To help you learn about the southern night sky, Sydney Observatory provides a guide and a sky map each month. This month’s guide is presented by Dr Andrew Jacob, Sydney Observatory’s Curator of Astronomy.
This month, learn how to find the autumn constellations. Tour the Milky Way galaxy from the Southern Cross to Orion in the north and discover the brightest stars in the sky.
Andrew also tells us how to find the planet Venus in the evening sky, when to see the next supermoon and how to watch Betelgeuse brighten after it faded last summer.
A special note for April 2020: This April podcast & transcript were recorded and written before coronavirus took hold in Sydney. We plan to continue writing Observatory blog posts for you and will try to maintain our Twitter and Facebook feeds. The Observatory exhibition and our Night Tours, however, are closed until further notice. But if your circumstances allow I hope you can find a little time to glance skyward this month – the stars are still shining down upon us, even if it may not feel like it right now.
See the Sky Chart
We provide an StarMap 04 April 2020(PDF) which shows the stars, constellations and planets visible in the night sky from anywhere in Australia. To view PDF star charts you will need to download and install Adobe Acrobat Reader if it’s not on your computer already.
Read the Guide
Hello, and welcome to the night sky for April.
This is Andrew Jacob, and I’m the Curator at Sydney Observatory, part of Sydney’s Museum of Applied Arts & Sciences. Come on a tour of the night sky with me. Learn what stars and constellations are visible, where to find the planets and what special events are happening overhead this month.
To make the most of this guide, you should begin by gathering a few items together. Firstly, you will need a star map. You can download a free one from Sydney Observatory’s website where you’ll find it in the Astronomy Resources section under Monthly Sky Guides. The star map will show you what stars and constellations are visible in the night sky this month and I’ll be referring to that star map in this guide.
As well as the star map, a torch with a red LED, or one covered with a few layers of red cellophane, will be very useful. The red light will allow your eyes to remain dark adapted during the evening, yet still allow you to read your star map.
Finally, a pair of binoculars or a telescope can be very handy. They’re not essential for following this guide but if you do have them they will help you see a few of the fainter objects more easily and in more detail.
Now that we have our equipment together we need to know a few directions and also how to measure angles across the sky.
You can find the cardinal directions – North, South, East and West – from a compass app on your mobile device, or just remember, of course, that the Sun rises in the east and sets in the west. And if the Sun is setting at your right shoulder, then you must be facing south. Another useful direction to remember is the zenith. This is the point directly overhead.
To find your way around the night sky, it helps to know how to measure angles across it. It makes no sense to say, for instance, that one star is “2 centimetres” to the left of another or that a shooting star left a trail “half a meter” long! Instead we should use angular measurements. The distance around the horizon, from North, through East, South, West and back to North is 360 degrees. And from the horizon vertically upwards to the zenith overhead is 90 degrees.
But how do we measure smaller angles? Well, despite the great variety of human form our fingers, hands and arms are all pretty much in the same proportions. If you hold your hand out at arm’s length and stretch out your little finger and thumb to make a hand span, they span an angle across the sky of about 20 degrees. A fist held out at arm’s length makes an angle across the sky of about 10 degrees. And a finger held up at arm’s length is about 1 degree, or twice the width of the Moon or the Sun. I’ll be using these measurements during this guide. So, a hand span at arm’s length is 20 degrees, a fist is about 10 degrees, and a single finger is about one degree across the sky.
One final point to note before we get started. The Earth rotates and so the sky changes hour by hour. My descriptions of the constellations and stars in this guide fit the time of one to two hours after sunset.
This month we return to the long nights of autumn following the end of daylight saving. The nights are long but not too cold for good observing. But get some warm clothes on and a beanie and grab a blanket to lie on. Just as we found in January the best constellations are high overhead. Lie down with your feet facing South-West, just to the left of where the Sun recently set. Look straight up towards the zenith. With your feet pointing south-west most of the constellations above will appear upright, rather than upside down as they often do from the southern hemisphere!
If you are in a dark location, and there is no Moon up, you will see the Milky Way, the edge-on view of our own galaxy, stretching from the south-eastern horizon on your left, passing overhead through or close to the zenith, and reaching the north-western horizon on your right. If you are near a city or large town and light pollution affects your view the Milky Way won’t be easily visible but you will still see a band of bright stars stretching across this part of the sky, from south-east to north-west. On your star map the Milky Way is indicated by the dot-dash line spanning the width of the map.
