Observations

July 2018 night sky guide and sky chart

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, don’t miss Mars – at its closest since 2003. And be sure to catch the total lunar eclipse before dawn on July 28. Learn how to find the winter constellations including the Scorpion and search for star clusters in the Milky Way.

Hear the Audio
You can subscribe to the podcast on iTunes or directly download this month’s guide to your favourite audio listening device.

See the Sky Chart
We provide a July 2018 Night Sky Chart (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 Guide for July 2018.

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.

And in July the nights are long and cold so please dress for the occasion!

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 from the horizon, you should be able to see two bright stars, almost side by side about three fingers-width apart. 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,” so-named because they help us find the Southern Cross by pointing towards it. We’ll get back to the Cross soon.

On your star map, the Pointers are clearly labeled. The Southern Cross is officially known as Crux, which is 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 telescopes the two close stars are usually 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 star 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.

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 lightyear 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 to the right, through Beta Centauri (the other Pointer star), and onward, you will reach the Southern Cross. Well, nearly so! The line passes just above the Cross. At first glance the Cross may look like a triangle but the fourth star soon becomes clear. In July the Southern Cross is at its highest point in the sky, standing upright and looking just as it does on the Australian flag. In fact, our flag can help you identify the Cross in the sky.

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 of the Cross 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 over the course of the night. In contrast to the northern hemisphere, where there is a North star, here no bright star marks the South Celestial Pole. 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 have found 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 (or an app that allows you to make ‘star-trail’ photos). 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, the one 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 come to ‘Mimosa’, then at the top of the Cross ‘Gacrux’ and finally on the right is delta (δ) Crucis.

Acrux, at the foot of the Cross, 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 binoculars use them to view Mimosa. Now, the star itself is not too interesting but look just to its left and you will see a small sparkling group, or cluster, of faint stars in the shape of a triangle. Astronomers call this type of object an “open cluster” and this one is named the Jewel Box. It is a group of young stars, not more than 15 million years old about 5000 light years away. This cluster looks much better through any telescope, when stars of different colours – red, yellow or just plain white – are seen.

If you have an eye for colour you may notice that Gacrux, at the top of the Cross, is orange – it is a cool red-giant star much larger than our Sun. It is only 88 light years away so is the closest of the four main stars of the Southern Cross.

The Southern Cross appears on the Australian flag, of course. 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 fainter in the sky than the four stars I have just named, and therefore smaller on the flag. This fifth star is called epsilon (ε) 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 & upper body lie above the Pointers, but if you can make out the features of a man’s head and torso here then I admire your imagination!

Just above the Centaur’s back is a 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 artificial light, with no Moon up and if your eyes have adjusted to the darkness. From a bright, light polluted, area you will need binoculars, at least, 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.

To orientate your star map to the western sky, rotate it so that the horizon labeled “West” is at the bottom.

The constellation of Leo the Lion, one of the constellations of the zodiac, 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, however it sets around 7:30pm this month. 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. This figure is associated with the virgin goddess Astraea of the Greeks and also with the Greek and Roman goddess of wheat and agriculture – Demeter to the Greeks and Ceres to the Romans. Her human form is hard to make out in the sky but the constellation’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, perhaps fresh from the harvest, reflecting the theme of agriculture but also of fertility.

The two stars, Regulus and Spica, lie on a line that crosses the sky called the ecliptic – you can see it drawn on your star map as a dashed line. This line marks the apparent path of the Sun through the sky and along this line you will also find the planets and our Moon – but more about them later.

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. Again, to orientate your star map, rotate it so that the horizon labeled “East” is at the bottom. The map should now match the eastern sky in front of you.

I’ve noted the colours of several stars so far. But I remember having great trouble seeing colour in stars when I first began looking at the sky. So if you don’t see the colours I describe 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 also 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 past 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. Finally, here’s a constellation that really does resemble its name!

Let’s move on. Below the sting of Scorpius is the constellation Sagittarius, which is supposed to represent an archer in the form of a centaur, but I’ve never been able to see an archer, let alone a centaur, 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 to allow your eyes to adjust to the darkness, 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 the center. Because 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 towards the edge of the galaxy.

If you haven’t already succumbed to the cold, and if you have binoculars handy here is your observing challenge for July. Slowly scan the region of Sagittarius, Scorpius and over to the Southern Cross and beyond. In this region there are many open clusters (like the Jewel Box), globular clusters (like Omega Centauri), multiple stars (like Acrux and Alpha Centauri) and nebulae (like the Orion nebula we met in January’s Guide). It shouldn’t take long before you discover at least one of each for yourself!

Now we’ve completed our tour of the sky for July.

