A significant telescope
If we were to talk about the most popular telescopes of all time, it is unlikely that you will be mentioning the Airy Transit Circle (ATC). You’ll probably recall space observatories like the James Webb Space Telescope and Hubble, and you might even spare a thought to the early versions made and used by Galileo Galilei and Isaac Newton. But ATC? Even if you aren’t aware of it, it is one of the most significant telescopes historically. And its claim to fame? Well, the prime meridian was first defined at the Royal Observatory Greenwich by the ATC!
A transit circle, also called a meridian circle, is a precision instrument for timing of the passage of stars across the local meridian, while at the same time measuring their angular distance from the zenith. The ATC, one such transit circle, was designed by English astronomer George Biddell Airy.
Mounted to move only in the plane of the meridian, the ATC had massive stone piers on either side for support. The first observation using the ATC was taken on January 4, 1851, and it remained in operation for over a 100 years! This included continuous usage for over 80 years until 1938, and the last observation using it was taken in 1954. It might no longer be put to use, but the ATC still stands in its original mounting, in the Transit Circle Room at the Royal Observatory.
The ATC still stands in its original mounting, in the Transit Circle Room at the Royal Observatory.
| Photo Credit:
Andres Rueda / Wikimedia Commons
Fundamental to creating star charts and setting clocks accurately before the advent of modern electronics, the ATC was recognised by scientists and astronomers of the time to be a very precise instrument. The ATC gained greater significance, however, following a conference in Washington D.C. in 1884 to establish an international system of time zones.
Owing to its huge popularity around the world, it was agreed upon that the cross-hairs in the eyepiece of the ATC precisely defined Longitude 0°. This, in turn, meant that it marked the start of the universal day, as the prime meridian — the imaginary line at the longitude — passes through it.
The movement of the Earth’s crust means that the exact position of the prime meridian also moves ever so slightly all the time. While the exact location of the line might now be either side of Airy’s meridian, the ATC holds pride of place as being the original reference of the prime meridian of the world.
The man who gave his name to ATC
Born on July 27, 1801, Airy graduated from Trinity College, Cambridge in 1823. Starting off as an assistant tutor in mathematics, he became Lucasian professor of mathematics at Cambridge in 1826. It was during this period that his interest in astronomy piqued, as is evident by the book that he published the same year, Mathematical Tracts on Physical Astronomy.
It wasn’t long before the mathematician turned into an astronomer as Airy became the Plumian professor of astronomy and director of the Cambridge observatory in 1828. By 1835, he was made the seventh astronomer royal or the director of the Royal Observatory, a position that he held on to until 1881.
Under his leadership, the Royal Observatory greatly flourished. He not only replaced and added apparatus, including, of course, the ATC, but also ensured that thousands of observational data wasn’t lost forever. He modernised their system of taking measurements, introduced specialist staff and even created new departments in the observatory.
Not all rosy
Even though Airy commanded a great deal of respect within the scientific community, especially in Britain, it wasn’t without its fair share of controversies. Chief among them was the failure on the part of British astronomers to search for what was then a new planet (Neptune) whose existence and probable location was predicted by British mathematician John Couch Adams in 1845.
When French mathematician Urbain-Jean-Joseph Le Verrier made a similar calculation the following year, it almost immediately led to the discovery of Neptune by German astronomer Johann Gottfried Galle. While the one year delay in the discovery of Neptune might even seem insignificant to us today, it led to a full-blown dispute in the scientific relations between the Brits and the French.
The blame for not acting was squarely laid on Airy, the degree of which might seem unfair to a modern observer and scholar. Airy, for his part, did go on to champion Adams’ contribution and Adams is now recognised as Neptune’s co-discoverer.
What are his other contributions?
Airy did plenty outside the observatory too. He supervised an experiment at Harton Colliery, South Shields, known for its incredibly deep shafts — about 1,290 feet or 390 m. The experiment involved swinging the same pendulum at the top and bottom of this mine. In turn, this allowed the experimenters to measure the change in the force of gravity at the top and the bottom. This meant that the strength of gravity could be established based on the distance below the Earth’s surface.
Just like most other astronomers of the time, Airy was also part of a number of eclipse expeditions, including the ones observed from Turin in 1842, Sweden in 1851 and Spain in 1860.
Knighted in 1872, Airy also functioned as the scientific advisor to the government and advised on the laying of transatlantic telegraph cable. He was involved in what remains to this day the most iconic symbols of Britain, advising on the construction of the chimes for Big Ben.
Airy was extremely methodical to the point of almost obsessing over orderliness by the time of his death in Greenwich, London, in 1892. The abstract of his biography published in Nature by his son in December 1896, in fact, mentions that “every line that Airy wrote; however trivial, found an assigned place in the Greenwich archives.”
Published – January 04, 2026 12:22 am IST
