What telescope did Cook use?

A brass reflecting telescope similar in appearance and optical design to those used by Captain Cook and his astronomer Charles Green at Tahiti to observe the transit of Venus on 3 June 1769. Courtesy Powerhouse Museum

I have received the following letter from Stuart:

I recently purchased and have just finished reading your book, Transit of Venus 1631 to the Present. I would like to pass on my appreciation for the obvious effort that went into publishing a book of such high quality.

I do have a question if you have some time. On page 74 of your book, the James Short Telescope is noted as a reflector. It looks like a refractor to me.

What is the optical design of this scope, reflector, sct or refractor?

Stuart is right that the James Short telescope illustrated in the Transit of Venus book, or the almost identical Dudley Adams telescope shown to the left, do resemble a refracting or lens telescope in that the eyepiece is at the bottom end of the telescope just as in a refractor. This is in contrast to the well-known Newtonian design of a mirror telescope where the viewing point is near the top of the tube at the side.
 

James Short’s telescopes had a Gregorian design. This design was due to a Scottish mathematician James Gregory who suggested a design for a reflecting telescope in 1663, but was unable to build it himself or get someone else to build it for him. Hence the honour of building the first reflecting telescope went five years later to Isaac Newton, who presented a working model of his own design to the Royal Society in 1668.

A cross-sectional drawing showing light rays inside a Gregorian telescope. Courtesy Wikimedia Commons and ArtMechanic

The Gregorian design due to James Gregory is based on two mirrors: a primary mirror of parabolic shape and a secondary mirror of ellipsoid shape placed after the focus point of the primary to reflect the light back down the tube. There it passes through a small hole at the centre of the primary mirror and is then examined through an eyepiece.

James Gregory has a connection with the transit of Venus in addition to the fact that James Cook used a telescope of his design. In the same 1663 book Optica Promota that Gregory suggests his new reflecting telescope design he also makes the comment in a Scholium to Proposition 87 that

Hoc Problema pulcherrimum habet usum, sed forsan laboriosum, in observationibus Veneris, vel Mercurii particulam Solis obscurantis : ex talibus enim solis parallexis investigari poterit.

Or in English

This prettiest of problems has a use, but perhaps a very laborious one, in the observations of Venus or Mercury obscuring a little part of the sun : indeed from such the parallax of the sun will be able to be investigated. (Translated by Ian Bruce)

Thus James Gregory did suggest using transits of Venus for solving the problem of the distance of the Sun long before Edmond Halley did in 1716. Halley receives credit as, unlike Gregory, he provided a practical method for making the measurement and not just a hint that transits could be used for the purpose.

James Short was born in Edinburgh, Scotland, in 1710, and was orphaned at age 10. Encouraged by a professor of mathematics after graduating from Edinburgh University, Short began making reflecting telescopes. The most difficult part in making telescopes was to grind the mirrors of speculum metal and he became much more successful than his contemporaries in giving these mirrors the required parabolic shape. His fame spread quickly and by 1736 he was summoned to London to teach mathematics to William, Duke of Cumberland, who was the younger son of the King and was later to be known as ‘Butcher Cumberland’ after the Battle of Culloden in Scotland. Within two years Short had moved permanently to London, where his telescopes commanded twice the price of those of his competitors.

Short observed the 1761 transit of Venus from London using one of his own telescopes and later as a Fellow of the Royal Society he made a detailed analysis of the various observations by British observers of that transit. Sadly he died in 1768 before the following transit that was observed by James Cook among many others around the globe. It is believed that he completed 1370 telescopes in his lifetime.

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