Astronomical Quadrant

Astronomical quadrants were most commonly used to measure the highest points at which stars appeared in the sky in their movement towards a known meridian. If the observer knew the latitude of the place where they were observing, they could obtain the exact location of a star or planet in the sky. With this information, it was possible to create a series of measurement tables allowing navigators to establish their latitude accurately.

The astronomical quadrant was a portable version of the large mural or wall quadrants employed by astronomers at the Royal Observatory in the first decades of the eighteenth century. Securely fixed in a north-south direction on the Greenwich meridian line, these instruments were used to measure the altitudes of celestial bodies at the line to derive base measurements by which to calculate geographical location elsewhere on the planet.

The size and fixed location of wall quadrants reduced the potential for observational error. The accuracy of astronomical quadrants was far more likely to be diminished by accidental warping or wear of its metal parts and could be seriously affected by extremes of heat and cold.

Cook used an astronomical quadrant built by the London instrument maker John Bird (1709-1776) to record the Transit of Venus in 1769. This instrument incorporated innovations designed to reduce observational inaccuracies described by Bird in his Pamphlet The Method of Dividing Astronomical Instruments'(London, 1767).