A planisphere is a disc calculator that displays the position of the stars on any day of the year, at any time, for a given latitude. Disc calculators, also called thumb wheel index charts, are analog computational devices widely used in the past in a multitude of fields requiring repetitive charted calculations. Disc calculators manage knowledge by matching data from two discs: the bottom disc is stationary, and a top disc rotates around the center of the object. The Miller planisphere has a 10.5 in. (26.5 cm.) diameter. Its bottom disc has a representation of the celestial sphere that depicts stars, planets, nebulae, galaxies, star clusters, constellations, etc. Its perimeter displays an index of the 365 days of the year, grouped by month. The top disc has an elliptical window that defines the field of vision of the celestial sphere from the surface of the Earth, and another index, also in its perimeter, divided into 24 parts -the 24 hours of the day. By matching a specific hour of the day with the desired day of the year, the planisphere'’s elliptical window frames the portion of the celestial sphere in sight that day, and the position of the heavenly bodies one can see in the sky. The Miller Planisphere puts the stars within everyone'’s reach, reads the tag line in the back of the bottom disc. Such sophisticated knowledge for just about 12$ (9.25 Euros) -–the retail price of this object- is certainly a bargain.
