Sidereal verge watch, c 1776.
© Science Museum / Science & Society Picture Library
Detail of dials. This verge escapement watch showing solar and sidereal time and with epicyclic maintaining power was made by James Green of London. The watch is in a silver pair case hallmarked 1776. It ndicates 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.