April  2017

Updated:   27 April 2017

 

 

Welcome to the night skies of Autumn, featuring Taurus, Gemini, Orion, Canis Major, Leo, Carina, Crux and Jupiter 

 

Our course in Astronomy "Understanding the Universe" is starting on Thursday, April 6 - one night per week for eight weeks, including observing through our robotic telescope (pictured below) on each clear night.
Click
here for details.

 

Note:  Some parts of this webpage may be formatted incorrectly by older browsers.

 

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

The 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 developed for our site with our equipment over the past year.

 

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.

A handspan at arm's length covers an angle of approximately 18 - 20 degrees.

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 exactly 100 times brighter than magnitude 6.0 (5 steps each of 2.51 times, 2.51x2.51x2.51x2.51x2.51 = 2.515 = 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. (The meridian is an imaginary semicircular line running from the due south point on the horizon and arching overhead through the zenith, and coming down to the horizon again at its due north point.) 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.

 

 

 Solar System

 

Sun:   The Sun begins the month in the constellation of Pisces, the Fishes. It leaves Pisces and passes into Aries, the Ram on April 18.   

 

 

Moon Phases:  Lunations (Brown series):  #1166, 1167 

 

First Quarter:        April 04         04:40 hrs          diameter = 32.0'
Full Moon:            
April 11         16:09 hrs          diameter = 30.0'         
Last Quarter:       
April 19         19:58 hrs          diameter = 30.1' 
New Moon:           
April 26          22:17 hrs         diameter = 33.1'  

First Quarter:        May 03          12:48 hrs          diameter = 31.6'
Full Moon:             May 11          07:44 hrs          diameter = 29.5'
Last Quarter:        May 19          10:34 hrs          diameter = 30.7' 
New Moon:            May 26          05:45 hrs          diameter = 33.4'       

 

 

Lunar Orbital Elements:



April 07:         Moon at ascending node at 19:18 hrs, diameter = 30.9'
April 15:         Moon at apogee (405 488 km) at 20:05 hrs, diameter = 29.5'
April 22:         Moon at descending node at 08:28 hrs, diameter = 31.2'
April 28:         Moon at perigee (359 341 km) at 02:08 hrs, diameter = 33.3'

May 04        Moon at ascending node at 20:45 hrs, diameter = 31.1'
May 13:          Moon at apogee (406 216 km) at 05:33 hrs, diameter = 29.4'
May 19:          Moon at descending node at 11:27 hrs, diameter = 30.7'
May 26
:          Moon at perigee (357 227 km) at 11:02 hrs, diameter = 33.4'
May 31        Moon at ascending node at 21:59 hrs, diameter = 31.5'


Moon at 8 days after New, as on April 05

Moon at 9 days after New, as on April 06

The two photographs above show the Mare Imbrium area in the Moon's northern hemisphere. They were taken a day apart, just after First Quarter. Mare Imbrium (the Sea of Rains) is a large lava flow caused by the Imbrium Event - a cataclysmic collision of an asteroid with the Moon many millions of years ago. A comparison of the two photographs will show how the appearance of lunar features changes with the angle of the Sun. 

In the first photograph, Mare Imbrium (left) is separated from Mare Serenitatis (right) by two ranges of mountains, the Alps to the north and the Apennines to the south. Two large craters at upper right are Aristoteles and Eudoxus. The straight Alpine Valley may be seen cutting through the Alps. Mt Piton (height 2000 metres) is visible as a bright spot with a shadow, due south of the southern end of the Alpine Valley. Archimedes is the large crater at left. It is a walled plain 80 kilometres in diameter with a flat floor. To its right are two bowl-shaped craters, Aristillus and Autolycus.  These craters are all formed by impact with large meteors. Apollo 15 landed close by the Apennines, in a small enclosed area to the right and below Archimedes, on the picture's central vertical axis.

In the second photograph, the sunrise line (called the 'terminator') has moved to the left, revealing a large walled plain in the Alps, known as Plato. South of Plato, an isolated mountain protruding through the lava flow is called Mt Pico. Ripples in the lava, called 'wrinkle ridges', are visible. The crater at lower left is Timocharis, 42 kilometres in diameter.

A detailed map of the Moon's near side is available here. A rotatable view of the Moon, with ability to zoom in close to the surface, and giving detailed information on each feature, may be downloaded  here.

