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. Transit circles are precision instruments specifically designed for measuring these two coordinates. They could also be used, and generally were, to determine the local time as well.
The Airy Transit Circle (ATC) carries the name of its designer, George Airy, the seventh Astronomer Royal. Ransomes and May of Ipswich were responsible for the engineering and Troughton and Simms of London, the optics. The instrument, like those it replaced, was mounted to move only in the plane of the meridian. The telescope tube, which is very nearly 12 feet in length, is supported by two massive stone piers – one to the east and the other to the west. The first published observation was made in 1851 and the last in 1954.
When the Airy Transit Circle came into use in 1851, it redefined the Greenwich Meridian and subsequently came to define the Prime Meridian of the World as well. It was used to determine Greenwich Mean Time until 1927, roughly the date of the picture shown here, when a smaller reversible instrument replaced it. Greenwich Mean Time became legal time in Britain in 1880 and was adopted in principle as the basis of universal time in 1884, when, at the International Meridian Conference in Washington, it was agreed that the universal day ‘…is to begin for all the world at the moment of mean midnight of the initial [Prime] Meridian’.
The stars all move across the sky in a similar way to the Sun, and like the Sun, they culminate, or reach their highest point, as they transit, or cross, the meridian. Because the Earth is spinning at a steady rate, each individual star crosses the meridian of the telescope at the same (sidereal) time each day.
Provided a transit instrument was accurately aligned to the meridian, the right ascension of a star could be obtained directly from the time of transit. It was recorded as a time, or hour angle, rather than in degrees. Declination was calculated from measuring what astronomers call the zenith distance (ZD) – the angle between the star, the observer and the zenith, the point vertically overhead.
Certain of the brighter stars, whose positions had been refined by repeated observation over a long period of time, were used as ‘clock stars’ to determine the errors of the transit clock – and hence the true time on the ground. This was done by comparing observed times of transit with theoretical ones. In 1851, some 67 clock stars were kept under observation for this purpose. The clock, which had been made some years earlier by William Hardy, was regulated to sidereal time. It was from this clock that the mean solar time at Greenwich, i.e. Greenwich Mean Time, was ultimately determined.
The meridian of the Airy Transit Circle, like that of the earlier transit instruments at Greenwich, was established from observations of the circumpolar stars – stars that never rise nor set.
These stars are always present in the sky and transit the meridian twice rather than once each day. When the telescope is correctly aligned, the measured interval between successive transits of any particular circumpolar star is constant.
The interval between successive transits of the Sun is about four minutes longer than the interval between similar transits of other stars. This is because at the same time that the Earth is spinning on its axis it is also orbiting the Sun, progressing about 1° around its orbit with each complete turn. Between one transit of the Sun – at midday – and the next, the Earth therefore has to turn through an angle of about 361° rather than the 360° required for the other stars – thereby accounting for the extra 4 minutes. Time measured by the stars is called sidereal time. Time measured by the Sun is called solar time.
The extra amount the Earth has to turn varies very slightly from one day to the next as a result of both the Earth’s elliptical orbit and the tilt of its axis. The exact interval between successive transits of the Sun varies in a periodic manner through the year; the longest being about 51 seconds greater than the shortest, the average length being 24 mean solar hours. It is because of this variation in the length of the natural day that ‘mean’ time is used for civil purposes.