By Duncan Steel 10/11/2019

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There will be a transit of Mercury – the planet Mercury will pass across the face of the Sun – taking place at sunrise in New Zealand on Tuesday, 12th November. It was by observing such an event 250 years ago that James Cook and his scientist colleagues were able to determine the longitude of NZ, and so put these islands in their proper place on the global map. 

Notwithstanding the fact that Polynesians had been in Aotearoa for about half a millennium before the arrival of HMS Endeavour (or HM Bark Endeavour) in these islands in October 1769, the mechanism whereby what was to become New Zealand was placed on maps of the world is often misunderstood. It was by observing the transit of Mercury across the face of the Sun on 9 November 1769 (from what is now Cook’s Beach in Mercury Bay on the eastern coast of the Coromandel) that James Cook and his colleagues were able to determine the longitude of that spot relative to the Royal Greenwich Observatory in London, and from that register the locations of all other points he mapped around NZ’s coasts.

The first aim of Cook’s expedition of 1768-71 was to observe the transit of Venus (ToV) across the solar disk on 3 June 1769 from Tahiti. The need for this observation was quite different from the utility of the transit of Mercury (ToM) five months later. The expedition led by Cook was part of a large international effort to time the duration of the ToV as seen from different locations. ToVs are rare, coming in pairs eight years apart with more than a century between them. The opportunity in 1761 had been missed for various reasons (wars and clouds among them), and the ToV in 1769 was the last occurrence until the pair in 1874 and 1882.

Britain sent expeditions to many different locations, such as Canada, with other European nations covering places such as the north of Norway, Russia, Baja California, and almost everywhere in between. People often imagine that Cook went to Tahiti, the ‘other side of the globe’, because the ToV could not be seen from Europe, but that is a nonsense: it was observed by King George III from Richmond, west of London.

The pivotal thing about Tahiti was that it is a long way south, and so the range of latitudes over which the ToV was timed was extended. June 3rd being less than three weeks before the summer solstice, it should be obvious to all that the ToV would have been visible if one went far enough north, into the land of the Midnight Sun. Cook’s job was to head as far south as feasible, with Tahiti having been chosen.

I have written about the ToV of 1769 in particular in my book Eclipse (various editions published in 1999, 2000 and 2002), but there are many more-extensive (and, I would admit, likely better) accounts published elsewhere. One of the chapters of a book published by Awa Press in Wellington and entitled Transit of Venus: How a Rare Astronomical Alignment Changed the World was penned by me, and a series of radio lectures on this and peripheral subjects recorded in 2004 (there were ToVs in that year and 2012) is still available on the Radio NZ website.

The fundamental concept behind the ToV observations was that the parallax obtained by having widely-separated (in latitude) teams watching the transit would enable the distance from the Earth to Venus to be determined more accurately than hitherto possible, and from that the distance between the Earth and the Sun would follow. With such a measurement it would be feasible then to calculate more-precise navigational tables, with a concomitant reduction in loss of life and ships at sea resulting.

(Parallax is the effect that comes from observing something at two slightly different angles, then being able to work out its distance away. A simple example is to hold up vertically one finger at arm’s length, and then blink between one eye and the other; your finger appears to jump sideways compared to a distant background. With both eyes open, the parallax delivered by your two eyes enables your brain to determine, more-or-less, how far away any object is, although only out to perhaps five or ten metres. If you don’t think parallax is important, try walking up and down a flight of stairs with one eye closed.)

In the end all those ToV observations – expensive both in terms of money and human life – did not lead to any great improvements in astronomical and navigational tables, but that is another story (which would need to wait another couple of years until Cook and his men made it back to England). Our story here is about the transit of Mercury, and that stems from the second aim of Cook’s expedition. After the ToV was watched, Cook opened his sealed orders that instructed him to sail west and search for Terra Australis Incognita, the anticipated but unknown great southern continent. As a result he ran into NZ first in October 1769. Recognising that this was a substantial land mass, he needed to locate it on the map so that others might be able to sail here again. And that is where the transit of Mercury comes in.

The transit of Venus was observed, then, in an attempt to improve knowledge of the distance to the Sun, and thence navigational and astronomical tables used for many purposes. Mercury is too far from Earth to be used in the same way, through timing a Mercurian transit. But a transit by Mercury is of great value in a rather different way.



