Vines� clock showing sidereal and solar times, 1836.

UNITED KINGDOM - AUGUST 11: Designed by Joseph Vines and made by Walsh of Newbury, Berkshire, this clock indicates solar time on one set of dials and sidereal time on the other. A sidereal day is the interval of time between one passage of a given star across the meridian in the southern sky and the next passage of the same star. It is nearly four minutes shorter than a solar day because the sun itself is moving slowly through the stars in a direction opposite to that of the apparent rotation of the stellar system round the earth�s axis. The clock also has a moon hand which shows the phases of the moon and its position relative to the sun. The timekeeping is controlled by a gravity escapement and gridiron pendulum. Shown in the Time Measurement Gallery at the Science Museum, London. (Photo by SSPL/Getty Images)