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The Greenwich meridian

This is one of a series of articles written for "Clocks" magazine by the late Noel Ta'bois, and reproduced with permission here as a memorial to him.

This article originally appeared in Clocks in April 1986

As time meridians are of great importance to diallists it is felt that a few notes on the history of the prime meridian will not be out of place on this page.

Greenwich Royal Observatory was founded in 1675 by decree of King Charles II, with John Flamsteed as the first Astronomer Royal, to plot the positions and study the movements of the heavenly bodies, an essential preliminary to being able to find longitude at sea. One of the pieces of apparatus used in this work was the transit instrument which is basically a telescope mounted to swing only in the plane of the meridian. Although the first such instrument was used in 1721 by Edmond Halley - of comet fame, and the second astronomer Royal - it was the transit telescope of James Bradley, the third Astronomer Royal, which from 1750 defined a line of zero longitude.

The transit instruments were improved by subsequent Astronomers Royal and the sixth, John Pond, set up on obelisk at Pole Hill in Chingford 11 miles due north of Greenwich. The photograph of figure 1, which I took early this year, shows both the obelisk and the large tree which now intervenes between it and Greenwich.

In 1850 Pond's successor, George Airy, built a greatly improved instrument known as a transit circle which was in use until 1954. So that transit observations could proceed without interruption, this new instrument did not replace that currently in use on Bradley's meridian but was installed in the adjacent room. This resulted in the Greenwich meridian being moved 19ft eastward. A change which is recorded in the plaque on the obelisk, seen in figure 2. Although the change was notified to the Ordnance Survey, for some unclear reason Bradley's meridian is still used for Ordnance Survey maps. In 1994 Airy's meridian became the Prime Meridian, the line of zero longitude for the whole world and the basis of the International Time Zone system.

The Greenwich Meridian runs through England from just north of the River Humber to Peacehaven on the East Sussex coast. This route is marked at many points by plaques or monuments of various kinds, and one such, which is quite spectacular, is in the Rose Garden at Waltham Abbey, shown in figure 3.

An interesting booklet entitled The Greenwich Meridian by Stuart Malin and Carole Stott is published by ordnance Survey, Southampton, Price 1.95.