Click here for a photographic animation showing the lunar phases. It also shows the Moon's wobble or libration, and how its apparent size changes as it moves from perigee to apogee each month. It takes a little while to load, but once running is very cool !<

 

 

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. This month we will describe the craters Eratosthenes, Copernicus and Stadius:

     

This area was photographed from Starfield Observatory, Nambour on October 10, 2016. East (where the Sun is rising) is to the right, north is to the top.
Eratosthenes is the crater at top right, Copernicus is at lower left.
The debris field caused by rubble from the Copernicus impact is at centre, where the ghost crater Stadius can be faintly seen.

        

Copernicus one and two days later. As the Sun rises, the shadows diminish. A high peak on the eastern rim throws a conspicuous shadow.

  
   

The rectangle shows the location of the Copernicus - Eratosthenes area.

These three impact craters are not far from the centre of the Moon, so they are seen almost directly from above, and are not foreshortened. Although there are some volcanoes on the Moon with craters at their summits, by far the vast majority of deep, circular features with raised rims that cover much of the Moon's surface are caused by the impacts of flying pieces of rock onto the lunar surface. In 1791 Johann Schröter named these features 'craters', from the Latin word for 'cup', because of their profile. He also named certain valleys 'rilles', from the German word for 'grooves'.

In the Moon's distant past, when the Solar System was very young, there was a lot of material circling the Sun which came into collision with the newly-formed planets and their satellites or moons. This material was in the form of rocks ranging in size from sand grains and pebbles up to car-sized and even as large as Tasmania. Each planet's gravity swept most of them up, but there are still plenty of them still flying free in space. We call them SSSBs (Small Solar System Bodies). The small ones are commonly known as meteors when they strike our atmosphere and burn up by friction; larger ones may develop tails of dust and vented gas when approaching the Sun and are known as comets, and the biggest ones are sometimes called asteroids. The largest asteroid, Ceres, has a diameter about a quarter of that of our Moon, and is called a 'dwarf planet'.

If one of these objects flying through space hits a rocky planet with little of no atmosphere to slow it down, then the impact creates a huge crater. Mercury, Mars, the Moon and satellites of the outer planets have areas which are covered with overlapping craters. Even the Earth has had numerous strikes, and the evidence of many is still visible today, despite two or three billion years of weathering and erosion. Two large 'astroblemes' in Australia are Wolfe Creek crater in the Kimberley District and Gosse's Bluff in Central Australia, where a collection of smaller craters at Henbury can also be found.

On the Moon, a large impact crater like those in the picture above is caused when a large rock flying through space - say about the size of Ayer's Rock or Nambour - hits the lunar surface when travelling at a typical 30 kilometres per second. This flying mountain has great mass and its speed gives it great kinetic energy. On impact with the Moon, the rock is stopped short in a tiny fraction of a second, and its kinetic energy is instantaneously converted into heat energy, which vaporises the rock in a stupendous blast that creates the crater. Great amounts of molten rock (called 'rock melt') are deposited all around the site of the impact, covering and blurring older features and clearly visible in the pictures. Rocks and boulders as large as city blocks are flung out in all directions for many hundreds (occasionally thousands) of kilometres. These rain down on the Moon's landscape, peppering the area with craterlets, many with shapes elongated radially from the impact. These types of craterlets are conspicuous in the central region of the picture above, some being in lines where a large block has bounced along the surface, leaving a craterlet each time it hit the ground.

The lunar surface was originally a light grey in colour, but as it ages, the continual rain of micrometeorites and particles from the Sun (mainly protons and ions) coats each soil particle with a very thin layer of metallic iron called nanophase iron (npFeo). This darkens each grain and hence the area of the surface in general, according to how long it has remained undisturbed. When there is a large impact nearby, material ejected from the new crater disturbs the surface in a pattern radial to the crater, bringing the light coloured subsoil to the surface and creating a pattern of light-coloured rays centred on the new impact crater. The craters Copernicus and Kepler have widespread ray systems. By far the most spectacular ray system on the near side of the Moon is centred on the southern crater Tycho, whose rays spread across the Moon for thousands of kilometres. They are very prominent around Full Moon. 

