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.
Andrew guides us through July stars, constellations and a globular cluster, and also includes intriguing facts such as this: if Antares (the star that is the heart of the Scorpion) were in the position of our Sun, it would engulf not only Earth but also Mars. Lucky for us it’s where it is, in Scorpius.
There is quite a choice of planets you can find this month with Andrew’s help, with all five unaided eye planets visible in the evening sky. To find out when and where to look for Mercury, Venus, Mars, Jupiter and Saturn in the evening, read the guide below.
See the Sky Chart
We provide a July 2016 night sky map (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 July. My name is Andrew Jacob, and I’m the Curator at Sydney Observatory.
To make the most of this podcast, you should begin by gathering a few items together. Firstly, you will need a star map. You can download one from Sydney Observatory’s website. Navigate your way to the Observation Blogs section and then to the Monthly Sky Guides to download the star map for the month of July. The star map will show you what stars and constellations are visible in the night sky for July and I will be referring to that star map during this discussion.
As well as the star map, a torch with a red LED will be very useful. The red light will allow your eyes to remain dark adapted during the evening to see the stars, yet still allow you to read your star map.
Next, a pair of binoculars or a telescope, if you have them, will allow you to view faint objects or more detail, although they’re not essential for following this podcast.
Finally, its winter, the nights are long and there is a lot to see in the sky, so do dress for the occasion!
Now we have our equipment together. Next, 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 is the zenith. This is the point directly overhead.
To navigate our way around the sky we use angular measures. Rather than say, for instance, “the Moon was 10cm below Jupiter” we would say “the Moon was 5-degrees east of Jupiter”. But how do we measure such angles? If you hold your hand out at arm’s length and stretch out your little finger and thumb, they span an angle across the sky of about 15 degrees. A fist held out at arms’ length makes an angle across the sky of about 10 degrees. And a single finger held up at arm’s length spans an angle of 1 degree. I’ll be using these measurements during this podcast. So, a hand span at arm’s length is 15 degrees, a fist, about 10 degrees, and a finger just 1 degree.
I will begin by discussing the interesting visible stars and constellations in the night sky, and at the end of this podcast, I will add the visible planets and interesting events for July.
Let us begin our tour of the night sky by looking towards the south. If you’re facing south in the early evening and you look high in the sky, 60 degrees or 3 hand-spans up, you should be able to see two bright stars, side by side. If you’re in a bright, light polluted location, such as central Sydney, they may be the only two stars you can see in that direction. Away from the city and other bright lights, they will be the brightest pair of stars you see towards the south.
These two bright stars, side by side, are known as “The Pointers,” for they help us find the Southern Cross by pointing towards it.
On your star map, the Pointers are clearly labelled. The Southern Cross is officially known as Crux, which is the Latin for cross. The brighter of the two pointers is called Alpha Centauri. The other one is called Beta Centauri. Alpha Centauri is the third brightest star in the night sky, while Beta Centauri is the 10th brightest star.
Alpha Centauri is a very interesting star. Although it looks like a single star to your eye, it is, in fact, a group of three, two stars are so close your eye cannot separate them and the third is too faint to see. Through a large telescope (one over 30-cm in diameter) two of the stars are visible. Both are very similar in size and colour to our Sun. They’re orbiting about each other with each orbit taking about 80 years. The third star in the system is called Proxima Centauri and it is the closest star to the Earth, after our Sun of course, but it is too faint to see by eye.
Proxima Centauri is a red dwarf and is only visible through large telescopes. It is believed to be orbiting the first two stars. Proxima Centauri is about 4.2 light years away, or approximately 42 million million kilometres.
I mentioned “light years” – what are they? A light year is a measure of distance, even if it sounds like a time. Light travels incredibly fast, about 300,000 kilometres every second. This means light can travel approximately seven and a half times around the Earth in just one second.
The distances in our Milky Way galaxy and our universe are vast beyond imagining. Light from the Sun takes about eight and a half minutes to reach us. The light from the Moon takes about one and a quarter seconds to reach us. The light from Proxima Centauri takes 4.2 years to reach us and therefore we see the star as it was 4.2 years ago. We can say then, that Proxima Centauri is about 4.2 light years away.
A light year is simply a distance, and it is equivalent to about 10 million million kilometres.
Now, let’s get back to the night sky. We’ll return to Alpha Centauri. If you draw an imaginary line from Alpha Centauri to the right, through Beta Centauri, and onward, you will reach the Southern Cross (Crux). In July it is at its highest point in the sky, standing upright and looking just as it does on the Australian flag.
The Southern Cross is very useful, as it can help us find the direction of true south. Hold your hand out at arm’s length, and measure the length of the long arm of the cross using two fingers, from the top star of the cross to the bottom star. Now, measure this distance downwards four times in the direction the long arm is pointing.
