Astronomical watch, c 1790.
© Science Museum / Science & Society Picture Library
George Margetts (1748-1808), a Freeman of the Clockmakers' Company of London, made a number of astronomical watches of this type. This watch shows mean time and tidal hours for ports around the country; it also indicates the position of the moon in the heavens, and the solar and lunar eclipses. The sun pointer rotates once in a 24 hour solar day, and the moon disc with its pointer once in a lunar day, which is 48 minutes longer.