A zenith telescope (or sector) is a refracting telescope designed to observe
stars as they pass overhead. Robert Hooke made the first instrument of this sort
in 1669, hoping to discover evidence of the earth’s annual motion about the
sun (or solar parallax). Hooke failed in this endeavor. So too did Samuel
Molyneux, who installed a zenith telescope made by George Graham in his
observatory in Kew in 1725. James Bradley, however, while observing with
Molyneux’s telescope, discovered the aberration of star light, and he later
discovered the nutation of the earth’s axis while observing with a smaller
instrument also made by Graham.
Portable zenith telescopes could be used for geodetic purposes. Graham made
an instrument of this sort for Pierre L. M. de Maupertuis, for his 1736–1737
expedition to Lapland to determine the length of a degree of latitude in
northern regions. The first zenith telescope in America was made by John Bird in
London in 1763, and purchased by Thomas Penn for Mason and Dixon’s survey of
the boundary between Maryland and Pennsylvania.
In 1834, while working on the boundary between Ohio and Michigan, Capt.
Andrew Talcott of the United States Corps of Engineers developed a more
efficient method of using zenith observations for latitude determinations.
Whereas earlier geodesists had observed only those stars that passed within a
degree or two of the zenith, Talcott observed pairs of stars that passed at
roughly equal distances to the north and south of the zenith, and within a short
period of time. The United States Coast Survey adopted Talcott’s method in
1846.
