Balmy summer evenings are a great time to go outside and enjoy some stargazing.
Find a dark location and let your eyes adapt to the darkness, which will take about 20 minutes. Ideally, when looking at the stars, it is best when there is no moonlight. As your eyes adapt and the skies darken, more and more stars will become visible.
Asterisms are groups of stars in the night sky which are often more prominent than the traditional stick figure constellations. They are the first to become visible as dusk progresses.
To orientate yourself, look for The Plough, which is shaped like an old-fashioned plough or saucepan. This is part of the constellation Ursa Major, or the Great Bear.
If you join up the two stars at the end of the bucket – Merak and Dubhe – and project the line, you will see the Pole Star or Polaris. This is north. Then look east, beyond Polaris and find the distinctive, wonky “W”-shaped constellation of Cassiopeia.
Slightly to the south of Cassiopeia you will see a child-like “house-shaped” pattern of stars. This is the constellation Cepheus.
These constellations are circumpolar – no-matter what time of year, they are always visible in the night sky, rotating around the Pole Star every 24 hours.
The summer months are best known for the iconic asterism, the Summer Triangle which comprises three of the brightest stars in the summer night sky and spans over three constellations. Look almost directly overhead and you will see the bright star Vega, then look for Deneb and Altair.
The first constellation, Cygnus, represents a swan and is easily recognisable. Its brightest star is Deneb, which is a white supergiant at a distance of 3,550 light years. “Deneb” means tail in Arabic, in this case referring to the tail of the swan.
The constellation of Lyra is one of the 48 constellations listed by the 2nd century astronomer Ptolemy and was often shown on star maps as a vulture or an eagle carrying a lyre. The brightest star, Vega, is unmistakable as it is the 5th brightest star in the entire night sky. It is a bluish-white dwarf star that is about 25 light years away and its name is derived from the Arabic for “swooping eagle”.
The constellation of Aquila sits on the celestial equator. It is Latin for “eagle” and it represents the bird that carried the thunderbolts aloft from Zeus, the almighty Greek god of the sky and thunder. Altair, the brightest star in Aquila, is a white dwarf some 16.6 light years away. Its name comes from the Arabic phrase meaning “the flying eagle”.
Once you have found the Summer Triangle, then look for the Summer Cross, which comprises: Deneb, Sadr, Albireo, Gienah Cygni and Al Fawaris. Albireo is quite faint, but it shows up easily with a pair of binoculars, and if you look closely, it is actually two stars, one a rich gold colour and the other a vibrant blue.
Now look for other asterisms. Start in the west with the Kite. This is easy to find. Look for the very bright orange star, Arcturus, which sits at the tip of the Kite. To find Arcturus, follow the line of the handle of The Plough away from the pattern of stars. Arcturus is a red-giant star, located about 37 light years away and is about 25 times the size of our Sun.
Next, look for the Keystone, which sits between the Summer Triangle and the Kite. It forms the centre of the constellation of Hercules. If you have a pair of binoculars, look for the Great Globular Cluster in Hercules. You will find it on the line between Eta Her and Zeta Her. These are tightly packed clusters of tens of thousands to millions of stars, all bound together by their mutual gravity. Finally, look towards the east and find the Great Square of Pegasus.
As it gets fully dark, you should see the most spectacular sight of all in the summer night sky, a milky-looking band of stars arching overhead. This is the Milky Way, which runs north to south and directly through the Summer Triangle. When looking at this band of stars, you are looking at the disk of our galaxy, edge on from our perspective, as our Solar system sits on the minor, Orion-Cygnus spiral arm of the Milky Way Galaxy.
Ancient cultures, without the modern light pollution of today, would have had a very deep connection to the night sky. Many ancient civilisations believed that the Milky Way was a bridge between Heaven and Earth. Spiral arms are home to dust and gas and the main birthplace of stars and planetary systems. If you look to the south, you will see a long sinuous dust lane – the Great Rift – snaking through the band of stars. With a pair of binoculars, start in the south and work your way along the Milky Way and you will see dense star clouds in Sagittarius and Cygnus.
Another spectacle not to be missed is the Perseid meteor shower, which is active from now until 24 August and peaks on the evening of the 12 August and into the early morning of the 13 August.
There will be a bright Moon overhead, but the brighter meteors will still be visible. You can also go out a few days before and after the peak.
The meteors are from the dusty remnants of the debris trail left by comet Swift-Tuttle, which last passed through the inner Solar System in 1992 and will make its next appearance in 2126.
You will find the radiant (where they appear to radiate from) towards the head of Perseus, so look in and around this general direction to see the meteors. The tiny particles, which are no larger than a grain of sand, hit the Earth’s atmosphere at speeds of up to 132,000 mph. At these speeds there is compression of the air in front of the meteoroid which heats up, causing the bright streak. The meteoroid itself also burns up and, depending on its composition, it can display different colours. The Perseids are often a vibrant green because of their magnesium content.
As we progress through the summer, the second-largest planet, the gas giant Saturn, will start to rise in the east. With a pair of binoculars, you will be able to see a round disk, but you might notice something appears missing!
Saturn is best known for its spectacular structure of icy rings encircling the planet, but they appear to have almost disappeared. This event is known as a “ring plane crossing”. It occurs approximately every 15 years and occurred this year on 23 March. Saturn takes 29.4 years to orbit the Sun and its orbit is slightly inclined with respect to Earth’s orbit around the Sun. Therefore, from our perspective on Earth, we sometimes see Saturn’s rings from below and at other times from above, when crossing from one viewpoint to the other, the rings are seen edge on and as they are, on average, only 10 metres in thickness, they appear to vanish.
Galileo Galilei first discovered Saturn’s rings in 1610. However, with his relatively modest telescope he was not able to understand what they were. He wrote to the Duke of Tuscany that “The planet Saturn is not alone, but is composed of three [planets], which almost touch one another”. He also described the rings as Saturn’s “ears”. In 1612 there was a ring plane crossing. Mystified, Galileo remarked “I do not know what to say in a case so surprising”. He mused, “Has Saturn swallowed his children?”. Galileo’s confusion was further compounded when the rings subsequently reappeared in 1613!
One final challenge for the summer is to find the Andromeda Galaxy, which is 2.5 million light-years away. From a dark location it can just be seen with the naked eye. Find the Great Square of Pegasus, find the three stars, Alpheratz, Mirach and Almach. They appear almost in line. Go to Mirach and then look vertically above the line to find Mu And, project the short line by the same amount again and you will find the Andromeda Galaxy. It will look like a faint, fuzzy patch. What you are seeing is the glow from about one trillion stars. Considering almost all stars have planetary systems, some of which most likely have intelligent life, do you think someone on a world in a distant galaxy might be looking towards our galaxy? And if so, what name might they give to our galaxy?
To find your way around the night sky there are lots of apps available for phones and tablets. To protect your night vision, they can be set to night mode. Look for Stellarium, Celestron Sky Portal and Sky Safari.
Dr Jean Dean, Astronomer