Flamsteed's own meridian line is a brass strip in the floor of the Meridian Building. Edmond Halley, Flamsteed's successor, who charted the comings and goings of the famous comet, worked out his own meridian, and the Bradley Meridian Room reveals yet another, standard from 1750 to 1850 and still used for Ordnance Surey maps. Finally, you reach a room that's spliced in two by the present-day Greenwich Meridian, fixed by the cross-hairs in Airy's "Transit Circle", the astrological instrument that dominates the room.

The exhibition ends on a soothing note in the Telescope Dome of the octagonal Great Equatorial Building, home to Britain's largest telescope. In addition, there are half-hourly presentations in the Planetarium (Mon-Fri 2.30pm; £2), housed in the adjoining South Building. Daily 10am-5pm; £6; www.rog.nmm.ac.uk.

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