Starwatch: naked-eye Pleiades count is rite of passage for astronomers

On a good night, up to 10 stars can be seen in cluster known as the seven sisters

This week's challenge is a longstanding rite of passage for amateur astronomers. It is also a quick and easy way to gauge the quality of a night's observing conditions whenever the constellation of Taurus, the bull, is visible.

It involves counting the number of stars in the Pleiades. One of only a handful of star clusters visible to the naked eye, the Pleiades is particularly special because we can resolve some of the stars that makes it up. Another name for the cluster is the seven sisters - but that doesn't necessarily mean that you can see seven stars.