planisphere

In astronomy, graphical device for determining the aspect of the sky for any date and time in the year. It consists of two discs mounted concentrically so that the upper disc, which has an aperture corresponding to the horizon of the observer, can rotate over the lower disc, which is printed with a map of the sky centred on the north or south celestial pole. In use, the observer aligns the time of day marked around the edge of the upper disc with the date marked around the edge of the lower disc. The aperture then shows which stars are above the horizon.