The astronomical quadrant used by Captain James Cook during his 1769 observation of the Transit of Venus in Tahiti was a 12-inch, brass, portable instrument made by John Bird
. Its primary function was to establish the precise latitude and, when used with an astronomical clock, the local time (longitude) of the observatory at Point Venus, which was essential for calculating the distance between the Earth and the Sun.
Function of the Quadrant
Determining Local Time: The quadrant was used to measure the meridian altitudes of the sun and stars to set the astronomical regulator clock.
Establishing Position (Latitude/Longitude): It helped fix the exact latitude and longitude of the observation site, "Fort Venus".
Complementary Tool: While the transit itself was observed with telescopes (like those by James Short), the quadrant provided the, crucial positional data needed to validate the timing of the transit phases.
Mode of Operation
Measurement Technique: The instrument measured the altitude of celestial objects (angles up to 90 degrees).
Stabilization: Due to the need for high precision, the quadrant was mounted on a sturdy pillar stand, which was placed on top of a heavy barrel filled with wet sand to ensure it was completely stable.
Alignment: It used two plumb bobs to ensure it was perfectly vertical (plumb) and was equipped with two detachable reading microscopes to accurately read the graduated scale.
Field Use: It was set up in a portable, wood-and-canvas tent observatory to protect it from the elements and local interruptions.
Cook and astronomer Charles Green used this equipment to measure the precise moment Venus entered and exited the Sun's disc on June 3, 1769, contributing to the Royal Society's mission to calculate the Astronomical Unit.