Introduction

On a clear, dark night several thousand stars can be seen at any one time. They form familiar patterns such as the Great Bear and Cygnus in the northern hemisphere and Scorpio and Crux in the south. The distances are so great that we see the constellation patterns essentially unchanged from those seen by the ancient Egyptians, for instance. This is partly due to the fact that some of the bright stars in constellations are in what are called moving groups - a loose association of stars moving through space together. More tightly bound are clusters of star such as the Pleiades or Seven Sisters which appears in the northern sky in the late summer. Eventually the moving groups and clusters of stars will gradually disperse because the distance between the stars is such that the gravitational attraction between the members is relatively weak.

Those with keen eyes will be able to see some close pairs of stars without optical aid. The most famous is Mizar and Alcor in the tail of the Great Bear. The first recorded "naked-eye" pair is v Sgr which was mentioned by Claudius Ptolemy in his famous Almagest catalogue of c. ad 140. It is described1 as "The star in the middle of the eye (of Sagittarius) which is nebulous and double". The angular separation of this pair is 13', or about the same separation as Mizar and Alcor. As a comparison, the apparent diameter of the Full Moon is 30'.

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