ROYAL ASTRONOMICAL SOCIETY / SCIENCE PHOTO LIBRARY ROYAL ASTRONOMICAL SOCIETY / SCIENCE PHOTO LIBRARY
Ferguson's epicyclic diagram. This diagram was published in 1756 by the Scottish astronomer and instrument maker James Ferguson (1710-1776), in his book Astronomy explained upon Sir Isaac Newton's Principles. Figure I (left) shows the Earth at centre, with the Sun tracing a circular orbit. The orbits of Mercury and Venus in this geocentric system are shown by a series of complex epicycles. Figure IIII (upper right) shows the phases of the Moon. Figure III (lower right) shows the Sun and Moon and the celestial equator and ecliptic, and Earth's axial inclination. Figure II (to the left of figure III) shows how the Earth's axial tilt causes the seasons.
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