Why did James Cook sail to Tahiti?

The visibility of the 1769 transit of Venus. Map from Transits of Venus by RA Proctor, 1874. Brian Greig collection

In Transit of Venus: 1631 to the present I include a series of wonderful visibility maps of each transit of Venus from 1631 to 2012 that were originally published by the British populariser of astronomy Richard Anthony Proctor in 1874. The maps show where in the world transits are completely visible, where they are visible until sunset, where visible after sunrise or where not visible at all.

There is, however, much more information encoded in the maps. For example, the information on the 1769 map can tell us why Tahiti was such a suitable location for observing that year’s transit of Venus.

Obviously for the transit to be visible at a particular place the Sun has to be above the horizon. What complicates the situation is that transits take a number of hours so that the distribution of day and night on the globe changes from the beginning to the end of the transit. That is indicated on the above map.

On the map the northern hemisphere is on the left and the southern hemisphere is on the right. For the northern hemisphere the distribution of light and darkness at the beginning of the transit moves clockwise to that at the end of the transit. In the southern hemisphere the movement of the shaded area moves anticlockwise. Completely shaded areas indicate regions from where the transit is fully visible and completely dark areas indicate regions from where the transit is not visible at all. From the shaded areas the transit is partially visible either before sunset or after sunrise.

Detail of the visibility map for 1769 transit of Venus showing the region of the South Pacific. The approximate location of Tahiti is indicated by the red circle. Original map from Transits of Venus by RA Proctor, 1874. Brian Greig collection

On the detail from Proctor’s 1769 map note the letters E’, H’ and I’ that denote key locations on the globe for that particular transit. H’ denotes the spot from where the duration of the transit from beginning to end is the shortest from anywhere in the world. That spot is important for the method of durations that had been proposed by Edmond Halley in 1716. In this method the duration of the transit from widely separated locations is compared in order to calculate the distance of the Sun. For the method to give reliable answers the durations have to vary as much as possible and so it was important to make observations from the vicinity of H’.

The point I’ is the spot where the beginning of the transit, ingress, takes place at the latest time while E’ is the spot where the end of the transit or egress takes place at the earliest time. These spots are useful for calculations based on the method suggested by the French astronomer Joseph-Nicolas Delisle in which just one observation of the time of ingress or egress is sufficient to be useful.

As E’, H’and I’ for the 1769 transit all lie near one area of the Pacific, the Astronomer Royal Nevil Maskelyne selected that area to which to send James Cook to make his observations. Fortuitously, the discovery of an island then named King George’s Island in the centre of the area was reported just as Cook joined the Endeavour. Hence Cook and his crew set sail for Tahiti on 25 August 1768.

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