As well as the blast going outwards, it also sends a powerful pulse of energy downwards into the Moon. When this pulse hits the solid bedrock, it bounces back up to the surface, fracturing and lifting the floor of the crater just formed. This is the origin of the clusters of central peaks found in the middle of the floor of a majority of impact craters, such as Eratosthenes above. Copernicus also has a cluster of peaks, but they are only 1200 metres high. In the image above, their summits, being 2560 metres lower than the surrounding rim, are yet to be illuminated by the sun's rays.

If the downward pulse of energy is powerful enough to fracture the bedrock, thereby releasing pressure on the hot rocks below, they immediately liquefy into molten magma which forces its way up to the crater floor, where it pools, sometimes overwhelming the cluster of new peaks and creating a flat floor to the crater. The large crater plain Ptolemaeus (described in January and visible in the small Full Moon image above as a circular flat area almost in the Moon's exact centre) was created in this way. There have been no craters larger than a kilometre in diameter formed in the 408 years since Galileo first looked at the Moon through a telescope in 1609.

 

Eratosthenes is the 60 kilometre wide crater at upper right, at the south-western end of a spectacular range of mountains called the Apennines. It is typical of a medium-sized crater, with a cluster of central peaks. It was formed between 1.1 and 3.2 billion years ago.

The crater Copernicus, with a diameter of 95 kilometres, is the largest crater in the image, at lower left. It is much more recent, only about 1 billion years old. The Sun was just on the point of rising over this crater when the main image was taken, and only the raised circular rim is illuminated by sunshine. The depressed interior is in deep shadow. Views of the crater one and two days later show how the appearance of all lunar features rapidly changes from night to night.

The oldest crater is 71 kilometre wide Stadius, found between the other two, and so ancient that numerous lava flows have filled it up and almost obliterated it, making what is called a 'ghost crater'.

 

______________________________________________________


Copernicus

Since the time of Pythagoras in the sixth century before Christ, all educated Greeks and Egyptians knew that the world was shaped like a sphere. Its actual size was still open to conjecture. They believed that the Earth was fixed and unmoving at the centre of the universe (a 'geocentric' view), and that the Sun, Moon, planets and stars revolved around the Earth once per day, being carried on transparent crystalline spheres. A few thinkers in the third and second centuries BCE, such as Aristarchus and Seleucus, thought that the Sun was at the centre of the universe (a 'heliocentric' view), and that the Earth was just another planet revolving around it in a year and rotating on its axis once each day, but they were not taken seriously.

At the beginning of the sixteenth century AD, the Renaissance was under way. The printing press has been invented, Columbus had discovered the New World, Vasco da Gama had found a sea route to India, and people like Leonardo da Vinci and Michelangelo were changing the art world. Niklas Koppernigk (Latinised to Nicolaus Copernicus, 1473-1543) was by 1506 a well-educated Renaissance man. He was a physician, economist, diplomat, mathematician, classical scholar, military leader, governor and artist. He worked as a Canon at Frauenburg Cathedral in East Prussia (now Frombork in Poland), but did not take holy orders. His hobby was astronomy.

He had read the Epitome of the Almagest by Regiomontanus, which described the fixed Earth and transparent spheres surrounding it, and thought the whole system as described by Aristotle and Ptolemy was mathematical sleight-of-hand. Though it gave fairly good predictions of planetary movements for astrologers, the whole complicated system with its epicycles, deferents and an equant seemed to be based on a lie. The more he studied Ptolemy's original Almagest when it was finally published by printing press in 1515, the more he realised that all the complications would vanish if the Sun replaced the Earth at the centre of the planetary orbits, and each planet would then snap into place.

He decided to begin work on a book of his own, using the Almagest as a model. In his book, he would clearly and patiently piece his theory together, fully explaining it with mathematical proofs and rigorous logic. He did not know it, but his book would restore interest in the heliocentric system suggested by Aristarchus in the 3rd century BCE, and lead directly to new laws of physics.

The book took about twenty years to write, but Copernicus was afraid to publish it as he felt that the Church, which strongly supported the geocentric view, would see his idea of an Earth moving around the Sun as an attack on the holy scriptures.  After all, Psalm 105:5 said quite definitely, "He set the Earth on its foundations; it can never be moved." In his last years, he finally agreed to publish 500 copies of it, and a copy fresh off the press was placed in his hand as he lay comatose on his deathbed. The book was called De Revolutionibus (On the Revolutions). It was indeed 'revolutionary', and gave us the word to use for anything that challenges the existing order of things.

