A sextant built by the English instrument maker Matthew Berge. Courtesy Powerhouse Museum
I have received a letter from Jonathan Milne-Fowler, Lieutenant-Commander RANR (Retired) regarding my book Transit of Venus: 1631 to the present. He says, ‘I’ve just finished reading the book on the Transit of Venus and found it well written and informative. That said I did find a couple of points on which I take issue and have written a commentary’. The two points both refer to Captain Cook’s first voyage that was mainly to view the 1769 transit of Venus from Tahiti. Here I just quote his commentary regarding Cook’s navigation and leave the section on the shape of Cook’s ship the Endeavour to another time.
Determining longitude: A commentary by Jonathan Milne-Fowler, Lieutenant-Commander RANR (Retired)
At page 48 of this book the statement is made that Captain James Cook on his first and most famous voyage (1768-1771), using the method of lunar distances to determine longitude, became the first navigator to know his position at all times. This assertion has been made by other authors but it is erroneous.
There is no doubt that James Cook was one of the few navigators at that time capable of performing the complicated calculations required to determine longitude at sea, but this was always subject to the vagaries of the weather allowing the necessary observations to be made. A bank of fog or cloud in the wrong direction, obscuring the horizon or the sun, moon or stars, may frequently frustrate the intentions of navigators intending to obtain a set of observations for the purpose of determining the position of their ship at sea. More than one hundred years after Captain Cook’s voyages ships supplied with chronometers still came to grief because masters had been unable to take sights needed to calculate latitude and longitude.
Among the first navigators able to determine their position at all times were those embarked in Trident submarines equipped with inertial navigation systems. GPS systems now enable navigators to determine their position with a degree of accuracy which was unimaginable to the likes of Captain Cook.
Lieutenant Commander Milne-Fowler is, of course, correct that saying that Cook knew ‘his position at all times’ is a little exaggerated for he could not make observations during times of bad weather. However, as he was the first to utilise the newly developed method of lunar distances to find the longitude of his ship, he had a better idea of his position than any previous navigator on a major voyage of exploration.
When those previous navigators were sailing to an island they would sail well to its east (or west), sail down a longitude line to the right latitude and then sail west (or east) until they found it. If they had estimated incorrectly and they were on the other side of the island to what they had thought, they would be sailing away and in trouble. In contrast, Cook could sail directly to the location he wanted.
The method he was using to find longitude was the method of lunar distances or lunars. To facilitate the use of this method Cook had with him on his ship the Endeavour Nautical Almanacs, newly published by the Astronomer Royal Nevil Maskelyne. These almanacs listed the angular distance of bright stars from the edge of the Moon at various times at Greenwich.
Cook and subsequent navigators using this method measured the angular distance between a star and the Moon with a sextant together with the elevation of the star and the Moon above the horizon. What made the technique difficult to use was that calculation had to be used to make the measured distance comparable with the tabulated distances.
First the navigator had to make the obvious corrections for the distance between the edge of the Moon and its centre and for the zero or index error of the sextant. Then came the tedious business of ‘clearing the distance’, which was applying corrections for parallax, that is working out what the measured lunar distance would have been if made from the centre of the Earth, and correcting for refraction, the shifts in the positions of the Moon and the star due to the bending of their light by the Earth’s atmosphere.
On his second and third voyages Cook had the benefit of the newly developed chronometers, but on his first voyage Cook’s excellent charts of Tahiti, New Zealand and the east coast of Australia were all due to his skill with lunar distances. He may not have known his position at all times, but he knew it when it mattered.
This blog post is simultaneously published on the Transit of Venus website