You should find yourself pointing at the sky about half way between the Cross and the horizon. This point is called the South Celestial Pole, the south pole of the sky. It’s the point in the sky about which all the stars appear to rotate, but there are no bright stars here. It’s just an imaginary point in the sky.
Now, if you drop a vertical line from the South Celestial Pole straight down to the horizon, you’ll find the direction of true south on the ground.
Here is something interesting you can do if you have a camera that allows you to leave the shutter open. If you take a photograph of the sky to the south, including the South Celestial Pole, and leave your camera shutter open for 10 minutes or more, you’ll find beautiful, circular star trails in your photograph.
Look back at the Southern Cross. The brightest star of the cross, at its foot, is called Acrux. It is labelled with the Greek symbol alpha (α) on your star map. Moving clockwise around the Cross we next come to Mimosa, then at the top of the Cross Gacrux and finally on the right delta Crucis.
Acrux, at the bottom, is another multiple star system like Alpha Centauri but it is about 320 light years away from us.
Mimosa, on the left, is about 108 light years away.
If you have an eye for colour you may notice that Gacrux, at the top, is orange – it is a cool red star although much larger than our Sun. It is only 88 light years away and is the closest of the four stars making up the Cross.
The Southern Cross appears on the Australian flag. The flag designers did a good job of representing the stellar Cross – the two axes are slightly skew and there is a fifth star, which appears smaller on the flag and is fainter in the sky than the four stars I have just named. This fifth star is called epsilon Crucis. It appears on your star map between bright Acrux and delta Crucis on the right.
Surrounding the Southern Cross is the constellation of Centaurus the Centaur. His front leg is The Pointer stars, his back arches over the Cross and his back leg hooks down to the right of the Cross. A Centaur is a half-man, half-horse creature holding a bow loaded with an arrow. His head and upper body lie above the Pointers, but if you can make out the features of a man’s head and torso I admire your imagination!
Just above the Centaur’s back is wonderful object called Omega Centauri. It is labelled on your star map, but you will only see it by eye if you are in a completely dark site, far from city, or other lights. From a bright area you will need binoculars to spot it. Omega Centauri is a globular cluster of stars, a ball-shaped group of several million stars, tightly packed and about 16,000 light years away. There are over a hundred of these globular clusters scattered around our Milky Way galaxy, but this one is the largest and brightest.
Well, that covers the southern sky. Let’s now turn to the west.
The constellation of Leo the Lion lies very close to the western horizon. Its brightest star Regulus, meaning “little King”, is about a hand-span above the horizon at around 6:30pm. Regulus is a hot star a hundred times brighter than our own Sun and about 77 light years away.
Above Leo is another zodiac constellation, Virgo. Literally, the Virgin but also associated with the virgin goddess Astraea or the Greek and Roman goddess of wheat and agriculture. Her human form is hard to make out in the sky but Virgo’s brightest star is Spica, at about 60 degrees, or three hand spans above the western horizon. Spica is Latin for “ear of wheat” and Virgo holds this wheat, or Spica, in her hand reflecting again the theme of agriculture but also of fertility.
The stars Regulus and Spica lie on a line across the sky called the ecliptic – you can see it on your star map. This line is the path of the Sun through the sky and along this line you will also find the planets and our Moon.
Let us now turn to the north.
You will notice an orange-red star due north, about 37 degrees or just over one and a half hand spans above the northern horizon. This is the star Arcturus in the constellation Bootes the Herdsman. But once again his human form is difficult to make out!
Finally we turn to the east.
Face due east, hold out your arm and measure three hand spans plus a fist upwards, to make 70 degrees, above the eastern horizon or almost overhead. You should see a bright, orange coloured star. If you’re using your star map, rotate it so that the horizon labelled east is at the bottom. This will orient the map to match the eastern sky in front of you.
I remember having great trouble seeing colour in stars when I first began looking at the sky. So if you don’t see the orange colour tonight, don’t worry.
This star is called Antares which means the ‘rival of Mars’ because of its reddish colour. It’s an enormous, red supergiant star, around 400 times the diameter of our Sun. If you placed it where our Sun is, it would reach out through the solar system and engulf the Earth.
Antares is about 604 light years away from the Earth. It’s a star coming to the end of its life. It will eventually die by exploding in a cataclysmic supernova, destroying itself in the process. However, it’s so far away that this will have no effect on the Earth, although it will be a spectacularly bright sight. Unfortunately, it will be a few hundred thousand years before this happens.
Antares is the heart of Scorpius, the Scorpion, one of the few constellations that really looks like its name. If you have your star map with you, hold it up towards the east with the east horizon at the bottom, as I described earlier, and locate the star Antares. Just above Antares, you’ll see a short arc of stars which represent the head and shoulders of the scorpion.