There was no theory of universal gravitation existing at the time to provide physical support for Copernicus’ system. Initially, the book made little impact. Reading it in Latin was heavy going, and not many people read it through completely; but before long most scholars were aware of the substance of the heliocentric theory, and a small but growing group of astronomers were convinced of its veracity and studied it in great detail. It was treasured by all who had a copy pass through their hands, and the wide margins of the copies still in existence are full of comments and notes. 275 of the 500 first edition copies are still with us, and priceless. In the end, De Revolutionibus was enough. It would be recognised as the greatest scientific book of the sixteenth century. It would change the world forever, but not until the invention of the telescope in 1608 proved that the Earth and other planets did orbit the Sun.

 

Eratosthenes

This crater is named after Eratosthenes, who was a custodian of the Great Library in Alexandria at the mouth of the Nile two centuries before the birth of Christ. Many of our Greek myths about the constellations come to us through his writings and those of Hyginus.

The story goes that Eratosthenes was told by a visitor to the Great Libray that at noon on the summer solstice (June 21), the Sun shone vertically down a well further up the Nile River at Syene, its reflection appearing in the water. Also, vertical sticks and tall columns were seen to ‘swallow their shadows’ around the time of the summer solstice. He checked this the following year and found that it was so, and that, at the same time on the same day, the Sun was 7.2º from the vertical at Alexandria.

Knowing the distance between the two observing sites to be 5000 stadia or about 800 kilometres, a stade or stadion being 157.5 metres (he hired professional ‘steppers’ to pace it out using a standard stride), he used this information to make a famous measurement of the diameter and circumference of the Earth, getting quite an accurate result. His method was simple: the 7.2º angle between the two towns is about one-fiftieth of a circle of 360º. This distance equals 800 kilometres.

Therefore the circumference of the whole circle (the Earth) equals  50 × 800 km  =   40 000 km. His calculation of the Earth’s circumference turned out to be very close to the true figure of 40  025 km (polar circumference), but it seems that he had more than his share of good fortune. The fact that Syene is slightly north of the Tropic of Cancer should have made the Sun shine down the well at a slight angle, not vertically. We are not told how deep the well was, and if it was exactly vertical. Would the use of their primitive measuring instruments at Alexandria have provided the quoted accuracy? How accurate were the measurements of the man or men who paced the distance? Did he use an 'Egyptian Stadion' or an 'Attic Stadion'? All of these factors would have introduced small errors into his observations and calculations, but luckily they appear to have cancelled each other out. In any case, he was the first man to measure the size of a planet, and all he used was sticks, eyes, feet and brains.


Stadius

One of the first astronomical almanacs to be based solely on the heliocentric system of Copernicus was the Ephemerides novae at auctae of Jan Stade (Latinised to Johannes Stadius, 1527-1579), published at Cologne in 1554. Avidly read and enthusiastically used by Tycho Brahe and Michel de Nostradamus, this set of planetary tables fearlessly stated that its calculations were made using the heliocentric system.

 

 

Geocentric Events:

April 1:           Mercury at greatest elongation east (18º 50') at 15:50 hrs  (diameter = 7.5")
April 1:           Limb of Moon 44 arcseconds north of the star Aldebaran (Alpha Tauri, mv= 0.87) at 20:21 hrs
April 6:           Jupiter 9.6 arcminutes south of the star Theta Virginis (mv= 4.38) at 13:08 hrs
April 6:           Saturn at western stationary point at 14:44 hrs  (diameter = 17.1")
April 7:           Moon occults the star Regulus (Alpha Leonis, mv=1.36) between 13:04 and 13:45 hrs
April 8:           Jupiter at opposition at 07:20 hrs  (diameter = 44.2")
April 9:           Pluto at western quadrature at 10:35 hrs  (diameter = 0.1")
April 10:         Mercury at eastern stationary point at 09:12 hrs  (diameter = 9.7")
April 11:         Moon 2.3º north of Jupiter at 09:58 hrs
April 14:         Uranus in conjunction with the Sun at 15:25 hrs  (diameter = 3.3")
April 15:         Venus at western stationary point at 20:19 hrs  (diameter = 48.7")
April 17:         Moon 3.3º north of Saturn at 04:53 hrs
April 18:         Moon 2.9º north of Pluto at 22:10 hrs
April 20:         Mercury at inferior conjunction at 15:50 hrs  (diameter = 11.7")
April 20:         Pluto at western stationary point at 17:53 hrs  (diameter = 0.1")
April 23:         Moon occults Neptune between 03:53 and 04:59 hrs
April 24:         Moon 4.7º south of Venus at 06:19 hrs
April 26:         Moon 3.4º south of Uranus at 02:50 hrs
April 26:         Moon 4.1º south of Mercury at 04:59 hrs
April 28:         Moon 5.1º south of Mars at 19:19 hrs
April 28:         Mercury 5.5 arcminutes south of Uranus at 22:17 hrs
April 29:         Limb of Moon 22 arcseconds north of the star Aldebaran (Alpha Tauri, mv= 0.87) at 03:06 hrs