You will need to align your star map with the real sky. Simply hold your map up in front of you and tilt it to the right so that the label “SW” (for southwest, of course) is at the bottom, lined up with your feet! Your map is now aligned with the stars overhead and is ready to assist you with the next part of this guide.
We begin with the Pointer Stars on your far-left. On your star map these are clearly labelled near the “SE”, or south-east, horizon.
These two bright stars appear almost side-by-side. The brighter of the two pointers, the one on the left, is called Alpha Centauri and it is the third brightest star in the night sky. The right-hand Pointer is Beta Centauri and it is the 10th brightest star. Later we will search for all ten brightest stars in the night sky.
Alpha Centauri is a fascinating star. Although it looks like a single star to your eye, it is in fact a group of three, with two of them too close together for your eye to separate and the third too faint to detect. Through a medium-sized telescope, two of the stars are visible. Both are very similar in size and colour to our Sun. These two stars orbit about each other once every 80 years or so. The third star is called Proxima Centauri, and it is the closest star to the Earth, after our Sun.
Unfortunately, Proxima Centauri is a faint red dwarf star and is easily visible only with large telescopes. It orbits the first two stars. Proxima Centauri is about 4.2 light years away, or approximately 42 million million kilometers.
Recently a planet was detected in orbit around Proxima Centauri. This makes it the closest planet beyond the eight in our own solar system – a very tempting target to send the first interstellar spacecraft to! But that is for the future.
A moment ago I mentioned “light years”. What is a light year? It’s a measure of distance, even if it sounds like a time. If you have a torch shine its light into the sky for a moment. The light from your torch travels incredibly fast. In just one second it goes almost 300,000 kilometers. That’s seven and a half times around the Earth or almost the distance to the Moon. In fact in just one and a quarter seconds your torch light would reach the Moon, 380,000 kilometres away. In eight and a half minutes the light would reach the Sun, 150 million kilometres from Earth. After five hours light from your torch would pass Pluto. Finally, after one year of time your torch light will have travelled one light year of distance. Yet we still haven’t reached the next nearest star!
Proxima Centauri, our closest star after the Sun, is about 42 million million kilometres away from us. Your torch light would take about 4.2 years to reach it. So we can say that Proxima Centauri is about 4.2 light years away from Earth. This also means that we see Proxima Centauri, the star, as it was about 4.2 years ago. We are looking into the past to see everything in the universe.
So a light year is simply a distance, and one light year is about 10 million million kilometers long.
Now, let’s get back to the night sky. We’ll return to Alpha Centauri. If you draw an imaginary line from Alpha Centauri across to the right through Beta Centauri, and onward, you will reach the Southern Cross. I hope you recognize it because it looks just like it does on the Australian flag.
The Southern Cross is the best known constellation in the southern skies. It is formally known as Crux, its Latin name, but we have also labelled it Southern Cross on your star map.
The Southern Cross is very useful, as it can help us find the direction of true south. Hold up your arm, and measure the length of the long arm of the cross using two fingers (your pointer and little fingers), from the top star to the bottom one. Now, measure this distance three and a half times downwards starting from the bottom (brightest) star of the Cross. This description works if you are lying down with your feet facing south-west. And it may help to close one eye as you measure!
The point you end up at is called the South Celestial Pole, the south pole of the sky. It’s the point in the sky about which all the stars are rotating, but there are no bright stars in this area. It’s just an imaginary point in the sky. It is labelled on your star map.
Now, if you draw a line from the South Celestial Pole to the nearest point on the horizon, you have found the direction of south on the ground. From now on you will never need an app or compass to find south.
Returning to the Cross you will notice, if you are in a dark site and the Moon has set, a dark patch below the Cross (or to its lower-left side if it was upright). This is the Coal Sack, a dark cloud of hydrogen gas and, mostly, carbon dust blocking light from more distant stars. In fact, you will notice many of these dark patches spread here and there along the Milky Way. They hold the raw material for making new stars.
Here is something interesting you can do if you have a camera that allows you to leave the shutter open (or an app that allows you to make ‘star-trail’ photos). If you take a photograph of the sky facing south, including the South Celestial Pole, and leave your camera shutter open for 10 or more minutes, you’ll find beautiful, circular star trails in your photograph. This shows how the stars appear to rotate about the South Celestial Pole. It is however the Earth which is really rotating.
