Twelve inch portable astronomical quadrant, 1760-1769.
© Science Museum / Science & Society Picture Library
This portable astronomical quadrant was made in London by the famous English instrument maker John Bird (1709-1776). It was sent with one of the expeditions dispatched by the Royal Society to measure the 1769 transit of Venus. Captain Cook (1728-1779), who observed the event from Tahiti on his first voyage to the South Pacific (1778-81), may have even used this particular instrument. An astronomical quadrant was employed to measure the angle of a celestial object from the zenith. Objects were sighted using a pivoting telescope fitted with cros-hair wire that moved along a radial scale graduated in degrees. The instrument's orientation in both the horizontal and vertical could be finely adjusted using the plumb bob and spirit level.