pole Star) to mark fairly closely the North Celestial Pole.

Another way to locate the South Celestial Pole is to draw an imaginary straight line joining Beta Centauri (a handspan above the south-south-eastern horizon) to Achernar (a handspan above the south-western horizon. At 7:55 pm at mid-month, both stars will be at similar altitudes and the line will be horizontal. Bisect this line to find the pole with an accuracy of two degrees.

Interesting photographs of this area can be taken by using a camera on time exposure. Set the camera on a tripod pointing due south, and open the shutter for thirty minutes or more. The stars will move during the exposure, being recorded on the film as short arcs of a circle. The arcs will be different colours, as the stars are. All the arcs will have a common centre of curvature, which is the south celestial pole.

A wide-angle view of trails around the South Celestial Pole, with Scorpius and Sagittarius at left, Crux and Centaurus at top, and Carina and False Cross at right.

Star trails between the South Celestial Pole and the southern horizon. All stars that do not pass below the horizon are circumpolar.

 

 

Double and multiple stars

 

Estimates vary that between 15% and 50% of stars are single bodies like our Sun, although the latest view is that less than 25% of stars are solitary. At least 30% of stars and possibly as much as 60% of stars are in double systems, where the two stars are gravitationally linked and orbit their mutual centre of gravity. Such double stars are called binaries. The remaining 20%+ of stars are in multiple systems of three stars or more. Binaries and multiple stars are formed when a condensing Bok globule or protostar splits into two or more parts.

Binary stars may have similar components (Alpha Centauri A and B are both stars like our Sun - B is even said to have an Earth-sized planet), or they may be completely dissimilar, as with Albireo (Beta Cygni, where a bright golden giant star is paired with a smaller bluish main sequence star).

     

 The binary stars Rigil Kentaurus (Alpha Centauri) at left, and Albireo (Beta Cygni) at right.

     

Rigel (Beta Orionis, left) is a binary star which is the seventh brightest star in the night sky.  Rigel A is a large white supergiant which is 500 times brighter than its small companion, Rigel B, Yet Rigel B is itself composed or a very close pair of Sun-type stars that orbit each other in less than 10 days. Each of the two stars comprising Rigel B is brighter in absolute terms than Sirius (see above). The Rigel B pair orbit Rigel A at the immense distance of 2200 Astronomical Units, equal to 12 light-days. (An Astronomical Unit or AU is the distance from the Earth to the Sun.)  In the centre of the Great Nebula in Orion (M42) is a multiple star known as the Trapezium (right). This star system has four bright white stars, two of which are binary stars with fainter red companions, giving a total of six. The hazy background is caused by the cloud of fluorescing hydrogen comprising the nebula.

Acrux, the brightest star in the Southern Cross, is also known as Alpha Crucis.  It is a close binary, circled by a third dwarf companion.

Alpha Centauri (also known as Rigil Kentaurus, Rigil Kent or Toliman) is a binary easily seen with the smallest telescope. The components are both solar-type main sequence stars, one of type G and the other, slightly cooler and fainter, of type K. Through a small telescope this star system looks like a pair of distant but bright car headlights.

Alpha Centauri A and B take 80 years to complete an orbit, but a tiny third component, the 11th magnitude red dwarf Proxima Centauri, takes about 1 million years to orbit the other two. It is about one tenth of a light year from the bright pair and a little closer to us, hence its name. This makes it our nearest interstellar neighbour, with a distance of 4.3 light years. Red dwarfs are by far the most common type of star, but, being so small and faint, none is visible to the unaided eye. Because they use up so little of their energy, they are also the longest-lived of stars. The bigger a star is, the shorter its life.

Alpha Centauri, with Proxima

Proxima Centauri

Knowing the orbital period of the two brightest stars A and B, we can apply Kepler’s Third Law to find the distance they are apart. This tells us that Alpha Centauri A and B are about 2700 million kilometres apart or about 2.5 light hours. This makes them a little less than the distance apart of the Sun and Uranus (the orbital period of Uranus is 84 years, that of Alpha Centauri A and B is 80 years.)

Albireo (Beta Cygni) is sometimes described poetically as a large topaz with a small blue sapphire. It is one of the sky’s most beautiful objects. The stars are of classes G and B, making a wonderful colour contrast. It lies at a distance of 410 light years, 95 times further away  than Alpha Centauri.

