Philips large revolving planisphere, 20" diameter
Dated to the middle of the nineteenth century, this planisphere was made by the London publishers, George Philip & Sons. The device made of paper and card, is a map of the heavens. It allows the user to display the appearance of the sky at any time of night or day throughout the year. The positions of the stars are plotted on a flat surface using the mathematical technique of stereographic projection. A round mask with a circular aperture covers the celestial map showing only those stars visible at the time and season indicated. The planisphere is also provided with a rule and circular scale on the round opening to indicate the elevation and direction of the stars shown.