SignedJohn Bird London
FunctionAn astronomical transit is used with a clock to measure the altitude and time that a star or celestial body crosses the meridian at a specific location. This gives the declination and right ascension of the star, which are that star's coordinates in the sky. The information can also be used to find the longitude of a location if one has knowledge of when the meridian crossing would happen at the zeroth longitude.
If one has a table listing when a particular star is to cross the meridian at a given location, one can use the transit to find the time at that location at that moment. For this reason, small table top transits like this one were often used by watchmakers to check the running of their watches and clocks.
Medium-sized and larger portable transits were used in the field by astronomers when they went on expeditions to observe an eclipse or Transit of Venus.
Historical AttributesOn May 11, 1767, the Harvard Corporation voted its thanks to Thomas Hollis, Esquire, of Lincolns Inn, London for his gift of £200, to be used to enlarge the philosophical apparatus at the College. Some of this money went to purchase this transit by John Bird. Benjamin Franklin acted as Harvard's agent in London. It cost 40 guineas.
Professor John Winthrop used this transit for his observation of the Transit of Venus in 1769.
Primary SourcesJohn Winthrop, Two Lectures on the Parallax and Distance of the Sun as Deducible from the Transit of Venus (Boston, 1769).
P. C. Le Monnier, Histoire celeste (Paris, 1741), LXXV-LXXXIV.
Published ReferencesDavid P. Wheatland, The Apparatus of Science at Harvard, 1765-1800 (Cambridge, MA: Harvard University Press, 1968), 38-39.