Let’s now turn our attention to the constellation of Orion, the Hunter in the sky, over on your right towards the north-west horizon. With the help of your star map locate the three stars of Orion’s belt – it is labelled. From left to right these are Alnitak, Alnilam and Mintaka. Turning your head to the right you will see, just above these three belt stars, the star Betelgeuse, glowing orange-red or sometimes pale-yellow. It’s not that the star changes colour, more that your perception of the colour changes with the clarity of the atmosphere and your eyes’ adaption to the dark. This enormous star represents Orion’s shoulder. It is a ‘red giant’ star coming to the end of its life, it is 425 light years away and it’s hundreds of times larger than our Sun. And yet to your eye it looks just like any other star. The stars are so far away that we can’t see the physical size of any of them – except for the one star that’s close to us, our Sun.
Just below Orion’s belt is the bright star Rigel, one of Orion’s knees. From Betelgeuse to Rigel is just over a full hand span or about 20-degrees across the sky. Rigel is also at a late stage in its life cycle, although not as late as Betelgeuse. It is about 1000 light years away and its surface temperature is about 11,000 degrees Celsius – about twice as hot as the surface of our Sun.
Now, if Betelgeuse and Rigel form Orion’s right shoulder and left knee respectively you should now be able to imagine the figure of a man, perhaps with the help of your star map. His head and shoulders are to the right, his body narrows to the three “belt stars” at his waist and his legs stretch out to the left. Between Orion’s legs, hanging from his belt, is Orion’s sword. To your eye this appears as a pair of (or maybe three to some of you) fuzzy stars. Through binoculars you will see three pairs of stars, plus many more fainter ones, with the middle pair surrounded by a faint hazy “cloud”. This cloud, or nebula, is the Orion Nebula, over 1500 light years away. It is also known to astronomers as “M42”. It is perhaps the most photographed object beyond our solar system and is a large cloud of, mostly, hydrogen gas which is producing new stars. The cloud is 15 light years in diameter and the stars you see embedded within it (with your binoculars) were formed from the collapsing hydrogen gas within the last million years or so.
Orion is a pivotal constellation – its stars are guides pointing to many other interesting stars and constellations.
If we extend the line of Orion’s belt stars one hand span (20-degrees) to the right we come to an orange star amongst a V-shaped pattern of fainter stars. This is Taurus the Bull. The orange star is Aldebaran, the eye of the Bull, and his head is the V-shape. In April Taurus is close to setting in the west and may be difficult to identify.
Returning to Orion’s belt we now follow the line of the belt to the left about one hand span. We reach the brightest star in the sky, Sirius. Its name means “scorching” or “brilliant” and you can see why! Sirius is also called the “Dog Star” because it’s the brightest star in the constellation of Canis Major, the Big Dog. With help from your star map you can identify the shape of the dog, the faithful hunting companion of Orion.
Just above Sirius, a hand span plus four fingers width, is the bright star Procyon in the constellation Canis Minor, the Little Dog. Procyon has a companion star a couple of fingers width to the right. However, this constellation looks nothing at all like a dog. In fact, few constellations clearly resemble their name!
Returning again to Orion, we now use his sword to direct us to our next destination. You might notice that Orion’s sword points directly left (towards the South Celestial Pole across the sky) and right (North). This is very useful to remember when the Southern Cross is low in the sky or hidden by cloud.
Following the direction of Orion’s sword towards the South, or left, about three handspans brings us underneath Canis Major and to a point above your feet and we reach another bright, white star. If you have a good eye for colour it will look pale-golden colour. This is Canopus (it’s labelled on your star map), 205 light years away and the second brightest star in the night sky after Sirius. Canopus is one of the celestial navigation stars that have been used by ships navigators for centuries. Appropriately it’s the brightest star in the constellation Carina, the Keel of a ship. On your star map you will see the other parts of the ship: the sails, Vela, the rear (or poop) deck, Puppis, and the ship’s compass, Pyxis. There is even a Flying fish, Volans, in this watery scene overhead! This ship – Carina, Vela, Puppis & Pyxis – used to be one huge constellation called Argo Navis, the Ship of the Argonauts. The Argonauts were mythological Greek heroes who accompanied the hero Jason on his quest to find the Golden Fleece. This is not an easy ship to identify in the sky so this is my observing challenge for April. Can you identify the whole of the ship – Keel, Sails, Rear Deck, Compass and all in the sky?
More easily identified is the False Cross. This is an “asterism” or star-shape not one of the 88 formal constellations. It lies on the border of Carina and Vela taking in stars from both constellations. It is larger and fainter than the real Southern Cross which lies further to the left.
