The Airy Transit Circle

Page 2 of 6

• 1. Airy's advances
• 2. New transit instrument
• 3. Chronograph
• 4. Innovations
• 5. Airy the man
• 6. Find out more
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New transit instrument

A new telescope was needed, Airy argued, because improvements in telescope design more generally meant discoveries were being made that the Observatory had no way of using or checking. Between 1801, when the first asteroid was discovered, and 1845, when Airy first proposed the new instrument, three new minor planets, or asteroids had been discovered. This figure continued to climb throughout the 19th century. The existing Troughton transit telescope was not powerful enough to see these. Yet, Airy argued, it was the duty of the Observatory to chart the position of all stars and planets so they might be used by navigators.

The transit instrument that Airy's transit circle replaced was made by Edward Troughton of London, a famous instrument maker of the 18th century. He worked independently, on commission, from his workshop-cum-showroom, with the help of one or two assistants and apprentices and using an object glass from the optician, Peter Dollond.

The construction of the Airy Transit Circle was completely different. For a start, Airy was in control, rather than the instrument makers involved. He drew up the designs, with advice from Charles May on engineering and William Simms on optics, to incorporate all the improvements he thought necessary to keep the Observatory at the forefront of positional astronomy. He then contracted out different parts of the overall construction to different companies. To that end, Ransomes and May of Ipswich were employed to carry out the engineering work on the telescope, such as the construction of the telescope tube and the mountings, while the optical work, and fine, detailed work of micrometer manufacture, was carried out by Troughton and Simms.

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Published: 2002-07-01