Astronomers knew when the 1769 ToV was going to take place several decades in advance, leading to the capability to plan and execute missions like that of Cook and the rest of the numerous observers who set out in 1768 and 1769 to be ready for 3rd June. The same is true for transits of Mercury: these occur 13 or 14 times a century, with the last being in 2016 and the next (after that tomorrow, on 12th November 2019) being in 2032; the next time viewers in NZ get a chance is not until 2039.

Since these ToMs occur at times that can be pre-calculated and tabulated, they provide a type of celestial clock. At the time of Cook’s first voyage, he had no mechanical clock that would keep accurate time on a ship, though when he came back to the Pacific in 1772-75, and again in 1776-79, he had one of John Harrison’s clocks aboard.

Seafarers can determine their position in terms of latitude easily enough, by observing the altitudes of bright stars above the horizon. Longitude, though, is a different beast. This had been a long-term quest, a story told memorably in Dava Sobel’s best-selling little book entitled Longitude.

Nowadays we use as our time references electronic clocks, supplanting even the excellent mechanical clocks made by Harrison and his followers. For real high-precision uses we employ things like the time signals from GPS satellites, or atomic clocks operated in physics laboratories. Back in the eighteenth and nineteenth centuries no such things were available, obviously. Even through the first decades of the twentieth century, still timekeeping was the province of the astronomical observatory.

In Cook’s days the actual location of the Sun in the sky compared to the overhead meridian was used (hence the terms “a.m.” and “p.m.”). This renders what is known as Local Solar Time; that is, the time at one locality according to the Sun in the sky. Later, in order to have a more uniform length of day, a construct was invented known as the Mean Sun: this is a theoretical object calculated so as to average out the effects of the non-circularity of Earth’s orbit, and the north-south movement of the sunrise position on the horizon across the year. The Mean Sun can be up to eight solar diameters (or sixteen minutes of time) away from the real Sun in the sky. (And now you can appreciate why I have repeatedly written “in the sky”!)

It is the Mean Sun that gave us Greenwich Mean Time (or GMT): that is, the time according to the Mean Sun on the Greenwich Meridian, the zero of longitude. As an aside, one really should not use the term GMT anymore, because it is not employed in earnest. Our international standard is Coordinated Universal Time (UTC), which is based on clocks in physics laboratories, not astronomical observatories. Every so often a leap second (which has absolutely nothing to do with leap years) is inserted, to correct for the Earth’s spin very slowly decelerating under tidal drag (mainly due to the Moon). In essence one might say that UTC and GMT get a little out of step, and a leap second is used in order to keep them within 0.9 seconds of each other.

An image of the planet Mercury obtained by NASA’s MESSENGER spaceprobe in 2008.


Let’s get back to Local Solar Time. Cook knew what that was simply by observing when the Sun crossed the meridian, just as his sailors worked in accord with the sunrise and sunset. As he (or his astronomers) saw the Sun crossing their meridian as they stood on the beach in Mercury Bay, they set their pendulum clocks – which would keep time on land, but not on a ship tossed by waves at sea – to noon. Each day gave a test of how those clocks were doing, in terms of keeping Local Solar Time.

However, the Local Solar Time in Mercury Bay was disconnected from the time back in London, because the travelling ship had no clock that would function on a sea voyage. The difference in the Local Solar Time for two different locations depends on their longitudes, just as New Zealand Standard Time is 12 hours ahead of the time in London (UTC). Cook knew what his Local Solar Time was, that was simple, just from observing the Sun; but he did not know where he was.

What Cook needed was some absolute reckoning of the time, and that’s what the transit of Mercury offered. His astronomical tables showed when it would occur if one were stood on the Greenwich Meridian. By observing when it took place from what is now called (in English) Mercury Bay, or Te-Whanganui-a-Hei in te reo Māori, and seeing how far that was from his Local Solar Time determined by watching the Sun cross the overhead meridian, and comparing the times using a pendulum clock set up on the beach, he would be able to determine his longitude.

And that’s how the transit of Mercury on 9 November 1769 enabled New Zealand to be properly located on maps of the world.



Footnote: There is a wonderful special exhibition on currently at the Auckland War Memorial Museum, entitled Voyage to Aotearoa: Tupaia and the Endeavour. It does a good job of explaining and exploring the similarities between ocean navigational methods as practiced by the Polynesians who were the first humans to arrive in these islands, and the methods applied by European seafarers like Cook.

