Yet another didactic blog post
VulcanSpirit
Richard & Alison Brunstrom
Sun 26 Jan 2014 11:11
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Another pretentious history lesson coming up.
Switch off now if you don't want to read the fascinating story of why Cook was here at Whitianga in November 1769,
observing the transit of Mercury.
Firstly, here is Cook's Beach taken from
Shakespeare Cliff (named by Cook after the white cliff at Dover). Vulcan Spirit
is moored in Mercury Bay, very near where the Endeavour was for two weeks. The
site of the observation is right at the far end of the beach in the
distance:
 Cook discovered NZ in late October 1769, probably
sighting the Gisborne area first, then moved up the east coast hoping to find
somewhere to observe the transit. But the story
begins much earlier, with the publication in 1716 by Edmond Halley (he of the
comet) of a method for determining the then totally unknown distance from
the earth to the sun using transits of Mercury and Venus. The orbits of the
planets, and their order and relative sizes, had been known for
some time, but the actual size was not. Wildly different guesses were
made about the distance to the sun, all of them far too small. No-one had any
real inkling of the enormity of the solar system.
Venus and Mercury lie closer to the sun than does
the earth, and their orbits are inclined slightly from the plane of earth's
orbit. This results in both planets occasionally and predictably crossing the
face of the sun seen from earth - a transit. Halley realised that these
transits, if carefully observed from a number of places widely spread over the
earth could, using parallax and simple geometry, yield the accurate
distance to the sun and hence the size of the entire solar system. He left
detailed instructions for observing the transits of 1761 & 1769, despite
knowing that he would be long dead by then (he died in 1642). The method
required wordwide observations; the British & French governments
collaborated deeply in achieving Halley's aims, despite being fully at war with
each other in 1761, with eventual great success.
The 1761 observations were marred by poor weather,
so efforts were redoubled for 1769 - and hence Cook's first voyage. Cook
was promoted for the purpose and sent off to observe both the June transit
of Venus and the November transit of Mercury from the South Pacific
(neither were visible from Europe). The observation required the face of the sun
to be watched carefully for several hours (a transit of Venus takes seven hours)
through a telescope, noting exact times as the small planet creeps across
the sun. Cook's observation of Venus in Tahiti was flawless, as was his
observation of the Mercury transit from New Zealand. Cook was also able to
use the known predicted time of the Mercury transit to establish his exact
longitude - before the invention of seagoing clocks by Harrison it was
almost impossible to calculate longitude exactly at sea - and thus create
accurate charts of NZ. As an aside, Harrison's clocks became available for Cook
on his second and third voyages which enabled Cook to produce charts of NZ
waters which were so accurate that some of them were still being used well into
the twentieth century.
Cook's data, combined with others, produced in
1771 a distance to the sun, known as an Astronomical Unit (AU), of
149.7 million kilometres, at the time a revolutionary figure. And an
astonishingly accurate one - the true distance is within a fraction of one
percent of this.
The site of Cook's observation at Whitianga was
assumed to be on Shakespeare Cliff and a monument was erected. Then some bright
spark in the Royal Astronomical Society had the brilliant wheeze of
back-calculating from Cook's original data which had survived, and was able to
demonstrate that the actual site was about a mile away at the other end of the
beach. So another monument was erected on the newly discovered site. Sadly in
the usual NZ fashion a rather dreary uninspiring concrete lump of a thing it is
too:
 (Shakespeare Cliff in the background).
Of course the miserable wet weather doesn't help,
nor does the temporary safety fencing from some sea protection work, but
even having said that it scarcely does justice to the quite amazing feat of
eighteenth century science that it commemorates. Putting the wrong date on the
plaque (it says 10 November; the actual observation took place on the 9th)
doesn't add much either. But at least something is
there.
It is a complete mystery to me how the eighteenth
century astronomers were able to compute the orbits of the planets and
predict transits (those of Mercury occur at intervals of 7, 13 & 33 years,
those of Venus at 8 & 120 years). I am in awe.
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