Shelton Mean Time Clock

Mean Time Astronomical Clock by Shelton.
Used by His Majesty King George III to obtain the exact time of the transit of Venus in 1769.

From "A Report on the Precision Clocks at Armagh Observatory" by Jonathan Betts, 1989.


8 day English regulator in mahogany case by John Shelton, London, c1769.

Maximum external dimensions (cms): Height:l97 Width:45 Depth:25

Typical substantial 'Shelton' style movement with six latched pillars, but now with a number of 19th century alterations.The movement now has a four wheel, 8 day train, a jewelled dead beat escapement and Harrison's maintaining power.The movement was originally month going with a five wheel train, the great/winding arbor positioned where the centre of the hour dial is now.The hours would have been indicated through a semicircular aperture, ('smiling’), on the centre of the main dial, with conventional motion work behind and would almost certainly have been signed:"John Shelton London" below this aperture and winding hole. The movement would have had bolt and shutter maintaining power; the slot for the lever remains on the right hand side of the dial.This would also have incorporated Shelton's 'drawback' device for lifting the bolt away from the centre wheel teeth.The movement would also have had stopwork in the form of a vertical pivoting bar between cocks on the left hand side of the movement, between the plates.

The pendulum is Shelton's original, but it is now suspended from a cast metal support screwed to the backboard instead of from the back cock.The brass covered driving weight is also a 19th century replacement.

Square silvered brass regulator dial with Arabic five minutes, seconds and on the 24 hour dial.The seconds dial has 'observatory marks' at five second intervals, probably a 19th century addition. Blued steel hands. The movement also has a wooden cover to protect it from dust within the hood, also probably 19th century addition.

The solid mahogany case is of the typical Shelton type, with a flat top hood, rectangular trunk door and base with a panel and a short double skirting below, with low pad feet. The hood has a sliding, jointed brass lamp/candle holder fixed to the right hand side, used to illuminate the dial during nightime observations. This, and a brass bar fixed across the trunk door opening about half way down are also 19th century additions.

Historical Note

According to the 1847 Observatory report, the Shelton came from the Kew collection and it was considered the best of them. However, a number of alterations were made by 'a London artist', under the direction of Sir James South, amongst which was the new jewelled escapement.The other alterations listed above were probably carried out at the same time. Subsequently, the jewelling in the escapement was replaced by Sharpe of Dublin, having worn prematurely. A note in the clock suggests this clock was No.322 in the Queen's catalogue; being the last entry: "Shelton's 8 day follower". The pendulum scale, (present in 1984, but missing in Dec.1988) is apparently scratch-marked: "January 1769 Shelton" which is very likely; Shelton often marked his clocks in this way. As at Kew, when in use at Armagh the clock was rated to solar time. According to another note on the clock, the gridiron pendulum was, at one stage replaced with an invar one, probably that pendulum now in store with an invar rod and a simple cylindrical bob.

Last Revised: 2014 November 10th