#1 observatory circle full#

The determination of precise time – Text of a lecture given in 1949 by the Astronomer Royal) that had been made to the instrument.Īiry’s 1847 Address to the Board of Visitors regarding the possible acquisition of a Transit CircleĪiry’s description of the Transit Circle (Appendix 1 of the 1852 volume of Greenwich Observations (plate 1 missing))Īiry’s description of the Transit Circle (Appendix to the 1867 volume of Greenwich Observations)ĭescription of the Airy Transit Circle from the introduction to the 1908 volume of Greenwich Observationsĭescription of the Personal Equation Machine In addition, each year from 1851 until 1909, a much shortened description was given in the Introduction to each of the volumes of Greenwich Observations, the text being amended as necessary to reflect alterations etc. A reprint with some modifications to the text (but the same plates), was subsequently published as an appendix of the volume for 1867.

#1 observatory circle full#

Airy himself published a full description of the telescope with 16 plates, in Appendix 1 of the 1852 volume of Greenwich Observations. Photograph from the former collection of the King of Afghanistan, who visited the Observatory on 22 March 1928This page is intended only to give a general overview of the telescope. His left hand is holding the finderscope. Between 18 when it was eventually retired, the Airy Transit Circle was used to make over 600,000 observations.Ī 1920s view of an unidentified observer at the eyepiece of the telescope. Its other functions continued as before until eventually being taken over by the Cooke Reversible Transit Circle, a new telescope that was ordered in 1931 and began its first trials at Greenwich in 1936. The Airy Transit Circle was used to determine the clock errors until 1927 when it was decided to use the small reversible transit instrument in the nearby Transit Pavilion instead. Once this error was known, the error of all the other clocks at the observatory (and elsewhere) could be found by simple comparison. By comparing their observed times of transit with their theoretical ones, the error of the clock being used to time their transit (the transit clock) could be determined. They were found by measuring the (sidereal) time at which the object crossed (transited) the meridian of the telescope and the angular height (the Zenith Distance) at which it did so.Ĭertain of the brighter stars, whose positions had been refined by repeated observation over a long period of time, were used as ‘clock stars’. Declination and right ascension give the position of a star in the same way that latitude and longitude give the position of a place on the Earth. The Airy Transit circle was designed to yield both the right ascension (RA) and the declination (Dec) of a celestial object during one observation. In essence, the Airy Transit Circle consists of a telescope attached to a graduated circle that has been mounted on an east-west horizontal axis so that although the telescope can point higher or lower in the sky, it can only do this in a north-south direction (in the plane of the meridian).

Brought into use at the start of 1851, it was the workhorse of the Observatory for in excess of 100 years. As well as defining the Greenwich Meridian, which it has done since 1851, it also defines the Prime Meridian of the World and is at the historical heart of our global system of timekeeping.ĭesigned by the Astronomer Royal, George Airy, after whom it is named, the Airy Transit Circle took over the functions of both the Troughton 10-foot Transit Instrument and the 6-foot mural circles.

From the 8 August 1885 edition of The GraphicIf it were necessary to choose just one telescope to epitomize the Observatory, it would have to be the Airy Transit Circle.

Above that, the dial of the transit clock can be seen set into the pier of the south collimator. The mercury trough for reflected observations is visible in the observing pit just above the observer's left shoulder. The Airy Transit Circle being used to observe the Sun.