It is a copy of a 1676 clock, one of the most accurate of the period, that was used to take timings of observations of the stars.
These were used by the Reverend John Flamsteed, the first Astronomer Royal, to determine accurate positions for stars in his catalogue, which was later used by navigators to establish their position at sea.
The reproduction clock, nine months in the making, will hang in the same place as the original in the Octagon Room at the Royal Observatory, Greenwich. The positioning has been made possible as many of the original holes for the mount fittings are still visible.
Sir Patrick Moore was at West Dean College last week to launch the start of the official testing period, at the end of which the clock will be installed on public display at Greenwich.