Binary stars may be widely spaced, as the two examples just mentioned, or so close that a telescope is struggling to separated them (Acrux, Castor, Antares, Sirius). Even closer double stars cannot be split by the telescope, but the spectroscope can disclose their true nature by revealing clues in the absorption lines in their spectra. These examples are called spectroscopic binaries. In a binary system, closer stars will have shorter periods for the stars to complete an orbit. Eta Cassiopeiae takes 480 years for the stars to circle each other. The binary with the shortest period is AM Canum Venaticorum, which takes only 17½ minutes.

Sometimes one star in a binary system will pass in front of the other one, partially blocking off its light. The total light output of the pair will be seen to vary, as regular as clockwork. These are called eclipsing binaries, and are a type of variable star, although the stars themselves usually do not vary.

 

 

 

Star Clusters


The two clusters in Taurus, the Pleiades and the Hyades, are known as Open Clusters or Galactic Clusters. The name 'open cluster' refers to the fact that the stars in the cluster are grouped together, but not as tightly as in globular clusters (see below). The stars appear to be loosely arranged, and this is partly due to the fact that the cluster is relatively close to us, i.e. within our galaxy, hence the alternate name, 'galactic cluster'. These clusters are generally formed from the condensation of gas in a nebula into stars, and some are relatively young.

The photograph below shows a typical open cluster, M7* . It is found in the constellation Scorpius, just below the scorpion's sting. It lies in the direction of our galaxy's centre. The cluster itself is the group of white stars in the centre of the field. Its distance is about 380 parsecs or 1240 light years.

Galactic Cluster M7 in Scorpius

Galactic Cluster M7 in Scorpius, high resolution

Outside the plane of our galaxy, there is a halo of Globular Clusters. These are very old, dense clusters, containing perhaps several hundred thousand stars. These stars are closer to each other than is usual, and because of its great distance from us, a globular cluster gives the impression of a solid mass of faint stars. Many other galaxies also have a halo of globular clusters circling around them.

The largest and brightest globular cluster in the sky is NGC 5139**, also known as Omega Centauri. It has a slightly oval shape. It is an outstanding winter object, but is also observable in autumn. Shining at fourth magnitude, it is faintly visible to the unaided eye, but is easily seen with binoculars, like a light in a fog. A telescope of 20 cm aperture or better will reveal its true nature, with hundreds of faint stars giving the impression of diamond dust on a black satin background. It lies at a distance of 5 kiloparsecs, or 16 300 light years.

The globular cluster Omega Centauri

The central core of Omega Centauri

There is another remarkable globular, second only to Omega Centauri. About two degrees below the SMC (see below), binoculars can detect a fuzzy star. A telescope will reveal this faint glow as a magnificent globular cluster, lying at a distance of 5.8 kiloparsecs. Its light has taken almost 19 000 years to reach us. This is NGC 104, commonly known as 47 Tucanae. Some regard this cluster as being more spectacular than Omega Centauri, as it is more compact, and the faint stars twinkling in its core are very beautiful. This month, 47 Tucanae is low in the south-south-west, and not clearly visible. By 10 pm Omega Centauri is high enough for detailed viewing.

Observers aiming their telescopes towards the SMC generally also look at the nearby 47 Tucanae, but there is another globular cluster nearby which is also worth a visit. This is NGC 362, which appears to lie above 47 Tucanae as we see it in mid-evening this month. It is less than half as bright as the other globular, but this is because it is more than twice as far away. Its distance is 12.6 kiloparsecs or 41 000 light years, so it is about one-fifth of the way from our galaxy to the SMC. Both NGC 104 and NGC 362 are always above the horizon for all parts of Australia south of the Tropic of Capricorn.

Globular Cluster NGC104 in Tucana.

The globular cluster NGC 6752 in the constellation Pavo.

 

*     M42:This number means that the Great Nebula in Orion is No. 42 in a list of 103 astronomical objects compiled and published in 1784 by Charles Messier. Charles was interested in the discovery of new comets, and his aim was to provide a list for observers of fuzzy nebulae and clusters which could easily be reported as comets by mistake. Messier's search for comets is now just a footnote to history, but his list of 103 objects is well known to all astronomers today, and has even been extended to 110 objects.

