Planisphere is a projection of a sphere on a plane. The name is practically applied to a device for ascertaining which stars will be visible on any given night. A chart of the heavens is fitted so that it can rotate under a cardboard cover, through a hole in which a portion of the chart can be seen. The days of the year are indicated by marks on the edge of the chart, and by means of these the relative positions of it and its cover can be adjusted so that the stars which appear through the opening are those which will be visible on that night. Some stars - such as the Pole Star and the Constellation of the Great Bear - will always be visible, but those near the horizon will change from day to day.