May 4:           Mercury at western stationary point at 02:24 hours  (diameter = 10.7")
May 4:           Moon occults the star Regulus (Alpha Leonis, mv=1.36) between 20:15 and 21:38 hrs
May 6:           Mercury at aphelion at 23:32 hrs  (diameter = 10.2")
May 8:           Moon 2.3º north of Jupiter at 09:10 hrs
May 14:         Moon 3.5º north of Saturn at 09:57 hrs
May 16:         Moon 2.6º north of Pluto at 07:36 hrs
May 18:         Mercury at Greatest Elongation West (25º 35') at 09:11 hrs  (diameter = 8.2")
May 19:         Saturn 19 arcminutes south of the star 58 Ophiuchi (mv= 4.86) at 14:43 hrs
May 20:         Moon occults Neptune between 16:12 and 16:41 hrs
May 22:         Moon 2º south of Venus at 23:01 hrs
May 23:         Venus 1.7º south of Uranus at 03:55 hrs
May 23:         Moon 3.1º south of Uranus at 17:49 hrs
May 24:         Limb of Moon 35 arcminutes south of Mercury at 12:59 hrs
May 26:         Moon 1.3º north of the star Aldebaran (Alpha Tauri, mv= 0.87) at 14:39 hrs
May 27:         Moon 4.6º south of Mars at 12:48 hrs







The Planets for this month:

 

Mercury:    On April 1, Mercury will be in the western twilight sky, setting 45 minutes after the Sun, before nightfall. This is not a favourable appearance, as it will not venture far from the solar glare. Mercury will reach its greatest angular distance from the Sun (18º 50') on April 1, when its angular size will be 7.5 arcseconds. The best time to look for it will be in the first week of April. To find Mercury on April 1, draw an imaginary line from the waxing crescent Moon downward and to the left to the planet Mars. The distance will be 23 degrees (one and a quarter handspans). Produce this line further on for another 15 degrees (almost a handspan) to find Mercury. On the morning of April 29, Mercury and Uranus will be only 10 arcminutes apart.

 

Venus:  This, the brightest planet, is the famous 'evening star'. It has dominated the western twilight sky since last September, but on March 25 it passed between us and the Sun (inferior conjunction), and left the evening sky. It has now reappeared in the pre-dawn sky as a 'morning star', rising in the east before the Sun, and has now become noticeable to even the most casual observer, rising in the east before the Sun. In early May the crescent shape of Venus will be visible in even the smallest telescope, as its angular diameter will be 36 arcseconds and its phase will be 27%, very similar to its appearance when the first photograph below was taken. At the beginning of May, Venus will rise in the east-north-east at about 3:15 am, three hours before the Sun. By the end of May, the angular diameter of Venus will have decreased to 25 arcseconds as Venus pulls away from us, but its phase will have increased to 47%, similar to that shown in the middle picture below. The phase of Venus will be exactly 50% (like a tiny half-Moon) on June 4. On May 31 Venus will rise at about 3 am, three-and-a-half hours before the Sun.

The waning crescent Moon will be 3.9 degrees below and to the right of Venus on the morning of May 23.

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

                           April 2017                              June 2017                         December 2017                      

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 the first half of 2015, Venus appeared as an 'Evening Star', but from August 2015 it moved to the pre-dawn sky and became a 'Morning Star'. Each of these appearances lasts about eight to nine months. Venus passed on the far side of the Sun (superior conjunction) on June 7, 2016, and has now returned to the evening sky and become an 'Evening Star' once again. It will leave the evening sky and become a 'Morning Star' again on March 25.

Because Venus was 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.

