The summer solstice is past and while most people are enjoying the long summer evenings amateur astronomers can look forward to the evening skies becoming darker a little earlier! I hope some of you managed to see the morning planetary alignment at the end of June.
The following charts represent the night sky at 10.00pm BST on the 8th of July and at 9.00pm BST on the 23rd July. To use the chart, face south at the appropriate time with the bottom of the chart towards the southern horizon and you will see the stars in the chart.
Our focus this month is The Summer Triangle. It is not a constellation but an asterism formed by three bright stars from three separate constellations. It is easy to find the brightest star by facing south and looking up to just below your zenith and there you find a very bright star. It might be easier getting a deckchair and lying down. This star is Vega, the 3rd brightest star visible from the northern hemisphere and you cannot miss it because of its brilliance. To its bottom left hand side there are four much fainter stars in the form of a parallelogram and together these stars make up the constellation Lyra- The Lyre or Harp. Vega is a white star of magnitude 0 at the relatively close distance of 25 light years. It is 50 times more luminous than our Sun. Vega forms a triangle with the top right hand star of the parallelogram and another star to its top left. This other star is called epsilon Lyra and those with good eyesight may be able to detect that it is a double star. But observation through a small telescope reveals that each of these is a double and they have become known as the celebrated Double Double.
Vega lies on the edge of the Milky Way represented by the lighter region in the chart to the left of Lyra. Here to the east you find a giant cross in the sky and this is the constellation Cygnus- The Swan. The bright star Deneb represents the tail of the swan which is flying down the Milky Way. Deneb is a supergiant but at a distance of 1,500 light years its magnitude is 1.3 and it is outshone by Vega. The star Albireo, representing the beak of the swan is another double star beautiful to see through a telescope.
Now from a line joining Deneb and Vega, look down about halfway to the horizon and you find another bright star Altair in the constellation Aquila- The Eagle. Altair is the 8th brightest star visible from the northern hemisphere shining at magnitude 0.8 and at just 17 light years away it is one of the closest bright stars to Earth.
The three stars Vega, Deneb and Altair form what is called the Summer Triangle, outlined in red on the chart. It is a great sight to see in the summer sky and a help in locating other objects.
Contained within the Summer Triangle near the bottom vertex is the constellation Sagitta- The Arrow. It is the third smallest constellation and with its brightest star at magnitude 3.5 it is a bit o a challenge for the unaided observer.
To the left of Sagitta and outside the Summer Triangle is another small but distinctive constellation Delphinus- The Dolphin. With stars similar in brightness to Sagitta you will need clear, dark sky conditions for observing.
From Altair, drop down the Milky Way and close to the horizon is another zodiacal constellation Sagittarius- The Archer. (See the second chart.)
It is difficult to see anything resembling an archer but what is much clearer is the asterism- The Teapot, outlined in red on the chart. Lying in the Milky Way as it does, Sagittarius is an area of the sky rich in star clusters and bright nebulae which hide the centre of our galaxy. As recently as May this year a supermassive black hole at the centre of our galaxy, named Sagittarius A*, was imaged and made headline news. No need to be alarmed, it is some 26,000 light years away.
Something to look out for
On Monday 4th July the Earth is at aphelion, its furthest distance from the Sun, and it is a reminder that our seasons are not governed by our distance from the Sun but by the tilt of the Earth’s axis to the plane of its orbit. During the summer months in the northern hemisphere the north pole is tilted towards the Sun meaning that the Sun is higher in the sky and we have more hours of sunshine compared to the winter months.
On a similar theme on Wednesday 13th July the Moon is at perigee, its closest to the Earth and the full Moon will be its biggest and brightest. It has become the norm to give a full Moon a name and this month it is the Buck Moon. Apparently this comes from the fact that the antlers of male deer (bucks) are growing fastest at this time.
On Friday 15th July the Moon and Saturn rise together just before midnight. Saturn will become better to view as time passes and it reaches opposition on the 2nd August. Similarly Jupiter reaches opposition on the 20th August so we will soon be able to enjoy both of them in the evening sky. Mars travels faster than both of these so it takes longer for the Earth to catch up and Mars will be at opposition on the 14th October.
Finally keep a look out for the first images from the James Webb Space Telescope due shortly. Exciting times!