November 9, the 244th
Anniversary of Cook’s
landing in New Zealand to
Observe Transit of Mercury
After exploring the islands of Tahiti, Captain Cook
sailed off searching looking for the rumored ‘great
Southern continent’. While in Tahiti, Cook and his
astronomer Charles Green observed the
transit of
Venus
across the face of the sun, Venus looking
like a tiny dot. The Royal Astronomical Society
knew of a second transit coming the same year,
the 1769 transit of Mercury
. The Royal Society
of London provided funding to send about ten
dozen astronomers throughout the world to
measure the transits using the relatively new
invention, the sextant.
Stormy seas caused damage to the Endeavour,
so Cook headed for the land discovered by Dutch
seafarer Abel Tasman in 1642. On 6 October,
1769, cabin boy Nicholas Young spotted the hills
of the New Zealand’s North Island and sailed
for land.
Over the next few months Cook mapped the
entire coastline, providing the first-ever ‘outline’
of New Zealand – and establishing that it was not
the Great Southern Continent. His charts proved
so accurate that some were still being used in the
20th century. When the predicted 9 November
transit day neared, Cook pulled ashore at
Te
Whanganui-o-Hei
, the bay on the north tip of the
Coromandel Peninsula.
Cook writes in his diary: “Thursday, 9th. Variable
light breezes and clear weather. As soon as it
This stamp, issued in 1997 by the NZ Post Office to celebrate the
navigational work of Captain James Cook, depicts his map of New
Zealand and his sextant. Sextants were used to determine a ship’s
latitude. They work by measuring the angle between two objects.
The observer sights along a half-silvered mirror at the horizon and
manipulates a second mirror until an image of the sun, moon, or a
star is directed onto the half-silvered mirror and overlays the horizon,
then the angle between the horizon and the celestial object is read
off the scale.
On 9 November 1769 James Cook and Charles Green determined the
latitude and longitude of Mercury Bay during their observation of the
transit of Mercury.
was daylight the natives
began to bring off
Mackrell, and more than
we well know what to do
with; notwithstanding I
order’d all they brought
to be purchased in order
to encourage them in this
kind of Traffick. At 8, Mr.
Green and I went on shore
with our Instruments to
observe the Transit of
Mercury.”
The ‘time’ was known
by the position of
the sun in the sky.
Many mathematical factors came into play
to geometrically determine the accurate
measurement of the distance between the Earth
and the sun. The combined measurements and
known locations of all observers were used to
determine the distance, which differs only 1/8th
of one percent from today’s measurements.
The exact sighting location was inaccurately
memorialised in 1919 by a memorial around
Shakespeare Cliff, but the actual site was the
eastern end of Cook’s Beach near the Purangi
estuary, where there is now also a small memorial
plinth.
And that is how Mercury Bay got its name.
