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 pasage of a given star acros the meridian in the southern sky and the next pasage 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 an unusual form of gravity escapement and Harrison's 'gridiron' pendulum.
© Science Museum / Science & Society Picture Library