Model of Flamsteed's 7ft Mural arc (by Abraham Sharp). Instrument supported on wall is painted black. Scale figure in a synthetic material is standing adjusting the instrument.


This model was made for the 1975 tercentinary exhibition and was displayed alongside models of Flamsteed's equatorial sextant (AST0021) and Halley's 5-foot transit circle (AST0023). The label accompanying these model in the exhibition read:

HOW STAR POSITIONS WERE MEASURED BY THE FIRST OBSERVERS AT GREENWICH
Flamsteed's Equatorial Sextant 1676
Soon after being appointed as the first Astronomer Royal, John Flamsteed with the help of his assistants busied himself with obtaining positions of stars, the Moon and plantes. This rather coumbersome device contained two telescopes whih coulc be moved to line up on two astronomical bodies so as to measure the angle between them.

Flamsteed's Mural Arc 1689
With this system, stars were observed with a stationary telescope as they were moved across the local meridian by the Earth's rotation. The local meridian is the imaginary line passing through the point on the horizon due south. through the zenith directly overhead and through the point on the horizon due north. The position of a star was found by measuring both the inclination of the telescope to the vertical and the time when the star crossed (transited) the meridian.

Halley's 5-foot Transit Instrument 1721
Like Flamsteed's mural arc, this was used to time astronomical bodies as they crossed the local meridian. The corresponding altitudes of the object were determined with another telescope. As did most later transit instruments, Halley's telescope rotated about a relatively long axis supported on two separate piers.