 The rear planisphere, similar in size to the one in the front but strictly circular, displays the brightest stars of the main circumpolar constellations (Ursa Major, Ursa Minor, and Cassiopeia).It is centred on the north celestial pole and, with its encircling silvered ring of 24-hour intervals, rotates once a sidereal day relative to a fixed meridian-index.Damage to the clock's mechanism was sustained during the fire of 9 July 1984; after 10 years' reparation work, vergers ceased winding it owing to inaccuracies of time-keeping.