 

Mars:   Having passed through opposition on May 22 last year, the red planet continues to shrink and fade as the speeding Earth leaves it behind. At opposition it reached magnitude -2, rivalling Jupiter in brightness, but by April 1 it has faded to 1.5 (one twenty-fifth as bright). In the same period, its apparent size has shrunk from 18.4 arcseconds to 4 arcseconds. It is a faint orange object low in the west as soon as twilight fades, as it is passing through an area of sky lacking in bright stars. It may be found on April 28 to the right of the crescent Moon. Mars is now approaching the far side of its orbit, about as far away as it can get.  

Mars is passing rapidly through the constellation Aries. On April 12 it will move into Taurus, and by April 21 will be three degrees to the left of the Pleiades star cluster. By the first of May, Mars will be alongside the Hyades star cluster, with its bright orange giant star, Aldebaran. By then it will be very low in the west as twilight ends. Mars reaches conjunction with the Sun on July 27.

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.

 

Jupiter:   This gas giant planet is now at its closest and brightest as it passes through opposition (directly opposite the Sun in the sky) on April 8. This month it may be easily seen in the eastern sky, rising soon after sunset. It is in the constellation Virgo, north of the first magnitude star Spica. The Full Moon will rise to the left of Jupiter and Spica as night falls on April 10.


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.

 

Saturn:   The ringed planet is an after-midnight object this month, as it was in conjunction with the Sun on December 10. It passed through western quadrature on March 18, when it rose at midnight. This month it will be visible low in the east just before midnight. The waning gibbous Moon will be just below Saturn just before midnight on April 16.

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 June 29, 2016. The shadow of its globe can 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. 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, mainly 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.

 

Uranus:  This ice giant planet is not in a good position for viewing in early April, as it will be only 12 degrees from the Sun and involved with the solar glare. Uranus will reach conjunction with the Sun on April 14, after which it will become a morning sky object, and still hidden in the light of the Sun. As it shines at about magnitude 5.8, a pair of binoculars or a small telescope is required to observe Uranus. Mercury will be only 10 arcminutes from Uranus on the morning of April 29.

 

Neptune:   The icy blue planet was in conjunction with the Sun on March 2, so will be a difficult object to observe this month. Neptune is located in the constellation of Aquarius, between the magnitude 3 star Skat (Delta Aquarii) and the four-star asterism known as the Water-Jar. As it shines at about magnitude 8, a small telescope is required to observe Neptune.

Neptune, photographed from Nambour on October 31, 2008

Pluto:   The erstwhile ninth and most distant planet is an early morning object this month, as it reached conjunction with the Sun on January 7. Located inside the 'Teaspoon' which is above the Sagittarius 'Teapot', it is a faint 14.1 magnitude object near the centre of Sagittarius. Pluto will reach western quadrature on April 9, when it rises at midnight. Pluto's angular diameter is 0.13 arcseconds, less than one twentieth that of Neptune.

 

  

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 for April:

 

Venus chases Mars

For the last few months, Venus has been chasing Mars in the western twilight sky. On February 3, Venus and Mars were at their closest, but two days later Venus gave up the chase and curved away from Mars. Venus performed a U-turn in the first two weeks of March, and headed back towards the Sun, which it passed on March 25. On April 15 Venus will perform another U-turn and will resume the chase after Mars, and will finally catch it on October 6, when the two planets will be less than 13 arcminutes apart in the morning sky, rising at 4:21 am in the constellation Leo.

On March 26, Mercury and Uranus were only 2.1 degrees apart, but by April 28 they will be extremely close, only 5.5 arcminutes apart. Unfortunately for us, this will occur at 10:17 pm, when neither will be above our horizon. By the time they rise the next morning (5:16 am), they will be 10.5 arcminutes apart, and only 12.5 degrees from the Sun, which will rise at  6:10 am. 

 

 

 

Meteor Showers:

Lyrids                              April 23                      Waning crescent Moon, 22% sunlit                         ZHR = 15
                                        Radiant:  Near the star Vega. 
      Associated with Comet Thatcher.

Pi Puppids                      April 24                      Waning crescent Moon, 13% 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'. Oan 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:



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.

The Eta Carinae Nebula is nearly a handspan to the right of the Southern Cross at 10 pm in mid-April.

 


The Stars and Constellations for this
month:

 

These descriptions of the night sky are for 9 pm on April 1 and 7 pm on April 30. They start at Orion, which is low in the west. 

