On 11 November 2019 scores of New Zealand astronomers trained their telescopes toward the sun to witness a relatively rare celestial event – the Transit of Mercury. Some 250 years earlier, almost to the day, Captain James Cook did exactly the same thing at what is now the Coromandel’s Cook’s Beach, to ascertain its longitude.

Astronomers have been aware of transits for centuries. The event involves a planet – seen as a tiny, silhouetted dot – moving across the face of the sun. From our perspective on Earth, only two planets can be observed in transit – Mercury and Venus – because they orbit between us and our sun.
Being closer to the sun, they also orbit more quickly: Mercury completes a lap every 88 days; Venus, further out, takes 225 days. We complete one every 365.25 days. While these circuits occur with clockwork regularity, we don’t get to see a transit very often because the ‘planes’ of the respective orbits rarely ‘line up’ perfectly. Mercury’s next ‘visible’ transit will be in 2032 – Venus’ in 2117.


Captain James Cook(1728-1779). Nathaniel Dance. BHC2628

As it happened, 1769 was a serendipitous year for visible transits – the Venus event was scheduled for June – and a few months later, on November 9, Mercury’s transit would also be visible. Essentially, Cook was tasked with using them to gain a better understanding of our solar system, and to refine a method for determining longitude.
For mathematicians and astronomers in the 18th century, the precision of this orbital merry-go-round offered a unique opportunity to solve what were then some of the scientific world’s most enduring mysteries: what’s the distance to the sun – in actual miles? How big is the solar system? While they knew six planets orbited the Sun (Uranus, Neptune and Pluto hadn’t yet been discovered), they only had a rough idea of the relative spacing between those planets.
In 1663 Scottish mathematician James Gregory  proposed an ingenious solution using relatively simple trigonometry. If a transit of Venus (or Mercury) could be observed from various locations on Earth – at precisely the same time – it would be possible to calculate the miles to the sun by using the distance difference between the locations on earth (the ‘solar parallax’). Having a glass of wine handy helps to get your head around this…
In 1716 astronomer Edmund Halley (he of comet fame) expanded on Gregory’s theory, pointing out that next transits of Venus would occur in 1761 and 1769. Sadly, Halley never got to enjoy the fruits of his arithmetical wizardry – he died in 1742 – but he galvanised the scientific community into action.
Venus’ 1761 transit involved 120 observers from nine nations in 62 locations – but the exercise was unsuccessful, mainly because of poor weather. The Royal Society of London was stung by this failure and with its customary modesty declared that the British “were inferior to no nation on earth, ancient or modern.” It was determined to make amends with the 1769 transit.
It selected three specific points for observing the transit: the North Cape at the Arctic tip of Norway, Fort Churchill at Hudson Bay, Canada – and a suitable island in the South Pacific. King George III signed off the plan, ordered the Navy to provide ships and allocated £4,000 in funding.
Tahiti – it was decided – would be an ideal South Pacific location, particularly because it was one of the few islands in the South Pacific with established, accurate coordinates. HMS Endeavour would take the astronomers and scientists to Tahiti – James Cook would be her skipper.
He was well aware that if the 1769 expedition failed, every astronomer on Earth would be dead before the next opportunity to observe a transit – in 1874. Talk about pressure!

On June 3, 1769 Cook – together with naturalist Joseph Banks, astronomer Charles Green and Swedish naturalist Daniel Solander – recorded the event at a makeshift observatory on Tahiti, at a point which came to be known as ‘Fort Venus.’
Cook recorded the event in his journal:
“This day prov’d as favourable to our purpose as we could wish, not a Clowd 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 Suns 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 my self, and we differ’d from one another in observeing the times of the Contacts much more than could be expected. Mr Greens Telescope and mine were of the same Magnifying power but that of Dr was greater than ours.”
Despite everything, the Royal Society was disappointed with the data. Because of the relatively unsophisticated telescopes and the ‘haze’ around Venus’ perimeter, it was difficult to determine the precise moment it began crossing/exiting the solar disc. This made for sloppy arithmetic.

Even with the fuzzy results though, astronomers/mathematicians calculated that “the mean distance from the Earth to the Sun was 93,726,900 English miles.” Modern technology shows the true distance is 92,955,000 miles (149,597,000km) – an error of less than one percent. Not too shabby for 18th century astronomers immersed in ‘island time’!
The Venus Mission accomplished, Cook turned his ship southwest to New Zealand – for the pending transit of Mercury. Determining longitude with unreliable clocks was still a tricky task in the late 1700s. Cook hoped the Mercury transit would help to pinpoint New Zealand’s location on the planet.

Transit of Mercury
He anchored at the then unnamed Mercury Bay and on November 3 wrote in his journal:
“My reason for putting in here were the hopes of discovering a good harbour and the desire I had of being in some convenient place to observe the transit of Mercury which happens on the 9th instant and will be wholly visible here if the day is clear. If we should be so fortunate as to obtain this observation the longitude of this place and country will thereby be very accurately determined.”
A granite cairn fitted with a plaque stands on Cook’s Beach to mark the spot at which this observation is believed to have taken place. Well, it used to. During a ferocious storm in July 2018 the beach was badly eroded and the cairn toppled into the waves. Fortunately, it wasn’t damaged and has been re-erected nearby, in a spot a little further from the menacing waves.
The plaque reads: Near this spot on 10 November 1769 James Cook and Charles Green observed the Transit of Mercury to determine the longitude of the bay.

Note: The date on the plaque is correct by today’s date-line convention, which was not used in Cook’s time. The transit occurred on 9 November as per Cook’s Gregorian calendar. Over the next four months he went on to draw a very accurate map of New Zealand, charting most of the coastline.
Perhaps less well-known about Cook’s visit is the ‘secret mission’ he was given. He had been instructed by the Admiralty – once his Venus Transit was complete – to continue exploring the South Pacific in the quest to find the legendary Terra Australis Incognita or ‘unknown land of the South’.
Many 18th century scientists believed such a continent had to exist to balance the great land masses of the northern hemisphere. This search was in vain – but the great navigator’s ‘cooking with planets’ left an enduring legacy.