Astronomical Clock by Thomas Earnshaw

Earnshaw's second clock has a 5-bar compensation pendulum of steel and zinc, an 8-day movement, rated at very nearly sidereal time. Its pallets and some of the pivot holes are jewelled.

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


8 day English regulator in mahogany case, signed by Thomas Earnshaw, London, C179O.

Maximum external dimensions (Cms): Height:l84 Width:45 Depth:3O.5

Rather conventional, 4 pillar movement reminiscent of the work of a company such as Thwaites, with a four wheel train with pinions of eight, with pallet and escape arbors only jewelled. Jewelled dead beat escapement and Harrison’s maintaining power.The train pivots do not appear to be particularly fine.There is a beat adjuster on the crutch and a 'shake' adjuster on the pendulum rod, both probably later alterations. Original five bar Zinc and Steel gridiron pendulum, suspended on a gold painted,cast iron pendulum support, firmly screwed to the backboard of the case.

The movement has had an electrical contacting mechanism fitted at some stage to enable the clock to deliver a signal every second, probably in conjunction with a drum chronograph. Subsequently, this has mostly been removed, leaving the attachment for the seconds contact on the pallet arbor, and the adjustable sliding mounting for the contacts on the left hand side of the movement.

10¼" square, silvered brass regulator dial with Arabic five minute and five second figures, and Arabic numerals on the reversed 24 hour dial. The dial is signed: "Earnshaw London" across the centre. Blued steel hands. The seconds dial has 'observatory marks' at five second intervals, probably added in the 19th century.

Solid, veneered mahogany case, not hermetically sealed, with a solid backboard having an extra board behind the movement for mounting the cast iron pendulum support. The flat top hood has an extension 'box' added on top, on the left hand side, to accommodate the electrical contacting mechanism. The large glazed sides to the hood are also possibly a later addition. The trunk has a plain rectangular mahogany door, and bombe sides to accommodate the swing of the pendulum. The plain base has a square panel and a double skirting below, the lower with ogee shaping to form feet at the front.

Historical Note

Supplied with No.1, the transit clock, this clock is of decidedly 'standard' quality, and was probably 'bought in' by Earnshaw to supply with his own transit clock. Nevertheless, along with its companion it represents Earnshaw's first involvement with regulator clockwork (according to Earnshaw) and is therefore still of the highest interest and value. The electrical contacts were probably fitted in the late 19th century, but apart from this the clock remains substantially unaltered. The clock is described in the contemporary notes as being set up "in the dome of the tower to the back of the north pillar of the equatorial instrument". It was rated by Hamilton to keep sidereal time, and he told Earnshaw he used it only as an assistant clock.

See also:
Thomas Earnshaw
Earnshaw No. 1
History of Armagh Observatory

Last Revised: 2014 November 10th