By Duncan Steel 02/06/2019

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On June 3rd occurs the 250th anniversary of the transit of Venus across the face of the Sun, the observation of which was the prime purpose behind the expedition of HM Bark Endeavour to the South Pacific, under the command of Lieutenant James Cook. Following the measurements of the transit made by Cook and the mission’s scientists in Tahiti, the Endeavour sailed west, leading to the claiming of New Zealand and then eastern Australia by the British. 

It happens that I have a particular – some would say peculiar – interest in calendars. This manifests itself in various ways, one of which is finding an excuse to have a celebratory drink on just about any day of the year. Last week I wrote about why 29th May is a special date, last Wednesday having been the centenary of the total solar eclipse of 1919, observations of which led to the confirmation of Einstein’s General Theory of Relativity as being a more-complete gravitational model than Newton’s theory.

The 3rd of June is another notable day, astronomically-speaking, and this year it is especially important, for the history of New Zealand. Two-hundred-and-fifty years ago, on 3rd June 1769, James Cook and the astronomers/other scientists on board the expedition of HM Bark Endeavour to the South Pacific conducted the first aim of that mission: as part of an international collaboration (with various European nations sending teams to other geographical locations) they observed the transit of Venus across the face of the Sun from Tahiti. Other British expeditions observed it from Norway and Canada, whilst King George III himself watched the event from his palace at Richmond, southwest of London.

Following the successful timing of the celestial conjunction, Cook opened his sealed orders, telling him to sail westwards across the Pacific from Tahiti to search for the suspected Terra Australis Incognita (the Unknown Southern Land/Continent). In consequence Cook and his men arrived off the coast of New Zealand a few months later, first setting foot in what is now known as Tūranganui-a-Kiwa/Poverty Bay on 7th October 1769. Subsequently the Endeavour spent some months in NZ waters mapping the coastline, the history of that episode being well known, including the claiming of the islands in the name of his king. The expedition then headed further west to the east coast of Australia, similarly mapping and claiming that coastline.

This year, then, marks the semiquincentenary* of Cook’s first arrival in the Pacific. From a limited astronomical perspective, the two pivotal dates are 3rd June (the transit of Venus), and 9th November (the transit of Mercury observed from what is now named Cook’s Beach in Mercury Bay on the Coromandel Peninsula). The differing importance of these transits is often misunderstood. The transit of Venus was significant because it could be used to determine the distance between the Earth and the Sun, and specifically what was needed was an accurate measurement of the duration of the transit from a known location (Tahiti).

(*A 250th anniversary may alternatively be termed a sestercentennial, a bicenquinquagenary, or simply a quarter-millennial event.)

The idea behind the observations of the transit of Venus in 1769 was that timings of the phases as Venus crossed the face of the Sun would render a precise determination of the length of the line/chord across its near-circular face. At different terrestrial latitudes distinct chords would be followed, as shown by the two straight lines, which are separated due to parallax. The analysis of their separation can then render the Earth-Venus-Sun distances. Cook’s expedition was planned so as to be as far south as possible, with observers in the northern hemisphere measuring other chords. At position/time I, Venus is just touching the exterior of the solar disk; at position II, Venus is just fully-immersed in the disk; at III, Venus is just touching the inside rim of the disk as it is leaving; and at position/time IV it has just left the disk. M denotes the mid-point of the chord.

The transit of Mercury was useful in distinct way, it being simply a celestial ‘clock’ in that the pre-computed time of any part of the transit (when Mercury entered face of the Sun, or when it left) would enable the scientists on Cook’s expedition to determine the longitude from which they were observing, and therefore have a reference point for their survey of New Zealand. Without that, all they would know with certitude would be the latitude of NZ (easily found from the altitudes in the night sky that charted stars attained), whereas the longitude was a far more difficult thing to measure. What was needed was a knowledge of the local solar time compared to the time at a reference longitude (the Greenwich meridian), and when Cook left England in 1768 there were no mechanical clocks available that would keep time on crossing the oceans. On his second and third voyages Cook did have such clocks, making longitude determinations much simpler. This story is told in the best-selling book Longitude by Dava Sobel.

The post I am writing now is about the event that occurred 250 years ago, the transit of Venus, so let me get back to transits of that variety. Transits of Venus (ToV) occur in pairs spaced by eight years, with no other ToV then occurring for more than a century. The idea that they could be used for measuring the Earth-Sun distance, and consequently giving a capability to prepare more-accurate navigational tables for seamen, had been around for some decades before the ToV of 1761 [sic], with Edmund Halley (of comet fame) having explained in 1716 how this could be achieved. In the event a range of mistakes and unfortunate events led to the 1761 opportunity being lost (as I explain in my book Eclipse), and so there was a determination that the anticipated ToV on 3rd June 1769 should be properly observed; hence Cook’s famous expedition.

In reality the timings of the 1769 ToV did not lead to the expected boons in astronomy and navigation, for a variety of reasons including difficulties in making those timings with sufficient precision due to the so-called Black Drop Effect.

One of the unexpected problems met by astronomers trying to watch the transit of Venus on 3rd June 1769 was the Black Drop Effect: as Venus entered the face of the Sun, the image of the planet appeared elongated, as depicted in this drawing.

Since then there have been ToVs in 1874 and 1882, and most recently in 2004 and 2012. I was really delighted to show those recent transits to hundreds of schoolchildren, using a small telescope to project an image of the Sun onto a screen so they could watch the dark circle of Venus slowly marching across the solar surface: in 2004 at Burnside Primary School in Adelaide, South Australia, and in 2012 at Canberra College in the Australian Capital Territory. (Yes, where my sons were pupils.)

If you would like to learn more about the other expeditions in 1769 (and 1761), linked here is a piece I wrote for The Conversation in Australia in 2012.

These recent ToV episodes did not pass unnoticed here in NZ. Awa Press in Wellington published a rather nice (indeed, award-winning) little book entitled Transit of Venus: How a Rare Astronomical Alignment Changed the World, and I was very happy to contribute a chapter about Science in Cook’s Time. Back in 2004 Radio New Zealand put together a series of six public lectures linked to the ToV, again with one from me on that subject, with some prominent kiwis contributing others, and those are still available as podcasts.

It you missed those transits of Venus in 2004/2012 then, sorry, you are out of luck, with no more being due until 11th December 2117, and then 8th December 2125. Mark those in your great-grandchildren’s diaries.

The difficulties with timing the transit of Venus in 1769 may have been linked to how Venus appeared during that event, as shown in this drawing. Around the black silhouette of the planet a bright ring was apparent, due to the extensive atmosphere of the planet refracting sunlight. Back 250 years ago, astronomers did not know that Venus is perpetually shrouded by a dense atmosphere, the pressure at the surface being almost a hundred times as high as on Earth. This bright ring seems to have contributed to the Black Drop Effect, limiting the accuracy with which the entry and exit of Venus onto the solar disk could be timed.

Of course, in New Zealand 3rd June 2019 is also a public holiday (the Queen’s [official] Birthday, marked annually in NZ on the first Monday in June), so that people here have another reason to celebrate. I have another excuse again to toast this date (June 3rd), however: it is the birthday of my much-loved partner, Jan; so Happy Birthday to her!


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