Autumn Stars

The three stars Vega, Altair and Deneb form a sixty-degree isosceles triangle frequently referred to as the "Summer Triangle". But summer is not a good time to be observing the night sky from high latitudes where nautical twilight (Sun 12º or less below horizon) lasts all night from Orkney between mid-May and late July. Far better, therefore, to identify the "Summer Triangle" in autumn when it comes to the meridian in mid-October at 20h (8 pm) BST*). Vega will be almost directly overhead with Altair at a comfortable angle of 40º above the southern horizon. Deneb is even higher and to the east (left) of the brilliant Vega. Vega and Deneb therefore form the base of the triangle which appears upside down.

All three stars are of the first magnitude, with Vega at 0.0, the fifth brightest star in the entire sky. Altair is a little under half as bright at magnitude 0.8, and Deneb even fainter at magnitude 1.3. [Visual magnitudes will be explained in an addendum to these notes to appear later.]

Fig 2: the sky looking due south at 20 30 GMT on November 1. Click the image for an enlargement.

Those wanting a challenge in star finding should attempt to see Fomalhaut, the brightest star in the constellation Pisces Austrinus (Southern Fish).

This star - about the same brightness as Deneb - comes to the meridian at 20 30 (8 30 pm) GMT on November 1. A mere 1.5º above the south point on the horizon, one must have an uninterrupted view. (I have glimpsed it from the car park at Midhowe on Rousay for a short time.) A useful pointer is to use the two stars at the front of the great square of Pegasus. Pegasus may be identified with reference to the summer triangle already mentioned (see fig. 2).

Due to its position in a sparsely populated area of sky Fomalhaut is a useful navigational star. It is also one of the four Royal Stars of ancient Persia, the others being: Aldebaran, Antares and Regulus (the leader).

Jupiter

Jupiter, the brightest natural object in the night sky (next to the Moon), may be seen rising north of east at approximately 30 minutes past midnight (BST) on the 15 October, and at 22 45 GMT on the 31st.

Saturn

Saturn rises a little further north of east at 20 30 BST on 15 October and at 18 30 GMT on the 31st.

A feature on Saturn will appear next month.

*British Summer Time ends at midnight on October 27.

JV
2000, October 10