A history of the Royal Observatory in six objects

1. Year-going clock movement

This clock movement is one of a pair that originally ticked away behind the panelled walls of the Octagon Room. As the Observatory building began to take shape in late 1675, the first Astronomer Royal John Flamsteed’s patron, Jonas Moore, ordered these highly accurate timekeepers from Thomas Tompion, the best clockmaker in London at the time.

Flamsteed needed such state-of-the-art timekeepers to prove that the Earth rotates on its axis at a constant rate (isochronal), a key assumption in all astronomical methods for measuring longitude. First, he improved his existing ‘Equation of Natural Days’ to smooth out variations in the length of the apparent solar day, caused by the tilt of the Earth’s axis and the elliptical shape of its orbit. 

By formulating an average or ‘mean’ day of exactly 24 hours, he set these clocks to show this new ‘Greenwich Mean Time’ (GMT). Next, he set up a 6ft telescope on the western wall of the Octagon Room to measure the time interval between successive crossings (transits) of the bright star Sirius. It was a highly consistent interval of 23 hours, 56 minutes and 4 seconds – one sidereal day – that enabled him to check the accuracy of the Tompion clocks. 

After months of hard work, Flamsteed did indeed prove the Earth’s isochronal rotation, although three centuries later the Observatory’s Shortt free-pendulum clock system would prove otherwise. But for Flamsteed, Tompion’s specially designed clocks had demonstrated their worth, with extra-long 13ft pendulums for accuracy that beat every 2 seconds. They also featured an innovative escapement design to reduce friction and heavy driving weights to keep them going for a year, reducing the need for winding and disruptive maintenance.

3. George Biddell Airy's dip circle

While people marvelled at the dramatic effects of the Carrington event in September 1859, one of the largest-ever known geomagnetic storms, Astronomer Royal George Biddell Airy was busy finishing the design of his own magnetic instrument. The changing vertical angle between the tip of a compass needle and the horizon, known as ‘dip’ or ‘inclination’, had been recognised by navigators and instrument makers since the sixteenth century, along with its horizontal counterpart. At the equator, the needle of a dip circle lies flat (0°) but at the magnetic poles it turns vertically (90°).

For Airy, changes in the Observatory’s dip results were proving problematic. In the winter of 1856 he received a letter from Professor Christopher Hansteen of Christiania (Oslo) University who was concerned by the anomalous dip results between Greenwich and other observatories in northern Europe. Airy then carefully examined the Observatory’s dipping needle and realised that the needle was positioned incorrectly, causing it to catch on the brass scale. 

Despite adjusting the instrument and following Hansteen’s advice to obtain better needles, systematic errors still appeared in the results, leading Airy to admit ruefully in his Annual Report for 1859, ‘In regard to the Dipping Needles, I am still very much perplexed.’ Responding in his typical fashion, thereafter Airy set about designing his own instrument, creating a dip circle that featured a more stable base, microscopes for reading the needle movements more closely and a revolving gaslight that illuminated the scales. The instrument was eventually installed in October 1861 and, after a few more years of adjustment, remained in use until December 1914.

5. Spider fork

Spiders have long had a duplicitous reputation at the Observatory, regarded as both friend and foe. Working in the 1600s, English astronomer William Gascoigne was apparently inspired by a wayward arachnid spinning a web across his telescope when he suggested that spider silks could be used as vertical lines in the field of view – similar to rifle crosshairs − to help observers measure the crucial moment of transit. Over the next two centuries, astronomers experimented with various materials, concluding that spider threads were stronger, thinner and more uniform than silk, human hair or metal wires. 

By the 1800s, supplying spider threads as ‘reticules’ for telescopes was part of everyday business for instrument makers. Replacement threads for Greenwich instruments were ordered from Troughton and Simms and the 1864 inventory notes ‘a box of cobwebs’ kept alongside the Airy Transit Circle. This box likely contained forks similar to the one shown, on which spiders were enticed to spin by rotating the gadget. About a dozen threads could then be kept taut or extracted as needed.

However, when journalist Frederick Knight Hunt visited in 1850, he described how Airy’s assistants struggled to keep a telescope tube clean because its ‘cool and dark interior was so pleasant to the spiders that, do what they would, the astronomers could not altogether banish the persevering insects from it’.