1. A planisphere comprising a plurality of concentric dials of progressively smaller diameter, a sky map on the smallest diameter dial having an Aries indicator portion extending radially therefrom, a Greenwich indicator dial having a Greenwich indicator portion extending radially therefrom positioned concentric with and adjacent the smallest diameter dial, a reference rule extending radially outwardly of the planisphere over the Greenwich indicator dial and plurality of dials, a sky mask dial positioned over and concentric with the Greenwich indicator dial having a transit scale indicator portion extending therefrom and means securing all the dials and reference rule together centrally of the dials for rotation relative to each other.