Shelton regulator clock, 1768-1769.
© Science Museum / Science & Society Picture Library
This clock is one of five made for the Royal Society in the 1760s by John Shelton (1712-1777) of Shoe Lane, London. It measures sidereal time (time measured from the apparent movement of the stars) instead of solar time (time measured from the apparent movement of the sun). It was originally used for timing the transit of the planet Venus acros the Sun, a rare astonomical event which occured in 1769 and was used to calculate the distance of the of the Sun from the Earth. Captain Cook (1728-1779) took this clock on his third voyage to the South Seas in 1776, and in 1828 it was used to compare the strength of gravity at the top and bottom of a mine in an attempt to find the density of the Earth.