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The Astronomy of James Cook
The First Voyage, 1768-70 – The Transit of Venus
On Cook's first voyage, science ruled supreme, for the primary function of the voyage was to accurately observe a transit of Venus from Tahiti. Observations from there were thought to offer the most effective means of determining that basic celestial yardstick, the Astronomical Unit (or AU), which is the distance from the Earth to the Sun.
Although the stated reason for the first voyage was to observe the 1769 transit of Venus, Cook’s ‘sealed orders’ revealed a secondary and, from a political perspective, even more important rationale: i.e. to search for (and if found to claim for Britain) the mooted ‘Great Southern Continent’.
An ex-collier refurbished and renamed Endeavour was chosen for the voyage, as were two astronomers: Lieutenant James Cook and Charles Green. Cook, born 1728, was a career seaman with a solid background in nautical astronomy and coastal mapping. In 1766 he had observed a solar eclipse from Newfoundland. Green was six years Cook’s junior and a former assistant at the Royal Observatory Greenwich. He had observed the 1761 transit of Venus, and in 1763 had taken Harrison’s chronometer on a voyage of evaluation to Barbados.
The Royal Society and the Royal Greenwich Observatory were careful to equip Cook and Green with the requisite scientific instruments, namely Gregorian reflecting telescopes; astronomical quadrants; sextants; astronomical, journeyman and alarum clocks; and pocket watches.
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<- A Gregorian Reflecting telescope of the type used for the transit of Venus observation from Tahiti. |
The expedition reached Tahiti in good time for the transit which was scheduled for 3 June 1769, and a small fort was erected at Point Venus in order to guarantee that there would be no local interruptions on the all-important day.
Fort Venus at Tahiti where the observations of the Transit of Venus were made.
Cook’s plan of Fort Venus showing the observatory (b) the clock (c) and other attributes of the observing station.
From Cook’s Diary:
“The astronomical clock made by Shelton and furnished with a gridiron pendulum was set up in the middle of one end of a large tent, in a frame made for the purpose at Greenwich. Without the end of the tent facing the clock, and 12 feet from it, stood the observatory, in which were set up the journey-man clock and astronomical quadrant: this last, made by Mr. Bird stood upon the head of a large cask fixed firm in the ground, and well filled with wet heavy sand. The Gregorian reflecting telescopes were to be used to observe the transit.”
Observatory tent (of the type used on the Second and Third Voyages).
Cook decided to establish two additional temporary observing stations. Cook, Green and Solander were stationed at Point Venus; Gore, Dr Monkhouse and his brother, went to Moorea; and Hicks, Clerke, Pickersgill and Saunders were at the second Tahitian station.
Cook’s chart of Tahiti and Moorea where the three observing stations were situated. Note point Venus to the north of the main island.
June 3 was fine. Cook reported in his journal:
“This day provided as favourable to our purpose as we could wish, not a cloud was to be seen the whole day and the Air was perfectly clear, so that we had every advantage we could desire in observing the whole of the passage of the Planet Venus over the sun’s disk: we very distinctly saw an Atmosphere or dusky shade round the body of the Planet which very much disturbed the times of the Contacts particularly the two internal ones. Dr Solander observed as well as Mr Green and myself, and differ’d from one another in observing the times of the contacts much more than could be expected.”
The page of Cook’s journal describing the first and second contact of Venus, 1769. Note the black-drop effect that was felt to be responsible for some measure of uncertainty in the timings.
The transit lasted for about six hours, and was also observed at the other two stations. Observations were published in the Philosophical Transactions of the Royal society. The timings that were deemed critical were of the second ingress contact and the first egress contact. Both Cook and Green had problems in accurately establishing these. Cook explains the black-drop effect and its consequences thus:
“Very difficult to judge precisely of the times that the internal contacts of the body of Venus happened, by reason of the darkness of the penumbra (i.e.. Atmosphere of Venus) at the sun’s limb, it being there nearly, if not quite, as dark as the planet. At this time a faint light, much weaker than the rest appeared to converge towards the point of contact, but did not quite reach.”
The problem was to decide precisely when Venus ‘broke free’ from the sun’s limb. Even with these timing uncertainties, there is no justification for the claims that the Tahitian observations were a failure as they did indeed produce a meaningful value for the AU.
