Autumnal equinox

Ken Kennedy, of the Dundee Astronomical Society, tells us what we can expect to see in the September sky.

“The summer constellations are now swinging westwards,” he says, “making way for those of autumn but before leaving Cygnus and Lyra, look downwards to the third and most southerly star of the Summer Triangle, Altair. Altair is the brightest star of Aquila, the Eagle, which mythology tells us carried the thunderbolts of Zeus.

“The constellation Aquila was mentioned as far back as the 4th Century BC by Eudoxus. Altair is the closest of the Summer Triangle stars being only 17 light years distant.

“The ecliptic, the path taken by the sun in the sky, is quite low in the evenings at this time of year and the constellations of the zodiac are those bordering the ecliptic. During September the zodiacal constellations are, from east to west, Pisces, Aquarius and Capricornus.

“Although covering a large area of sky, none of these constellations contain any bright stars and do not make a great impression on the observer. With urban street light pollution it is difficult to see anything of the patterns of stars made by these constellations. However, they sit low in the sky below the rising autumn constellations of Pegasus and Andromeda, both of which are much more recognisable and interesting.

“As early as mid-August I was aware of the great square of stars forming the body of the inverted horse, Pegasus, rising above neighbouring houses with its neck and head stretching out towards the meridian by the early hours. By September the globular cluster, Messier 15, which is placed just west of the nose of Pegasus, will be in an ideal position, almost due south at 10pm, to view with binoculars or small telescope.

“September is the month of the autumnal equinox when the sun passes over the equator and heads southwards. A wet summer will soon be giving way to darker, colder days and clearer nights.

“The moment of equinox this year is 9.03 pm (summer time) on September. 22. Because of the relative angles of the axes of the earth and sun, both spring and autumn equinoxes often produce more aurorae than at other times of the year. However, the sun has been becoming increasingly quiet and I suspect that we will not be seeing many auroral displays this September.

“None of the planets is particularly well placed for observation during September but you may just be able to pick out Mercury, low in the east rising about one and a half hours before the sun around the 12th of the month.

“Venus is quite low in the eastern morning sky rising two hours before the sun but is bright and easy to spot. To find Mercury around the 12th look downwards and slightly left of Venus.

“Mars also rises about an hour and a half before the sun and will be low in the east, close to Mercury on the 6th. Jupiter sets less than an hour after the sun so will be low in the south-west and difficult to see. Saturn is very low in the southwest by 10pm throughout the month.

“The moon is full on the 6th, at last quarter on the 13th, new on the 20th and at first quarter on the 28th.”