Transit instrument

This transit instrument was commissioned by Astronomer Royal James Bradley in 1749 and remained in use until 1816. It was erected a little to the east of Halley's earlier meridian instruments marking out a new north-south line (or meridian) referred to today as 'Bradley's meridian'. It was on Bradley's meridian that the first Nautical Almanac, published in 1767 was based. It was also the meridian used for the first ordnance survey maps based on triangulation carried out in 1787.

A transit instrument is a telescope pivoted on a stand so that it can only move in one plane. Normally this 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. These types of telescopes can be used to create star charts, since every star can be viewed and so plotted. The time at which each star crosses the north-south line, or meridian gives one co-ordinate, the angle the telescope must point at to see it (its angular height) gives the other. It can also be used to find the time by the stars. This transit instrument has no angle scale so a separate instrument (setting semi-circle AST0980.2) must be used along side this telescope to find the angular height of the star.

The telescope is made up of a 2.4m (8ft) tube this originally had a common object glass of 2.4m focal length and aperture 0.07m aperture stopped down to 0.04m by an elliptical reflector. The object glass was replaced by an achromatic one by Peter Dollond in 1772. The new object glass has a focal length of 2.46m and an aperture of 0.07m.

Object Details

ID: AST0980
Collection: Astronomical and navigational instruments
Type: Telescope
Display location: Not on display
Creator: Bird, John
Date made: circa 1749
Exhibition: Ships, Clocks & Stars: The Quest for Longitude
Credit: National Maritime Museum, Greenwich, London
Measurements: Overall: 2438 mm
Parts: Transit instrument

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