Pensford Field Night Sky, May to August 2010
At the summer solstice on 21 June, the longest day, there are over 16 hours of daylight and less than 8 hours from sunset to sunrise. But the period of darkness is much less because twilight lasts a long time in the summer (the "twi" is an old form of "two", the two lights being daylight and starlight). On the equator twilight is short; at the north pole it stays light all night; and we are somewhere in between.
On 7 May there is a good opportunity to see the International Space Station (ISS) which is as bright as Venus. There is often a crew of half a dozen on board. The ISS will rise in west at 9.50 pm BST, move rapidly east up to the zenith and continue east; then it gets into the earth's shadow and disappears before reaching the horizon. The crossing lasts about 6 minutes. On 8 May it crosses again, rising at 10.15 pm and moving on the same path.
The 7 stars of the Plough straddle the zenith at the start of this period. The American name of Big Dipper refers to the big ladle used for taking the stew from the pot and suits the shape very well. The pointers, the two stars furthest from the handle, point to the Pole Star about which the heavens or 'celestial sphere' appear to rotate anticlockwise. If you take a time exposure of the stars, each one traces out a circle around the Pole Star. You can use the handle of the Plough as a pointer to the bright star Arcturus, lower down in the east; it is the brightest star in the northern hemisphere and will be visible throughout this period. It is the lowest star in the constellation of Bootes, often described as kite-shaped; the other stars of the kite are much less bright. Extend the line of sight beyond Arcturus towards the horizon about the same distance again to find the star Spica in the constellation Virgo. Spica is bright and there are four less bright stars nearby, higher in the sky. The Plough also serves as a pointer to Regulus in Leo; but you have to use not the usual pointers but the other two stars of the body, and follow them a long way in the other direction.
In the early months of the year there was a huge triangle of bright stars to see (Sirius, Procyon, Betelgeuse). One by one these three are now leaving our view; they will soon be replaced from the east by a big new triangle. The first of the new three to look for is Vega, high in the eastern sky around 10 pm. on 1 June; and near the zenith at midnight on 1 July. The second star of the triangle, Altair, lies between Vega and the horizon. Look for the third star, Deneb, to the left of Vega and higher in the sky than Altair. The corner of the triangle at Vega is a bit less than a right angle. Deneb is the body of the flying swan constellation Cygnus and a very remarkable body it is. Although looking less bright than Vega it is a supergiant, at least 100,000 times brighter than the sun; and its light has taken at least 2,000 years to get here. We see it now as Augustus and Jesus Christ saw it (and they saw it as it appeared to the last builders at Stonehenge).
The planet Venus continues to be a bright object in the western sky at sunset during May but will gradually be getting too low to see. Mars is to the right of Regulus and will move slowly to its left. By August it will be too low to see. Saturn is found between Spica and Regulus but it too will be getting too low by the end of this period. Jupiter rises in the east early enough to be seen before midnight in July and August.
There are new moons on 14 May, 12 June, 11 July and 10 August; look for them in the west an hour or two after sunset, two or more days later when the thin crescent has become thick enough to be seen. Full moons are on 27 May, 26 June, 26 July and 24 August. See them rising in the east shortly before or after sunset on, or the day before or after, these dates.
Roy Singham, 29 April 2010