Airy's Transit Circle

4 January 2001 marked the 150th anniversary of 'first light' for the Airy Transit Circle – the first observation of a star using the meridian which now defines the Prime Meridian of the World. This online exhibit explains how the transit circle was used to set Greenwich Mean Time, and gives an introduction to the Victorian observatory.

I think it worthy [of] the careful consideration of the Visitors, whether meridional instruments carrying larger telescopes should not be substituted for those which we possess. Whatever we do, we ought to do well. Our present instruments were, at the time of their erection, the best in the world; but they are not so now; and we actually feel this in our observations.
George Airy, in the Report to the Board of Visitors, 1847

Transit Circle Mounted in the Transit Circle Room,defining the Greenwich Meridian since 4 January 1851.Makers: Ransomes & May of Ipswich (engineering)Troughton & Simms of London (optical and instrumental)to the design of G. B. Airy. Lens: 8.1 inch aperture objective glass Focal Length: 11 feet 7 inches Magnification: 195 (180 for the Sun).

The Airy Transit Circle was designed by George Biddell Airy, and came into use 150 years ago. The first observation was taken on 4 January 1851, three days later than Airy had intended due to the English weather. The circle remained in continual use until 1938, and the last ever observation was taken in 1954.

The transit circle sits in its original mounting, in the Transit Circle Room. It was immediately recognised by astronomers as an instrument of great accuracy. But its importance for everyone dates from a conference held in Washington DC in 1884 to create an international time-zone system. It was agreed that the meridian line marked by the cross-hairs in the Airy Transit Circle eyepiece would indicate 0° longitude and the start of the Universal Day.

Making a transit measurement

The watcher who wishes to observe the passing of a star must note two things: he must know in what direction to point his telescope, and at what time to look for the star. Then, about two minutes before the appointed time, he takes his place at the eyepiece. As he looks in he sees a number of vertical lines across his field of view. These are spider-threads placed in the focus of the eye-piece. Presently, as he looks, a bright point of silver light, often surrounded by little flashing, vibrating rays of colour, comes moving quickly, steadily onward -- 'swims into his ken,' as the poet has it. The watcher's hand seeks the side of the telescope till his finger finds a little button, over which it poises itself to strike. On comes the star, 'without haste, without rest,' till it reaches one of the gleaming threads. Tap! The watcher's finger falls sharply on the button. Some three or four seconds later and the star has reached another 'wire,' as the spider-threads are commonly called. Tap! Again the button is struck.
Maunder, 1900.

A transit circle is used to make two measurements - the time at which a star crosses the meridian, as it travels from East to West across the sky, and the angle of the star above the horizon. From these measurements an astronomer can calculate the celestial coordinates of the star - its precise position in the sky, relative to other stars. If the star's coordinates are already known, then the transit of a star across the meridian can be used to set the clocks at an observatory - the sky can be used as a giant clock.

To use the Airy Transit Circle, first the roof of the Observatory was opened to allow the telescope to observe the sky. Next, a star was chosen for a measurement, and the telescope was tilted up to the angle of that star in the sky. The observer then waited for the rotation of the Earth to carry the star across the field of view of the eyepiece of the telescope. As the star crossed the meridian, its position was measured using a series of wires (actually threads of spider's web) in the eyepiece of the transit telescope.

As the star moved from East to West, it would move across a series of vertical wires. Each time the star crossed a wire, the astronomer at the telescope pressed a button to send a signal to the drum chronograph. This recorded the precise times at which the star crossed the wires. From these measurements, astronomers could calculate precisely when a star had crossed the meridian, and convert this time into the celestial longitude of the star, or Right Ascension.

In 1915, the precision of the Airy Transit Circle was improved by the addition of the system illustrated in the animation above. As the star being observed crossed the innermost pair of vertical wires, the astronomer followed it with a movable wire. Electrical contacts within the eyepiece sent a signal to the chronograph when the moving wire moved past the fixed vertical wires. This removed human errors involved in the key-tapping system, greatly improving the accuracy of the observations.