What are the special events and highlights in July 2018?
Let me first note that all the times I mention here are in Eastern Australian Standard Time, or the time a clock would show in the eastern states. Please make the appropriate adjustments for your time zone where necessary.

This month we are in for a planetary bonanza with five naked-eye planets all visible at once; Mars at its best since 2003; Venus dominating the western horizon. Plus there’s a partial solar eclipse for Tasmania and a total lunar eclipse, for everyone, which turns the Moon red! It’s a very busy month so I’ll describe these chronologically I think and end on Mars.

But first…a historical anniversary. July 4 marks the 150th anniversary of the birth of Henrietta Leavitt. She was an astronomer working in the early 1900s at the Harvard College Observatory, in the United States who discovered that some stars that vary in brightness have a very distinct relationship between their average brightness and the period over which they change their brightness. This is now known as the “Leavitt Law” in her honour. The relationship was later used to establish that other galaxies were “island universes” in their own right, not just spiral clouds in our own Galaxy. Overnight, our concept of the size of the universe was transformed!

Let’s begin with the Moon phases. The month begins with a Last Quarter Moon on Friday 6th at 5:51pm. New Moon falls on Friday the 13th at 12:48pm. First Quarter is on Friday 20th at 5:52am. And finally Full Moon occurs on Saturday 27th July at 6:20am.

The Moon is the brightest object in the night sky when it is up, no matter what phase it is in. It’s 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 the Milky Way and the constellations it is best to avoid moon-lit hours. How can you do this? If the Moon is between New and Full (i.e. 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 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.

July 2018 starts out with Earth at its furthest point from the Sun, or aphelion. This happens on Saturday July 7 at 2:47am. At that time we will be just over 152 million kilometres from the Sun.

On July 13 there is a partial eclipse of the Sun. Only the very southern parts of Australia get to see any part of it. From south-eastern South Australia and the coastal areas of Victoria from the South Australian border through Melbourne to Lakes Entrance barely a sliver of the Sun is covered by the Moon. Tasmania gets the best view although even then it is the tiniest of ‘bites’ out of the Sun. From Hobart the eclipse begins at 12:52pm and ends at 1:24pm. *Please remember this is a solar eclipse so eye protection must be used when trying to view it.*

Venus is the evening planet at present shining brightly (brighter at night than anything except the Moon) in the western sky after sunset. On the 10th Venus is just 1-degree (a finger’s width or two Moon widths) to the right of Regulus, the brightest star in Leo. This may help you identify Regulus which I noted earlier. On July 16 there is a much nicer pairing – Venus will be just to the right and above the thin crescent Moon.

Venus is joined in the evening sky by Mercury (low in the west), and the planets Mars, Jupiter and Saturn. In fact, from July 14 through to July 23 all five naked-eye planets plus the Moon will be visible in the evening sky at once.

In the morning sky Mars and Saturn, having been up all night, will be setting in the west.

And now we come to Mars. The highlight of the highlights this month and beyond.

Mars is at its closest and brightest to Earth and, through a telescope, looks larger than it has at any time since 2003 and it won’t look this good again until 2035 – 17 years from now. If you have a telescope, or book a Night Tour at Sydney Observatory to view with our telescopes, you might see the polar icecaps, surface features and maybe even a dust storm. Don’t miss it!

Mars is further from the Sun than Earth and so takes longer to orbit the Sun. Every roughly two years and two months Earth catches up and overtakes Mars like an Olympic runner on the inside track. At this point the planets are relatively close, Mars appears bright in the night sky and we see Mars in detail when we use a telescope. When the Sun, Earth and Mars are lined up we call this an “opposition” – Mars is opposite the Sun in the sky. This year the opposition of Mars occurs on July 27 at 3:13pm. But Mars will look good through a telescope throughout July and August and astronomers loosely refer to Mars being at opposition through this whole period of time.

But why is Mars so good this year? Mars has a particularly elliptical (or oval) orbit – well, if you drew the orbit on a piece of paper you wouldn’t see the oval shape, but it is more oval than all the other major planets, except Mercury. Earth’s orbit is also elliptical and in July Earth reaches its furthest point from the Sun. This all means that at every seventh or eighth opposition, or every 15 to 17 years, Mars and Earth are particularly close and we have a “favourable” opposition. Back in 2003, at the last favourable opposition, Mars appeared at its largest and closest for 60,000-years. This year it will appear 97% as big as it was back then – so its still well worth a look.

And it gets even better, at present Mars is high overhead from Australia – we are in the ringside seat. From Europe and North America Mars is low to the horizon and difficult to see.

