Troughton's 10-Foot Transit Telescope -- Royal Observatory, Greenwich, London, UK
N 51° 28.673 W 000° 00.088
30U E 708215 N 5707233
This telescope at the Royal Observatory, built by astronomer John Troughton, defined the Prime Meridian from 1816-1850.
Waymark Code: WMT461
Location: London, United Kingdom
Date Posted: 09/22/2016
Views: 3
This odd looking telescope is on display next to George Airy's MASSIVE Transit Circle telescope at the Royal Observatory in Greenwich.
From the Royal Observatory website: (
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"Telescope: Troughton 10-foot Transit Instrument (1816)
Erected in 1816, the Troughton 10-foot Transit Instrument was a replacement for Bradley’s 8-foot Transit Instrument of 1750. It was itself replaced by Airy’s Transit Circle in 1850. It defined the Greenwich Meridian from 1816 until 1850.
Mounted in the plane of the meridian, the Troughton 10-foot Transit Instrument was used in conjunction with an accurate pendulum clock, to ascertain the right ascension of a heavenly body. This was done by measuring the (sidereal) time at which it transited (crossed) the meridian.
Certain of the brighter stars, whose positions had been refined by repeated observation over a long period of time, were used as ‘clock stars’. By comparing their observed times of transit with their theoretical ones, the errors of the transit clock could be determined.
Dimensions
Constructed by Troughton, the overall dimensions of the telescope were determined by two factors. Firstly, It incorporated as its object-glass, a pre-existing achromatic lens that had been made by Peter Dollond. This belonged to a telescope acquired by Maskelyne for the Observatory in 1793 at a cost of £150 (RGO6/22/30). The object-glass dictated the length and aperture or the telescope. Secondly, the new instrument had to be capable of being mounted on the same piers as its predecessor, albeit raised in height.
The length of the telescope is about 10 feet, and the clear aperture of the object-glass 5 inches. The length of the axis (between the extremities of the pivots) is 4 feet. . . "