planisphere

A planisphere, also known as a star wheel, is an analog computer used to determine the positions of stars in the night sky at any given date or time. It consists of two independently rotating discs, one containing a star chart calibrated for use at a particular latitude, the other a moving window that enables the user to display only the portion of the chart that corresponds to the visible sky at the time and date desired. This is used by (mostly amateur) astronomers to orient themselves and get a sense of what to expect and where to look for it during observations.