There is just one problem I would point out here. Throughout this exhibition, which deals only with Cook’s first Pacific voyage, he is labelled as “Captain Cook”. He wasn’t, at that time. I love alliteration as much as anyone else, but during the 1768-71 voyage he was simply Lieutenant James Cook, only having been advanced even to that rank so as to give him sufficient seniority to command the Endeavour’s party.

One of the placards in the Voyage to Aotearoa exhibition at Auckland Museum, incorrectly designating James Cook as “Captain Cook”. During his first Pacific voyage, he wasn’t. (But the exhibition is great, so don’t let me put you off visiting it!)



Information on the Transit of Mercury, 12th November 2019 (NZDT)

If you would like to see details of the times at which the transit of Mercury begins and ends, here is the best website I know about. Fred Espenak makes available all sorts of information about eclipses and transits (which are really just a type of eclipse) on that site. Just note that the times are given in UTC, and we are 13 hours ahead of that on NZ Daylight Time; thus the website says 11th November, but it is already 12th November here in NZ. Other wonderful web sources concerning this event include this NASA page, and this one or this one at Various ‘star parties’ are planned in New Zealand.



Addendum, 11th November 18:23: I really should have written and indeed emphasized above that you should not look at the Sun directly, either with the naked eye or using an instrument such as a telescope or pair of binoculars. Experienced observers know how to stop down an aperture or use suitable filters. For anyone else, a good way to see Mercury in transit is to use a small telescope to project an image onto a screen, as shown in this lovely nineteenth-century sketch:

0 Responses to “How NZ was put on world maps using a transit of Mercury”

  • Did Cook record the local date of the transit as 12 hours ahead or behind Greenwich time? One of Cook’s maps lists the NZ east coast longitude at 183W (of Greenwich), hinting he was accumulating latitude longitude as he sailed westward from UK and probably adjusting local time incrementally as well. Does this mean Oct 8 1769 was recorded by Cook 12 hours behind Greenwich, so that the 250th anniversary of his landing may have been commemorated 24 hours too early?

    • Hi Mike, and thanks for the interesting comment.

      First of all, I think (actually, I am pretty sure) that you meant to write ‘longitude’ where you had ‘latitude’. Thus in your comment I have struck-through ‘latitude’ and inserted ‘longitude’ in bold type. If I have erred there, please let me know!

      Second, on your question (the first sentence): off-hand I do not know, and I am snowed under with various things at the moment so I am unable to look it up. Surely it would be well-known, though. One person to ask/place to look would be Wayne Orchiston’s paper(s) about the 1769 transits. I’d need to reply similarly to the question posed in your final sentence.

      Third, on that sentence where you wrote: “as he sailed westward from UK”… Well, he would have known the longitude of Tahiti from previous determinations (that’s how he found his way there), but also from observations whilst there (by Green, Cook, Solander, Banks, others). In Tahiti they could determine Local Solar Time, but also ‘absolute time’ from some celestial phenomenon that had been pre-calculated. The transit of Venus is the obvious example! However, there are other possibilities, such as (a) A lunar eclipse (though the uncertainty in the timings due to ‘fuzziness’ mean that the ToV rather more-distinct ingress and egress times can be more-precisely observed); and (b) Eclipse phenomena of the Galilean satellites of Jupiter (e.g. the instant when Io goes behind the jovian disk could be known in advance). My discussion here is somewhat trivial, in that there will be others who know far more than me about this; but I just wanted to point you/others in a suitable direction.

      Thus Cook’s party would have known the longitude of Tahiti (Point/Fort Venus), and reckoning from there to NZ would have been more-accurate than any reckoning from England by sea.

      On your final sentence/question, I noted above that I do not have an immediate answer. One thing I would say is this: any date is a moot point in that in that era there was no international standard of longitude (the Greenwich Meridian was never agreed on by almost-everyone until the conference in Washington DC in 1884, and in fact France has never agreed to it, I think!). That same conference also defined the International Date Line, and of course that is not a single meridian at 180 degrees from Greenwich, but rather there are some deviations such as that around the Chatham Islands, plus it seems that every decade or so one of the Pacific island nations decides it wants to jump to the other side/a different day.

      On that score: hey, it’s all good fun, and interesting to discuss. Just never read Umberto Eco’s The Island of the Day Before.

      Final point: given the present Brexit nonsense, I note that the UK did not really exist in 1769. I think that it came into being essentially on 1st January 1801, but I stand to be corrected by constitutional lawyers.