Clocks, Regulator Vines' clock showing sidereal and solar times (l'horloge de Vines donnant le temps...

Clocks, Regulator Vines' clock showing sidereal and solar times (l'horloge de Vines donnant le temps sideral et le temps solaire), 1836. Detail of hood and dials. 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 earths 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 an unusual form of gravity escapement and Harrisons gridiron pendulum. ©SSPL/Science Museum