The sundial chosen to commemorate the Tercentenary of the founding of the Observatory at Greenwich (in 1675) is called a reclining equiangular sundial. The angle of reclination was deliberately chosen so that the gnomon would be vertical. In selecting a dial for this historic occasion it was decided that the dial should be capable of giving the time in GMT correct to a minute, that it should be easy to read by members of the public and that if possible it should be of a novel design. The first two of these criteria have certainly been achieved and possibly the third also since, as far as is known, there is no other reclining equiangular dial in existence at the time of writing.

The dial was erected at Herstmonceux Castle, the old home of the RGO, and was inaugurated by Princess Anne as part of the tercentenary celebrations. It is now re-erected to the east of the RGO's new building in Cambridge. The time marks are at 5 minute intervals on a circle of radius 161 cm. Thus on this circle the shadow of the gnomon moves 7 mm a minute. The time is read by the position of the centre of the shadow of the gnomon on this circle. The sundial is adjusted in two ways. Firstly the dial itself can be rotated to allow for the equation of time. The maximum daily change is only half a minute. It should be noted that the longitude correction is also allowed for by a similar one--off adjustment. Secondly the gnomon carriage can be moved along the N–S diameter of the ring to compensate for the changing declination of the Sun. The maximum daily change is 9 mm.

The sundial may not be accurate to its design of one minute when you see it. This is because the RGO has not got the manpower necessary to perform such tasks as often as they are needed.

The Greenwich water tanks

The two large lead tanks on the eastern side of the clubhouse are relics of the Royal Observatory at Greenwich. Dated 1790, they are water tanks for domestic supply provided by the Board of Ordnance, at that time responsible for the observatory. The Board's arms (three cannon) are displayed as an indication of ownership.

They were moved with the RGO from Herstmonceux Castle where they were installed in the central courtyard. They have been used for floral arrangements for at least the last 30 years.