Astronomical clock. 19th-century diagram of the dial-plate of an astronomical clock, showing the daily motions of the Sun, Moon and stars. The outer ring gives the hours of the day in Roman numerals. Next is the approximately 29.5 days of the lunar cycle. Two pointers are labeled with the Sun and Moon, and rings show the twelve months and signs of the zodiac. At center around the Earth are the ecliptic, horizon, equinoctial, tropic of Cancer, and tropic of Capricorn. Diagram from "The Works of James Ferguson, F.R.S." (1823) containing works by Scottish astronomer and instrument maker James Ferguson (1710-1776).