A quadrant, such as the one above, measures the altitude of celestial objects and was one of the key pieces of equipment the astronomer Charles Green needed to observe the transit of Venus from Tahiti in 1769.
Used in combination with an astronomical clock, the quadrant was vital in confirming local time and helping establish longitude – that all-important metric still contested by scientists at the time.
The chronometer – a reliable watch that remains accurate over long periods – had already been invented by John Harrison but the Astronomer Royal, Nevil Maskelyne, still favoured his own lunar distance method.
Therefore Green – who had previously worked for Maskelyne, staying behind at the Royal Observatory in Greenwich while his master sailed to St Helena to witness the 1761 Venus transit – had to rely on the quadrant and the rest of the equipment issued him for the 1769 observation.
Green was the HMB Endeavour’s official astronomer but the ship’s captain James Cook was also a highly accomplished practitioner – an essential and relatively rare skill for any prudent skipper of the age.
Cook and Green were supplied with one or more quadrants of one-foot radius by John Bird (right), like the one above, and they were both well versed in Maskelyne’s lunar distance technique.
This involved “accurately measuring the angular distance between the centres of the moon and of the sun, or of the moon and some star,” according to the preface written by Sir William Wharton to an 1893 edition of Cook’s NEndeavour journal.
Because the moon is moving, this angular distance changes by the second and so, using previous astronomical calculations made at Greenwich, it was possible to work out from anywhere in the world what time it was back there. At the least that was the theory.
“By well-known calculations the local time at any spot can be obtained, and when this is ascertained, at the precise moment that the angular distance of sun and moon is observed, the difference gives the longitude.”
Wharton goes on to point out that none of this was straightforward, firstly due to the complexity of the calculations themselves, then because the astronomical tables of the day – like most sailors expected to use them – weren’t really up to the job.
Cook was, however, and luckily for him and Green, their task was made a little easier by Maskelyne’s own labours.
In 1763 – two years before he was appointed Astronomer Royal – Maskelyne published the lunar distance method of longitude calculation in The British Mariner’s Guide. In it, he suggested that to find longitude at sea (with the aid of a sextant), lunar distances should be calculated beforehand for each year and published in a form accessible to navigators.
This proposal was approved by the government and The Nautical Almanac for 1767 was published in 1766, overseen by Maskelyne. He subsequently persuaded the government to print his observations annually and so when it came to 1769, the Endeavour’s men were armed with this essential aid.
Using Bird’s quadrant, an astronomical clock or regulator (right) made by John Shelton, plus Maskelyne’s almanac, Green and Cook could figure out precisely they were in the world, enabling them to make more accurate observations of another one – Venus.
To observe the transit itself, they had two reflecting telescopes made by the Scottish instrument maker James Short, one of which was fitted with an object-glass micrometer (below) made by Peter Dollond.
The Navy Board also supplied a Gregorian telescope Cook had used previously and which he also had fitted with a micrometer. Dr Daniel Solander, the Swedish assistant to chief naturalist Joseph Banks, also had a Gregorian telescope.
The remainder of the Endeavour’s astronomical apparatuses comprised three clocks made by Shelton: one the all-important astronomical clock; one, an alarm clock; the third a journeyman clock. A fourth clock or rather watch made by George Graham, by one account was loaned to Cook by Maskelyne – rather curiously given his apparent contempt for “mechanics”.
Finally, there was a brass John Hadley sextant courtesy of Jesse Ramsden, used to determine the elevation of celestial objects with respect to the horizon usually while at sea, plus two thermometers from Bird and a stand for his quadrant.
The Endeavour also carried portable observatories similar to the one below – tents made of wood and canvas, in which the clocks and instruments would be set up to protect them from the weather and the natives. The tents had openings in the roof, which could be rotated to follow the stars and planets with the telescopes.
It was a lot of paraphernalia – high tech stuff at the time – and the stakes were high. It had taken the Endeavour more than a year and a half to reach Tahiti, all in the hope of finding clear skies there for a few hours on June 3 so that Green and Cook could get an uninterrupted view of Venus passing across the Sun. In witnessing this brief event, the Royal Society hoped that their observations would help towards establishing the size of the solar system.
Despite an early scare when one of the Tahitians made off with the quadrant in the middle of the night, everything was looking good as the countdown began.
“We are now very busy in preparing our Instruments, etc., for the Observations, and Instructing such Gentlemen in the use of them, as I intend to send to other parts to observe, for fear we should fail here,” Cook wrote in his journal on May 30, 1769.
But Banks, despite his own lack of interest in astronomical matters, summed up the atmosphere within Fort Venus rather better.
“The day of Observation now approaches. The weather has been for some days fine, though in general since we have been upon the Island we have had as much cloudy as clear weather, which makes us all not a little anxious for success,” he wrote on May 31.