Below the False Cross and Carina you will notice, if you are in a dark site away from city lights and with no Moon in the sky, two cloudy patches. These are the Large Magellanic Cloud (LMC on your map) and Small Magellanic Cloud (SMC). These two “clouds” are companion galaxies to our own Milky Way galaxy. But our galaxy is bullying and harassing these smaller companions, tearing off shreds of hydrogen gas. Their future is uncertain – they may either be absorbed in the distant future by the Milky Way or perhaps just pass on by.
The Small Magellanic Cloud is the lower of the two, if you are still lying down with your feet to the south-west. Just to its right is another bright star. This is Achernar, meaning the River’s end. It is the star at the end of the constellation Eridanus, the River in the sky.
Let’s review what we’ve seen tonight. We begin from the south-east, on your left, and we’ll travel along the Milky Way. First are the Two Pointer stars and then the Southern Cross. Moving northwards, to the right, we pass the False Cross between Vela the Sails and Carina the Keel. Then comes Canopus, the navigation star, and below it the Magellanic Clouds and Achernar. Next along is Canis Major and the brightest star, Sirius, followed by Orion the Hunter with his belt and sword. Then comes Taurus the Bull. What a grand sight!
It’s an even grander sight with binoculars. If you sweep the Milky Way passing all the objects I’ve mentioned tonight you’ll also discover, in between, hazy gaseous nebulae where stars are born, sparkling clusters of young stars and curious star patterns here and there. At first it seems overwhelming but if taken in bite sized chunks, month by month, there’s a lifetime of observing to be done.
But wait there’s more! Most of the brightest stars in the night sky are visible during April nights. Sirius, the Dog Star, is the brightest followed by Canopus, the navigation star. Third brightest is Alpha Centauri, the brighter of the Two Pointers. Next are Arcturus, Vega and Capella none of which is visible on April evenings. Seventh brightest is Rigel, Orion’s knee. Next is Procyon in the Little Dog. Ninth is Achernar at the end of the River Eridanus. Finally, Betelgeuse, Orion’s shoulder, is the tenth brightest star in the night sky. That’s seven of the ten brightest stars all visible at once on autumn evenings
We haven’t yet tried to find Gemini the Twins, or Cancer the Crab, or Leo the Lion in the northern sky. But we have covered a lot already so I shall leave it to you, with your new star-map reading skills, to identify these three Zodiac constellations in the April evening sky. Anyway, looking for them would require moving and abandoning the comfy position you are in. Why not stay a while, contemplate the universe, and wait for a meteor or two…
Now, let’s have a look at the special events and highlights for this month.
What are the special events and highlights for April 2020?
Let me note that all the times I am about to mention are in Australian Eastern Daylight-saving Time (AEDT) or Australian Eastern Standard Time (AEST) as appropriate. Daylight saving, in states where it is recognised, comes to an end on Sunday April 5 — at 3am put your clocks back one hour! For all times I mention please make the appropriate adjustments for your time zone and state if necessary.
Let’s start with the Moon phases.
We begin April with a First Quarter Moon on the very first day of the month at 9:21pm. Full Moon is then on Wednesday 8th at 12:35pm followed by Last Quarter on Wednesday 15th at 8:56am. And finally, we have a New Moon on Thursday 23rd at 12:26pm.
The Moon is the brightest object in the night sky when it is up, no matter what phase it is in. It is well worth observing its changing phases or looking closely at the craters, plains and other features with binoculars or a telescope. But to get the best views of other astronomical objects such as the Milky Way and the constellations it is best to avoid moon-lit hours. If the Moon is between New and Full (we call that waxing) wait for it to set before observing the Milky Way and stars. If the Moon is between Full and New (waning) observe before it rises. You don’t really need a daily list of rise and set times – just watch the Moon for a few days and you will soon learn to predict its behaviour.
So this month wait for the Moon to set in the first and last weeks. Otherwise look for the Milky Way and constellations in the hours before the Moon rises.
What planets are visible in April 2020?
In the evenings this month it’s all Venus, shining brightly as the “Evening Star” above the western horizon every night. Venus orbits the Sun closer than Earth so it shows phases, just like the Moon. Sometimes it is crescent shaped, sometimes “half-Moon” shaped and sometimes a gibbous shape – but you need binoculars to see these phases. Venus also overtakes Earth – just like an Olympic runner on the inside track. In April the planet is approaching Earth on its way to passing between us and the Sun in June so that during the month it grows larger and also more crescent shaped. It is fascinating to watch it with binoculars for the month to see its changing phase and size – just as Galileo did in 1610 when he was the first person ever to see the phases of Venus with his newly constructed telescope.