 

This month, Orion (see below) is low in the west, about a handspan above the horizon. By mid-month, Orion will have set by 10.00 pm. Canis Major (the Large Dog) is above him, with the brilliant white star Sirius (Alpha Canis Majoris) showing the Dog's heart. Sirius, the Dog Star, is the brightest star in the night sky and is about fifty degrees above the western horizon. Directly overhead is the constellation, Puppis, the Stern (of the ship, Argo).

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 is straddling the west-north-westerly horizon, but its main features, the Pleiades and Hyades clusters, will have set by 9:10 pm. 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. On the evenings of April 1, the waxing crescent Moon will be close to Aldebaran and passing through the Hyades cluster. On April 28 a similar event will occur, but the Moon will have set from Australia. Below the Hyades, the Pleiades will have disappeared by 7.40 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 the nebula which formed the Pleiades can be seen around the brighter stars in the cluster.


Above the north-western horizon lies the constellation of Gemini, the Twins. The two twin stars at its north-eastern end, Pollux and Castor, are very distinctive, Pollux being vertically above Castor. These two stars are about one handspan above the horizon, but will have set by 10.30 pm at mid-month.

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-north-east is another zodiacal constellation, Leo, the Lion. The bright star Regulus (Alpha Leonis) marks the Lion’s heart, and is presently due north. Denebola, the star marking the lion's tail, is high in the north-east. We see the lion upside-down from the Southern Hemisphere. Regulus is the highest star in a pattern called 'The Sickle' (or reaping-hook). It marks the top of the Sickle's handle, with the other end of the handle, the star Eta Leonis, directly underneath. The blade of the Sickle curves around clockwise from Eta Leonis. The Sickle looks like an upside-down question mark, and forms the mane and head of the lion.


About four degrees to the right and below Eta Leonis is a beautiful double star, Algieba or Gamma Leonis. With a total magnitude of 2.61, the two stars are only 4.3 arcseconds apart, and may be distinguished with a small telescope. Both are orange in colour.

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 is a good sight in binoculars.

Rising in the north-east is a particularly beautiful orange star with a fine name: Arcturus. This is the fourth brightest star (after Sirius, Canopus and Alpha Centauri). It is a K2 star of magnitude -0.06, and lies at a distance of 36 light years. It is the brightest star in the constellation Boötes the Herdsman (only partially risen) and therefore has the alternative name of Alpha Boötis.

High above the eastern horizon (about 45 degrees up) 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. Virgo is presently dominated by the brilliant planet Jupiter, three times brighter than even Sirius. Jupiter is this month located close to Spica, and is brighter than anything else in this evening's sky apart from the Moon.

Below Spica and to its right is the faint constellation of Libra (the Scales). It has only two main stars, both about magnitude 2.7, but they have spectacular names: Zubenelgenubi and Zubeneschamali. Libra is low in the east, and as the night progresses it will be followed above the horizon by one of the brightest and most easily-recognisable constellations, Scorpius the Scorpion, which will be described next month. The two planets Mars and Saturn are presently close to Scorpius.

Above Spica and approaching the zenith is the constellation Corvus the Crow, shaped like a quadrilateral of third magnitude stars. A large but faint constellation, Hydra (the Water-snake), winds its way from near Procyon through the zenith and over the top of Corvus and Virgo to Libra, which has just risen above the eastern horizon. Hydra has one bright star, Alphard, mv=2.2. Alphard is 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 bright stars nearby. Tonight it is about 20 degrees north of the zenith, near culmination. About a handspan to the south-east of Alphard is a bright planetary nebula, the 'Ghost of Jupiter' NGC 3242. It is the remnant left when the central star exploded (below).
 

 

The planetary nebula NGC 3242


Between Hydra and Leo is a faint constellation, Sextans, (The Sextant), named by Johannes Hevelius to commemorate his favourite observing instrument which was destroyed when his observatory was burned by arsonists. Between Sextans and Corvus is another, Crater, (The Cup). Just starting to appear above the east-south-eastern horizon and below Saturn is the head of Scorpius, the Scorpion. This famous zodiacal constellation will be fully risen by 10.00 pm at the beginning of the month.

