Classical Astronomy & Astrology

This simulator animates Ptolemy's geocentric astronomy as described in the Almagest (c. 150 AD). Each planet rides an epicycle — a small circle whose centre moves along a larger deferent circle centred near Earth. The overlapping rotations reproduce retrograde motion, the varying speeds of the planets, and the seasons, all without invoking a moving Earth.

Ptolemy's model rests on the Two-Sphere Model: a stationary Earth at the centre surrounded by a rotating celestial sphere carrying the fixed stars. The coordinate systems of both spheres are direct mirrors of each other — declination equals latitude, right ascension equals longitude (convertible via 15° per hour). Every point in the sky has a precisely corresponding Geographic Position (GP) on Earth's surface at every instant in time.

This one-to-one correspondence between the celestial and terrestrial spheres is the foundation of celestial navigation. A navigator measures a star's altitude, looks up its GP in the Nautical Almanac, and computes their distance from that GP. Two or three such observations intersect to fix position at sea. The Nautical Almanac, the graticule of lat/lon, and the intercept method of navigation all descend directly from Ptolemy's two-sphere framework — open the Info tab to learn more.