Venus is in the constellation of Taurus, the Bull, this month. And on April 3rd and 4th it passes in front of a group of stars named The Pleiades or sometimes The Seven Sisters – a cluster of young stars. With binoculars you may be able to see both the planet and the stars – but this may be a tricky observation with both being in the bright evening twilight sky, and getting close to the horizon. Nevertheless, give it a try on the 3rd and again on the 4th to see just how far Venus can move across the sky in only one day.
Enough of Venus. What can we see in the morning sky?
The morning planets this month are Mercury, Mars, Jupiter and Saturn. In fact, Mars, Jupiter and Saturn are visible after 1am all month but you will have to wait till after about 5am to see Mercury.
Mercury remains low in the east rising shortly before sunrise. Even the Moon can’t help us locate Mercury this month, being just one day before its New phase – and therefore too thin a crescent to detect – as it passes Mercury on the 22nd.
Jupiter is much easier to find. It’s the brightest of the three outer planets we see by eye – Jupiter, Saturn and Mars. Jupiter rises before midnight for most of the month and is the brightest object in the eastern sky, bar the Moon, so it should be easy to spot. On the 15th the Last Quarter Moon sits just above it.
Saturn is about 5-degrees below Jupiter. But you may notice it steadily moving closer to Jupiter during the month as these planets approach their very close conjunction in December. Saturn begins the month on April 1st side-by-side with Mars – they are just two Moon-widths apart. Mars, of course, is the reddish one while Saturn looks yellowish. Thereafter Mars steadily slips further towards the horizon each morning. On the 16th the Moon sits between Saturn and Mars.
We have another supermoon this month. You may be aware a supermoon is a Full Moon that occurs very close in time to when the Moon is closest in distance to Earth during its elliptical orbit about our planet. The actual definition is unclear and almost completely arbitrary. In any case, on Wednesday April 8 at 4:09am the Moon is at its closest to Earth, called perigee, at just 356,907km from Earth. And this is its closest perigee for the year- so this is the best supermoon for the year. Just a few hours later, at 12:35pm the Moon is Full (and now 357,030km away). Later on the same day, for Australia, when the Moon rises (at 18:06pm for Sydney) it may look spectacular. But I think any Full Moon rising over the horizon, reddened and maybe passing behind clouds, looks spectacular! Do you think it looks larger tonight?
What else is happening in April 2020?
The dimming of Betelgeuse. Earlier I said Betelgeuse, the red star that is the shoulder of Orion the Hunter, was the tenth brightest star in the sky. But that’s true only on average! Betelgeuse varies in brightness slightly, sometimes it can be the 8th brightest star, and at other times it dims to become 12th brightest. However, strangely over this last summer Betelgeuse faded in a way its never before been seen to do – and by mid-February it became the 23rd brightest star in the sky. To anyone familiar with the constellation it did look a little strange. But now it’s brightening again. Its brightening is subtle and slow, but if you watch it once a week you may notice it change. Presently (mid-March when we recorded this podcast) it is about as bright as the stars of Orion’s belt, but at its brightest it rivals Rigel – the knee of Orion. Check your starmap to identify these stars.
April 12 is International Day of Human Space Flight, sometimes called Yuri’s Night in celebration of the first human space flight. Take a moment to consider how far we have come in the past century – from the wood & canvas biplanes of 1920, to Yuri Gagarin’s first orbit of Earth in 1961, Neil Armstrong’s step upon the Moon in 1969 and on to the verge of commercial space travel for everyone – at least at first for everyone wealthy enough!
And that wraps up the special events for April 2020.
An excellent companion to Sydney Observatory’s monthly Night Sky Guides is the annual “Australasian Sky Guide” by Dr Nick Lomb. It’s jam-packed with monthly night star maps and astronomical information, including rise and set times for the Sun, Moon and planets, plus tide times and a detailed look at our solar system and upcoming astronomical events. It’s available from Sydney Observatory or the MAAS store, or you can purchase it online, for which additional costs apply.
And if you’re in Sydney visit the Observatory in the Rocks area. Book in for a night tour to view the skies through our telescopes. Or tour our exhibition for free and discover the history of Australian astronomy.
And that brings to an end this Night Sky Guide from Sydney Observatory and from me, Andrew Jacob. Thank you for listening and I wish you clear skies until next time.