To help you learn about the southern night sky, Sydney Observatory provides an audio guide/podcast, transcript of that audio, and a sky map or chart 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, some at night, some early in the morning. To find out when and where to look for Venus, Jupiter and Saturn in the evening and for Mercury in the morning, read the transcript below.
HEAR THE AUDIO
Audio coming soon!
SEE THE SKY CHART
We provide an embedded sky map (below) and a July 2015 night sky chart as a printable 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.
BUY THE BOOK
Our annual book, ‘The 2015 Australasian sky guide’, by Dr Nick Lomb has more information and star maps for months from December 2014 until December 2015 inclusive, plus information about the Sun, twilight, the Moon and tides, and a host of other fascinating astronomical information. You can purchase it ($16.95) at Sydney Observatory and Powerhouse Museum shops or other good bookshops, or online through Powerhouse Publishing (additional packing/postage costs apply).
READ THE TRANSCRIPT (after the jump)
Transcript of the July 2015 monthly sky guide audio
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 Astronomy & 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 labeled. 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 kilometers.
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 kilometers 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 kilometers.
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 labeled 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 center 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 center. At the center 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.
Now we’ve completed our tour of the sky for July.
What are the special events and highlights in July 2015?
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 Full Moon which falls on Thursday the 2nd at 12:20pm. Last Quarter is on Thursday 9th at 6:24am. New Moon occurs on Thursday, 16th July at 11:24am. First Quarter is on Friday 24th at 2:04pm. And finally there is another Full Moon on Friday 31st at 8:43pm. This second Full Moon in the one calendar month is termed a “Blue Moon” although it won’t look at all blue.
What planets are visible in July 2015?
Mercury begins the month low in the east before sunrise, in Taurus. It moves into Gemini in the second week as it disappears into the morning twilight.
The greatest show this month is provided by Venus and Jupiter. They spend time together in the west just after sunset for the whole month. They begin the month, on July 1, with a close approach or conjunction. For a few days binoculars will show both planets at once – they appear closer together than the width of our Moon. If you have binoculars see if you can detect the crescent shape of Venus and the moons of Jupiter. On the 18th and 19th the crescent Moon joins them making for a spectacular photographic opportunity. Throughout the month both planets are also very close to the star Regulus in the constellation Leo. By eye Venus will be the brightest of the group, in fact it is at its brightest on July 10. Jupiter is slightly fainter and has a yellowish colour. Regulus is the faintest of the group.
Saturn spends the month high in the sky in Libra, but not far from the head of Scorpius.
There are a few other noteworthy events happening in July 2015. The New Horizons space probe is approaching Pluto. For two weeks, until closest approach on the evening of July 14, we will receive increasingly detailed images of this dwarf planet. For over 80 years Pluto has been little more than a mere dot amongst the stars. Soon it will transform into a fully-known world with craters, mountains and valleys and probably a few surprises too. During the two hours of closest approach we will learn nothing because the spacecraft will be busy recording data. Following the flyby it will spend the next two years returning all its data before flying on to investigate the frozen outer reaches of our solar system.
Pluto is not visible to the naked eye but it is high in the sky in the constellation of Sagittarius this month.
One final event for July is the Earth will reach its furthest point from the Sun, or aphelion, on July 7 at 05:40am. We will be just over 152 million kilometres from the Sun.
If you have enjoyed this podcast and think you might want to regularly check out what’s in the night sky, why not purchase a copy of Sydney Observatory’s book, “The Australasian Sky Guide.” It not only contains detailed monthly sky guides, but is jam packed with astronomical information, including rise and set times for the Sun, Moon, and planets, tides and a detailed look at our solar system and upcoming astronomical events.
Only $16.95 from Sydney Observatory, and Powerhouse Museum shops, or you can purchase it online, for which additional costs apply.
For more astronomical information, check our website and blogs at Sydney Observatory. You may also like to explore our Facebook and Twitter accounts.
This has been Andrew Jacob from Sydney Observatory with the podcast for July, 2015.