History of the Northumberland Telescope

The Northumberland Telescope is familiar to the many members of CUAS who have used it for observations, as well as those who have used it on public observing evenings. While some aspects of its history (such as how it was used by Professor James Challis to fail to discover Neptune) are well known to its users, others are not, and are not reflected in the standard account of this telescope on the IoA web site.

Most notably, it seems to be little known that Airy, who designed the telescope, wrote a book about it, Account of the Northumberland Equatoreal and Dome, attached to the Cambridge Observatory, privately distributed in 1844. (Although the distribution of the book on its own was private and it did not go on general sale, it was also bound with the Cambridge Observations for 1843 (volume XV) as Appendix II, and the Cambridge Observations were sold to subscribers.) This contains nineteen plates with sixty-seven figures showing almost every part of the telescope and its original dome in exquisite detail, down to individual nuts and bolts, with some of the fine details drawn being as little as one eight-hundredth of an inch across, and thirty-nine pages of text explaining these figures and the construction of the telescope and dome in great detail. (The diagrams are not yet available from the online version here.)

I have placed a photocopy of the book in the South Annexe (ante-room C in Figure 2 in the book) of the dome for the use of those using the telescope. Some of the beautiful fine detail has been lost in the copying process, but the copy should be adequate for most purposes when actually using the telescope. I will endeavour to ensure that, when the diagrams are made available online here, they do reflect the level of detail of the diagrams in the book.

The telescope cost 15000 francs for the object-glass and £1938. 9s. 2d. for the rest, according to Airy’s autobiography (which also mentions his book about the telescope, although the autobiography seems somewhat better known).

Extensive contemporary papers concerning the construction of the telescope and the writing, production and distribution of the book are preserved among Airy’s papers in the Royal Greenwich Observatory Archives, in RGO 6/157 (for the telescope itself) and RGO 6/158 (for the book), and may be studied in the University Library.

Among the facts about the telescope that are not well-known are:

One story which does, however, seem to be at least somewhat accurate, is that on setting up the telescope Airy made some adjustments, then tightened some bolts and said that no-one should ever loosen them. No-one ever has, and those bolts have since been painted over; the use of the bolts is explained in his book. However, the correspondence between Airy and Challis shows that much of the adjustment was done by Challis.

Documents available here:


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Contact: Joseph Myers (jsm@polyomino.org.uk)
Last updated: 5 January 2016