Starwatch: Alderamin on course for pole position – in the year 7500


This week, track down the northern constellation of Cepheus, the king of Ethiopia in Greek mythology. Although somewhat faint, the constellation is large and relatively easy to locate because of its proximity to the W-shaped constellation of Cassiopeia, who was Cepheus’s wife.

The chart shows the view all week looking roughly south from London and up towards the zenith. The brightest star in Cepheus is Alderamin, located 49 light years from Earth. Because of the precession of the Earth’s rotation pole, in the year 7500 Alderamin will function as the north star.

The most astronomically significant star in the constellation is Delta Cephei, at a distance of 887 light years. In October 1784, the young astronomer John Goodricke discovered that Delta Cephei changed its brightness in a regular, repeating way. So-called variable stars were a sensation at the time and Goodricke followed its variations almost every night until the end of that year.

Today the star is the prototype of a class of variable star called Cepheid variables. Because their period of pulsation is linked to their absolute brightness, they can be used to gauge distances across the universe. Cepheus is so far north that it is impossible to see from most of the southern hemisphere.