Exhibition dates: 6 February–16 July 2004

Glass negative of the transit of Venus from Luxor, 1874. Repro ID: B5948-F ©National Maritime Museum, Greenwich, London

A 'transit of Venus' is when the planet Venus can be seen from Earth passing in front of the Sun's disc. It occurs twice every 113 years.

In 1716 the second Astronomer Royal, Edmond Halley, suggested that this phenomenon could be used to find the size of the solar system. He proposed that the transit should be observed from different points on the globe simultaneously. Applying trigonometry to the results would produce the distance from the Earth to Venus and to the Sun, and thereby (with the help of Kepler’s equations) the size of the solar system.

In 1761 the Royal Society sent Nevil Maskelyne (later the fifth Astronomer Royal) and Robert Waddington to make observations on St Helena, and Charles Mason and Jeremiah Dixon to the Cape of Good Hope, South Africa. Maskelyne and Waddington’s observations unfortunately failed due to bad weather, and so a comparison between the two sets could not be made.

Captain Cook in Tahiti

Captain James Cook, 1728-79, by William Hodges, 1775-76. Repro ID: BHC4227 ©National Maritime Museum, Greenwich, London. Acquired with the assistance of the National Heritage Memorial FundJames Cook's very first voyage to the Pacific in 1768 was initiated by the Royal Society, expressly to observe the 1769 transit of Venus from Tahiti. Nevil Maskelyne, the Astronomer Royal, working for the Royal Society also organized the sending of observers to Hudson's Bay, Cornwall, Ireland and Norway. After the failure of the 1761 observations it was the astronomers last chance until 1874.

This time the project was relatively successful. The only serious problem was something called 'black-drop effect', which was thought to be caused by the atmosphere of Venus. It prevents observers seeing exactly when the planet first crosses the Sun's limb (edge). As it does so the planet – which looks like a black dot – seems to elongate into a 'drop'. It was a problem that was to affect 19th-century expeditions as well.

Cook made three voyages to explore the Pacific, then little known to Europeans. The first (1768–71) took him to Tahiti, New Zealand and Australia. The second (1772–75) probed south towards Antarctica and extended knowledge of the south and central Pacific. He was killed on Hawaii during the third (1776–80), after exploring the coasts of north-western America and beyond the Bering Strait. On the last two voyages he took Larcum Kendall's copy of John Harrison's fourth timekeeper.

Both Harrison's timekeeper and the Kendall copy are on display at the Royal Observatory.

The 19th-century expeditions

The Observatory’s preparations for the 1874 transit of Venus. Equipment being tested in Greenwich Park before embarking to station A in Egypt. Image ID: B1006-N. Copyright: National Maritime Museum, Greenwich, LondonBy the 19th century scientific apparatus had advanced in accuracy, while improved ships and navigational technology allowed easier travel. In time for the 1874 transit of Venus, George Biddell Airy, the seventh Astronomer Royal, organized and equipped five expeditions to different parts the world to improve on the observations of Cook and his colleagues in 1769. The 1874 transit was the first to be photographed.

Apparatus was first assembled here at the Royal Observatory to check that it was complete and all worked. It was then packed and shipped with very detailed instructions from Airy to the five sites chosen: Egypt (station A), the Sandwich Islands (Hawaii) including Honolulu (station B), the islands of Rodriguez in the Indian Ocean (station C), New Zealand (station D) and Kerguelen Island in the Indian Oceann (station E).