The microscopes for measuring the altitude of the Airy Transit CircleThe microscopes for measuring the altitude of the Airy Transit Circle Once a transit observation had been made, the observer measured the angle at which the telescope was tilted by using a system of microscopes to read a high precision scale on the side of the transit circle. This measurement gave the angle of the star above the horizon, which could be converted into its angle above or below the celestial equator, or declination. The Airy Transit Circle could be used to measure the declination of stars to an accuracy of 0.01s of arc.

The drum chronograph

The drum chronograph in use at the ObservatoryThe drum chronograph in use at the ObservatoryAn astronomer making a star map takes two measurements: the angle at which the telescope is positioned to see the star cross the meridian and the time at which this happens. Traditionally the second measurement was estimated by listening to a clock facing the astronomer. With Airy's chronograph, the time was recorded electrically by tapping a key on the eyepiece of the transit circle.

Chronograph Record 1926 Chronograph Record, Greenwich, June 30th 1926

As the drum of the chronograph turned, a pen traced a line along a sheet of paper mounted on it. A standard clock, connected to the chronograph, sent impulses every two seconds, which were recorded by the pen. When a signal was received from the observer at the transit circle, it was also recorded. The time of transit of a star could then be calculated by measuring the position of these marks relative to the marks left by the clock.

This picture shows a detail of a chronograph record, from 1926. The horizontal lines are the lines traced by the chronograph pen. The vertical columns of marks are the regular timing signals from the clock, two seconds apart. The marks between the regular signals show the times at which stars crossed the vertical wires in the eyepiece of the transit circle.

Chronograph Record 1926 Chronograph Record 1926

Certain stars, whose postions in the sky were precisely known, were designated as "clock stars". The transit times of these stars could be calculated very precisely from their coordinates, in advance of the actual observations. This allowed astronomers to check the accuracy of the standard clock, and set it to the correct time. In this way, observations of stars with the transit circle were used to set Greenwich Mean Time.

Edwin Dunkin

The Airy Transit Circle in useThe Airy Transit Circle in use Edwin Dunkin was one of Airy’s most famous assistants. Dunkin rose from temporary computer to Chief Assistant and in his spare time wrote a number of articles and books popularizing astronomy. In The Midnight Sky Dunkin includes a whole chapter on the Royal Observatory including this illustration of the Airy Transit Circle in action.

Sir George Biddell Airy, KCB (1801–92)

After graduating from Trinity College, Cambridge in 1823 Airy worked as an assistant tutor in mathematics. In 1826 he became Professor of Mathematics at Cambridge, his interest in astronomy developed in this period as is reflect in his book, Mathematical Tracts on Physical Astronomy published the same year. He was elected Professor of Astronomy and Director of the Cambridge Observatory in 1828. In 1835 he became Astronomer Royal.

George Biddell Airy in 1852, the year after the Airy Transit Circle came into use.

The observatory expanded under Airy. Although he saw the proper role of the Observatory as providing data that could be used for the Navy and Empire (rather than as a research institution) he appeared to stretch this role to its limits. He replaced and added apparatus including the altazimuth telescope in 1847 and the Airy Transit Circle (thus providing the Observatory with its 4th meridian line - it is this line on which GMT is based). He introduced specialist staff to the Observatory and he introduced new departments, including a department for magnetic and meteorological data in 1838. He also introduced photographic registration in 1848, an electric device to time transits in 1854, spectroscopic observations in 1868 and daily observations of sunspots using the Kew heliograph in 1873. Outside of his Observatory work he supervised an experiment at Harton Colliery, South Shields to measure the change in the force of gravity with distance below the Earth's surface. Like many astronomers of the time, Airy went on a number of eclipse expeditions including Turin in 1842, Sweden in 1851 and Spain in 1860.

Airy also worked as a scientific advisor to the Government, supervised a catalogue of geographical boundaries advised on the laying of transatlantic telegraph cable and on the construction of the chimes for Big Ben.