Geodetic transit theodolite, made by Troughton & Simms for James Ludovic Lindsay

This large theodolite, made by the renowned London instrument makers Troughton & Simms, was made for the wealthy amateur astronomer James Ludovic Lindsay, later 26th Earl of Crawford and 9th Earl of Balcarres (1847-1913). In the early 1870s his private observatory at Dun Echt in Aberdeenshire was fitted out with first class instruments. At the same time, he was planning a visit to the island of Mauritius to view the 1874 Transit of Venus across the Sun's disc.

This instrument, with its finely engraved gold scale, was used on the Mauritius expedition both as a geodetic transit and as a portable altazimuth. Its measurements enabled the observing site's co-ordinates to be determined very accurately.

The day of the Transit, 9 December, started cloudy, but later cleared to allow a range of observations. However, under the very difficult conditions of viewing it proved impossible to determine the precise moment when the planet's limb (or edge) coincided with the Sun's disc. This disappointing result was found to be the case with all the other expeditions, and it effectively put an end to the serious use of Venus transits for the determination of solar distance.

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