The ellipticity of Mars orbit means it reaches its closest point to Earth a few days after its opposition on July 27. So while Mars is opposite, or in line with, the Sun on July 27, we wait till July 31 for it reach its closest point when it will be 57.6 million km from us.

What should you look for by eye? By eye we will see Mars in the constellations of Capricornus. Throughout most of July it will be brighter than Jupiter. But, despite its name, the Red Planet never looks traffic light red particularly at these opposition times – it’s more a pale orange or yellowish colour. By eye you will also see Mars in “retrograde” motion. What’s that? Mars, like the outer planets Jupiter & Saturn, slowly drifts past the stars (along the ecliptic) towards the east. Observing it weekly should be enough to spot this. But as the Earth passes between the Sun and Mars the planet appears to reverse direction and move westwards. This backwards, or retrograde, movement is just a relative effect like perceiving a car on a highway to be moving backwards as you overtake it. Mars moves retrograde from June 29 through to August 28.

What can you see through a telescope? Only during favourable oppositions are the surface features of Mars readily visible. Through a telescope you have the opportunity to see the polar ice caps, morning frosts, clouds, dark markings (only some of which correspond to physical surface features) and maybe even a dust storm.

Right now it’s spring in the southern hemisphere of Mars. You should spot the remains of the south polar ice cap, which has been shrinking over the last month or two. Pale frosts might appear near the morning edge of the planet. Sometimes clouds are seen streaming downwind from the tops of the volcanoes of the Tharsis region. Or even from Olympus Mons itself – a 25km high volcano three times higher than Mt Everest whose peak is as good as in space compared to the plains surrounding it. One of the most obvious surface features is Syrtis Major, a dark triangular projection that crosses the equator. It is in fact an enormous shield volcano – a shallow broad volcano. Dust storms often blow up in the Martian spring. They spread across the plains covering and disguising familiar features. Some become global and veil Mars in a planet-wide yellow-orange coat obliterating completely all view of the surface!

Remember that Mars is half the diameter of Earth and is still 57 million kilometres away. Surface features, clouds and so on can be hard to detect at first. Observing it takes patience and persistence. Ideally wait till Mars is high overhead to get the best “seeing” or the best view through Earth’s seething atmosphere. And Mars rotates in almost the same time as Earth does, 24-hours and 38-minutes, so to see its various faces watch it for a few hours on one night or at the same time for successive nights.

Make the most of this favourable Mars opposition period – while we are spoilt for images from spacecraft and landers this is the best view human eyes will get for some time.

As if all that planetary activity wasn’t enough the solar system throws in one last event to keep you sleepless for July. Early on the morning of July 28, just hours after Mars reaches opposition, there is a total lunar eclipse. Look to the west from about 5:30am (Sydney and eastern states), 5:00am for Adelaide & Darwin and 3:30am for Perth. The Moon will be beside Mars and over the next hour and a half the Moon moves into Earths shadow and turns a dark red colour. The Red Planet accompanied by the reddened Moon will descend together towards the western horizon. From some locations (Brisbane, Sydney, Canberra) both set before the eclipse ends.

So, a total lunar eclipse and Mars at opposition within 24-hours of each other (and on the same day in some parts of the world). How rare is that? I made a careful check of some planetary opposition and lunar eclipse tables. And such a pairing has not happened since the year 0 and won’t occur in the next thousand years. As best as I can determine it is a once in three-thousand year event!

And one last thing before you collapse from a July astronomy overload…This will be the longest total lunar eclipse this century (from 2001 to the year 2100) at 102 minutes and 57 seconds because the Moon passes almost through the centre of Earth’s shadow and is also at its furthest from Earth (apogee) where it travels at its slowest.

What a month! Phew!

If you’ve enjoyed this Night Sky Guide and want to regularly check out what’s in the night sky, why not purchase a copy of Sydney Observatory’s annual guide, “The Australasian Sky Guide” by Dr Nick Lomb. It’s jam packed with monthly astronomical information, including rise and set times for the Sun, Moon, and planets, tide times and a detailed look at our solar system and upcoming astronomical events. It’s available from Sydney Observatory and the MAAS store, or you can purchase it online, for which additional costs apply.

For more astronomical information, check our website and blogs. Keep in touch via our Facebook and Twitter accounts.

And if you’re in Sydney visit the Observatory in the Rocks area. Join a tour to view the skies through our telescopes. Visit our Space Theatre or the Sydney Planetarium. Tour our exhibition for free and discover the history of Australian astronomy. But please check our website for details and be aware not all activities are free and some require bookings.

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.

5 responses to “July 2018 night sky guide and sky chart

Leave a Reply

Your email address will not be published. Required fields are marked *

Related