English astronomer and surveyor.

Assistant to James Bradley, the astronomer royal, at Greenwich (1756-60). With Jeremiah Dixon, observed the transit of Venus for the Royal Society (1761) and determined the boundary line between Pennsylvania and Maryland (the “Mason-Dixon Line”) on behalf of the Calverts and William Penn (1763-68).

Returned to England (1768), where he conducted further astronomical observations for the Royal Society and the Board of Longitude. Extensively revised Tobias Mayer’s Lunar Tables; the final edition (London, 1787) long remained the standard work.

Son of Charles and Anne (Damsel) Mason of Gloucestershire. Married twice; eight children.