Transit instrument by George Graham

Transit instrument

George Graham

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Transit instrument

The earliest telescope at the Royal Observatory Greenwich that can definitely be associated with its history, apart from the remains of John Flamsteed's well telescope, is this 5ft transit instrument purchased by Edmond Halley, the second Astronomer Royal. When Halley arrived at the Observatory in 1720 he discovered that Flamsteed's widow had removed all the instruments, which she saw as personal property. Halley managed to obtain a grant of Õú500 from the Board of Ordnance for re-equipping the observatory and this was one of the first items that he ordered.

Instruments of this sort were a relatively recent invention by Olaus RÔòá┬¬mer, professor of astronomy at the University of Copenhagen, and Halley's may have been the first made in England. Lighter and less expensive than a mural quadrant, their disadvantage was that they could easily become misaligned through slight changes in temperature. However, Halley's instrument proved useful and remained in service, with modifications, until 1750.

A transit instrument is a telescope pivoted on a stand so that it can only move in one plane. Normally it would be aligned north-south so that, as the Earth turns, every star visible from the latitude of the telescope can be seen to rise and fall over the course of a year. Such telescopes were used, together with a very precise clock (an astronomical regulator), to create accurate star catalogues and charts. The co-ordinates of each star are provided by the time at which it crosses the meridian and the angle at which the telescope is pointing when it is viewed. This transit instrument has no angle scale so a separate instrument, such as Halley's mural quadrant (AST0970), must be used alongside.

This telescope consists of a brass tube, 5ft (168cm) in length, with an object glass aperture of 1.5 inches (4cm) and magnification of less than x 40. The object glass survives but the eyepiece is missing. It is mounted on an off-centre axis supported by braces that would have attached to two north-south aligned walls or piers. The maker is not known, although one account suggests that Robert Hooke (who died in 1702) may have been responsible for at least part. George Graham, who made Halley's mural quadrant, may have completed or mounted the instrument.
George Graham

  • Image reference: F8673

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