**    NGC 5139: This number means that Omega Centauri is No. 5139 in the New General Catalogue of Non-stellar Astronomical Objects. This catalogue was first published in 1888 by J. L. E. Dreyer under the auspices of the Royal Astronomical Society, as his New General Catalogue of Nebulae and Clusters of Stars. As larger telescopes built early in the 20th century discovered fainter objects in space, and also dark, obscuring nebulae and dust clouds, the NGC was supplemented with the addition of the Index Catalogue (IC). Many non-stellar objects in the sky have therefore NGC numbers or IC numbers. For example, the famous Horsehead Nebula in Orion is catalogued as IC 434. The NGC was revised in 1973, and lists 7840 objects. 

The recent explosion of discovery in astronomy has meant that more and more catalogues are being produced, but they tend to specialise in particular types of objects, rather than being all-encompassing, as the NGC / IC try to be. Some examples are the Planetary Nebulae Catalogue (PK) which lists 1455 nebulae, the Washington Catalogue of Double Stars (WDS) which lists 12 000 binaries, the General Catalogue of Variable Stars (GCVS) which lists 28 000 variables, and the Principal Galaxy Catalogue (PGC) which lists 73 000 galaxies. The largest modern catalogue is the Hubble Guide Star Catalogue (GSC) which was assembled to support the Hubble Space Telescope's need for guide stars when photographing sky objects. The GSC contains nearly 19 million stars brighter than magnitude 15.

 

 

Two close galaxies

 

High in the south, to the left of Achernar, two large smudges of light may be seen. These are the two Clouds of Magellan, known to astronomers as the LMC (Large Magellanic Cloud) and the SMC (Small Magellanic Cloud). The LMC is to the left and above the SMC, and is noticeably larger. They lie at a distance of 160 000 light years, and are about 60 000 light years apart. They are dwarf galaxies, and they circle our own much larger galaxy, the Milky Way. The LMC is slightly closer, but this does not account for its larger appearance. It really is larger than the SMC, and has developed as an under-sized barred spiral galaxy.

From our latitude both Magellanic Clouds are circumpolar. This means that they are closer to the South Celestial Pole than that Pole's altitude above the horizon, so they never dip below the horizon. They never rise nor set, but are always in our sky. Of course, they are not visible in daylight, but they are there, all the same.

The Large Magellanic Cloud - the bright knot of gas to left of centre is the famous Tarantula Nebula (below)

These two Clouds are the closest galaxies to our own, but lie too far south to be seen by the large telescopes in Hawaii, California and Arizona. They are 15 times closer than the famous Andromeda and Triangulum galaxies referred to above, and so can be observed in much clearer detail. Our great observatories in Australia, both radio and optical, have for many years been engaged in important research involving these, our nearest inter-galactic neighbours. 

 

 

Why are some constellations bright, while others are faint ?

 

The Milky Way is a barred spiral galaxy some 100000 – 120000 light-years in diameter which contains 100 – 400 billion stars. It may contain at least as many planets as well. Our galaxy is shaped like a flattened disc with a central bulge. The Solar System is located within the disc, about 27000 light-years from the Galactic Centre, on the inner edge of one of the spiral-shaped concentrations of gas and dust called the Orion Arm. When we look along the plane of the galaxy, either in towards the centre or out towards the edge, we are looking along the disc through the teeming hordes of stars, clusters, dust clouds and nebulae. In the sky, the galactic plane gives the appearance which we call the Milky Way, a brighter band of light crossing the sky. This part of the sky is very interesting to observe with binoculars or telescope. The brightest and most spectacular constellations, such as Crux, Canis Major, Orion and Scorpius are located close to the Milky Way.

If we look at ninety degrees to the plane, either straight up and out of the galaxy or straight down, we are looking through comparatively few stars and gas clouds and so can see out into deep space. These are the directions of the north and south galactic poles, and because we have a clear view in these directions to distant galaxies, these parts of the sky are called the intergalactic windows. The southern window is in the constellation Sculptor, not far from the star Fomalhaut. This window is too low in the south-west in the early evenings this month for useful viewing. The northern window is between the constellations Virgo and Coma Berenices, roughly between the stars Denebola and Arcturus. It begins to rise in the east-north-east at 8 pm at mid-month, and is well placed for viewing at midnight.

Some of the fainter and apparently insignificant constellations are found around these windows, and their lack of bright stars, clusters and gas clouds presents us with the opportunity to look out across the millions of light years of space to thousands of distant galaxies.   

 

 

  

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