The Transit of Venus - June 8^th  2004

There are only two planets capable of appearing to us to transit the face of the sun, - Mercury and Venus, as both circle within Earth's orbit. Mercury, being the closest to the sun, averages 13 to 14 transits per century, always either in May or, more often, November,  (it was the first planet to be observed transiting in 1631). Venus, however, has a rather more interesting pattern of behaviour. Its transits occur in pairs 8 years apart, and the period between pairs alternates between 105.5 years and 121.5 years, and occur either in early December or June. The next Venusian transit, the first since 1882, will occur on Monday, June 8^th over much of the world, but apart from a brief encounter of the Venusian disc with the sun's at 5-06pm, not here in NZ. However, if you happen to be in Australia, you're in luck after 5pm there. 

One of Captain James Cook's orders on his first expedition was to observe the transit of Venus, which he did in Tahiti in the winter of 1769, thus contributing immensely to the accurate calculation of the distance between Earth and Sun, the scale of the solar system (using Kepler's Third Law) and navigation generally. Sailing up the east coast of New Zealand as far as the Coromandel Peninsula, he anchored in a bay and observed a transit of Mercury in November of that year. Hence the name Mercury Bay and Mercury Islands. He then sailed through the Colville Channel into the Hauraki Gulf. 

It was Edmund Halley, (whose brilliant comet graced our eastern night sky here night after night 18 years ago, who realized that the careful timing of transits could help accurately measure the Earth-Sun distance. The technique relied on observations made simultaneously from the far corners of the globe, hence Cook's voyage. In turn, this relied on accurate clocks, and from memory, several Harrison Clocks were carried by Cook. (The late Bob Harrison was reputed to be descended from the clockmaker's brother).

A transit of Venus appears as a disc 1/32 the diameter of the sun's disc. As a consolation perhaps, it is easily possible to make 3D glasses and go to Mars via the NASA website and see that planet in sharp, colourful detail, both grand and minute, as the rovers crawl in and out of various craters.

Don Armitage ©

(First published in ‘The Island Breeze’ 9^th June, 2004).