Look back through Antares and out to the right. You will see a curving arc of stars reaching across to the right, the scorpion’s body, then an arc of stars hooking down and back to the left, his tail. At the very end of his tail, you can see his sting quite clearly. Scorpius really does look like a scorpion.
Let’s move on. Below the sting of Scorpius is the constellation Sagittarius, which is supposed to represent an archer, but I’ve never been able to see an archer when I look at this set of stars. All I can see is a rather triangular teapot. On your star map, Sagittarius is highlighted as the Teapot. Can you see the Teapot pouring tea all over the tail of Scorpius?
Just off the tip of the Teapot is an interesting point in the sky. If you’re away from bright city lights and you’ve been outdoors for more than 15 minutes or so, you might have noticed the Milky Way stretching overhead. It’s a band of faint, milky light stretching from the Southern Cross past the two Pointer stars and continuing overhead through the tail of Scorpius, through Sagittarius and finally down to the eastern horizon.
The centre of our Milky Way galaxy lies just above the tip of the Teapot not far from the sting of Scorpius. It’s a good thing we’re out here near the edge of the Milky Way galaxy and a long way from its centre. At the centre of our galaxy, 27,000 light years away, lies a very large black hole over four million times the mass of our Sun. We’re quite safe from it here, out on the edge of the galaxy.
What are the special events and highlights for July 2016?
Let me note that all the times I mention are in ‘clock’ time, the time a clock would show or Eastern Australian Standard Time.
Let’s start with the Moon phases. We begin with a New Moon which falls on Monday the 4th at 09:01pm. First Quarter is on Tuesday 12th at 10:52am. Full Moon occurs on Wednesday 20th July at 08:56am and finally Last Quarter is on Wednesday 27th at 09:00am.
What planets are visible in July 2016?
The month begins with three bright planets – Mars, Jupiter & Saturn – overhead. A few days into the month Venus joins them in the north-western sky and from the middle of the month Mercury is also visible in the north-west. So all five naked-eye planets – the planets that have been known for millennia – will be visible at once.
Mercury emerges out of the evening twilight around the middle of the month low in the north-west. It spends the second half of July in Leo.
Venus begins the month low in the north-west, moving from Gemini to Cancer and finally into Leo. On 5 July, with Venus close to setting, a thin crescent Moon may just be visible above and to the right of the planet. On 17 July Venus and Mercury are within about one Moon-width of each other. This is a good opportunity to identify Mercury which can be difficult to find otherwise. Binoculars will help here.
Jupiter remains in Leo for the month and in the north-western sky during July evenings. On 9 July the crescent Moon approaches the planet and they appear very close as they both set. Viewers in southern Africa and Madagascar will see the Moon continue to approach Jupiter and hide, or occult, it.
Saturn remains in Ophiuchus, not far from Antares the orange-coloured star at the heart of Scorpius.
Mars shines brightly just off the head of the Scorpion. If you haven’t seen it yet with a telescope this month will be your last chance for a good view. Earth is drawing away from Mars so it will appear smaller each week. It won’t look as large or as bright for another two years.
There are a few other noteworthy events happening in July 2016.
Jupiter and Juno: The spacecraft Juno is due to arrive at Jupiter on July 5 (it will still be July 4 in the USA). Juno will search for water within the clouds, measure the gravitational field and investigate Jupiter’s intense, and huge, magnetosphere. Every fortnight the spacecraft skims low over the planet, just thousands of kilometres above the cloud tops. It will likely return spectacular images but is in danger of being destroyed by the damaging radiation trapped near the planet. The mission is planned to last about two years.
The Solar-System Barycentre: When we talk about the planets orbiting the Sun we often forget that the Sun also has its own small orbit caused by the gravitational attractions of all the other planets combined – just like a hammer thrower at the Olympic Games. Everything moves around the centre of mass or barycentre of the Solar System. The barycentre moves about in a complicated spiralling path within and close to the Sun. Sometime between July 15 & 20 this year it will move outside the Sun, remaining there until 2027. In fact it spends most of its time outside the Sun largely due to the gravitational influence of the massive planet Jupiter.
This year, 2016, professional astronomers in Australia mark the 50th anniversary of the formation of their society, the Astronomical Society of Australia. Also open to anyone with a demonstrated dedication to the advancement of astronomy the Society was established partly through the efforts of Harley Wood, Government Astronomer at Sydney Observatory. Amongst the events to celebrate the anniversary will be the Society’s annual conference in early July, a time capsule to be opened on the100th anniversary and the opening of a small native garden at Sydney Observatory dedicated to Harley and his wife Una.
One final event for July is the Earth will reach its furthest point from the Sun, or aphelion, on July 5 at 02:24am. We will be just over 152 million kilometres from the Sun.
This has been Dr Andrew Jacob from Sydney Observatory with the podcast for July 2016.