Well up in the south-south-east, Crux (Southern Cross) is at an angle of about thirty degrees. Just below the second brightest star in the Cross (Beta Crucis) is a brilliant small star cluster known as Herschel's Jewel Box. In the centre of the cluster is a red supergiant star, which is just passing through.
 

Beta Crucis (left) and the Jewel Box cluster

Herschel's Jewel Box


The two Pointers - Alpha and Beta Centauri - lie below Crux, Alpha being below Beta. Crux will have rotated clockwise to a vertical position by 11.00 pm at mid-month. Surrounding Crux on three sides is the large constellation Centaurus, and the Pointers are its two brightest stars. They are also known as the Guardians of the Cross.
 

At top right - the Eta Carinae Nebula. Centre - Crux (Southern Cross) tilted on its side. Beneath Crux - the Coalsack. Bottom - the two Pointers, Alpha and Beta Centauri.


To the right and below Crux is a small, fainter quadrilateral of stars, Musca, the Fly. Out of all the 88 constellations, it is the only insect. Below Alpha Centauri is a (roughly) equilateral triangle of 3rd magnitude stars. This is the constellation Triangulum Australe, the Southern Triangle. It is well 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 huge 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 in the eighteenth century Nicholas Lacaille divided it into three sections - Carina (the Keel), Vela (the Sails) and Puppis (the Stern).

One and a half handspans south of Sirius is the second brightest star in the night sky, Canopus (Alpha Carinae). Although appearing almost as bright as Sirius but a little more yellow, the two stars are entirely dissimilar. Sirius is a normal-sized star that is bright because it is close to us - only 8.6 light years away. Canopus, on the other hand, is a F0 type supergiant, over 100 times brighter than Sirius, but 36 times further away (312 light years).

On the border of Carina and Vela is the False Cross, larger and more lopsided than the Southern Cross. The False Cross is two handspans above Crux and to the right, and is also tilted to the left at this time of year. It has passed culmination, and is beginning to head for the south-south-western horizon. 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.

Between the Southern Cross and the False Cross may be seen a glowing patch of light. This is the famous Eta Carinae Nebula, which is a remarkable sight through binoculars or a small telescope working at low magnification. It is a turbulent area of dark dust lanes and fluorescing gas. There is a peculiar unstable star in its centre.
 

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


Close to 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 will have set by 9.00 pm at mid-month.

High in the south, about 30 degrees above the horizon, the Large Magellanic Cloud (LMC) is faintly visible as a diffuse glowing patch. It is about a handspan below (south of) Canopus. About a handspan below and to the left of the LMC is the Small Magellanic Cloud (SMC), a smaller glowing patch. The LMC and SMC are the closest other galaxies to our own Milky Way Galaxy, and are described below.

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

 

 

 

The season of the Hunter and his Dogs

 

Two of the most spectacular constellations in the sky may be seen in the western half of the sky 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


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 foot stars to the south. It is 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 deep sky objects). 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 nose 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, which 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.

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, 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 pm at mid-month, this small constellation is a little more than two handspans above the north-western horizon. 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. To the left of Orion is a small constellation, Lepus the Hare. Between Lepus and the star Canopus is the star group Columba the Dove. Underneath the star Canopus are the faint constellations Pictor (The Painter's Easel), Caelum (The Chisel), Reticulum (The Net) and Horologium (The Pendulum Clock). These were all invented by Nicolas de Lacaille and named after items in his workshop. Eridanus the River winds its way from near Orion west of the zenith to Achernar, very low in the south-south-west. The LMC lies in the constellation Dorado, with the faint constellations Mensa (Table Mountain) and Hydrus (The Water Snake) to its left. The South Celestial Pole is in the very faint constellation, Octans.

Below the False Cross is the faint constellation of Piscis Volans (The Flying Fish), called Volans for short. Underneath and to the left of Volans is Chamaeleon, (The Chamaeleon Lizard). The Lizard has his tongue extended to catch the adjoining Fly, Musca.

 

 

 

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.

To find this point, first locate the Southern Cross. 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 pm on April 1, both stars will be at similar altitudes and the line will be horizontal. Bisect this line to find the pole.

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, like 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

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 lies 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


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 close to the horizon in summer. 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 9 pm Omega Centauri is high enough for detailed viewing.
 

Globular Cluster NGC 104 in Tucana


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.

*     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-south-west, below and to the left of Canopus, 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 right 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, 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 below the horizon in the early evenings this month. 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 sunset 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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