On the morning of Tuesday, November 16th, Mercury will pass across the Sun’s disk. If the evening’s clouds dissipate by morning—as they have often done so far this Spring, then we’ll see it from Hamilton, New Zealand starting about 10:20 NZDT. But just barely: Mercury clips the edge of the solar disk, its edge a fraction of an arc second inside the solar limb, and the whole show will be over in less than 1 hour. Observers in Canada and the northern US will see up to a 6-arc second gap between Mercury and the limb at maximum transit.

Will anyone be watching? Transits are notable now for their rarity: only Mercury and Venus can transit the Sun, and a particular, precise orbital alignment is required. Mercury’s orbit is inclined 7° and Venus’ a bit more than 3° to the Earth’s, so transits cannot occur except near at their ascending or descending nodes. Mercury’s always occur in May and November, Venus’ in June and December. Transits come in pairs, for example, every eight years on average for Mercury, and 14 in all this century. I recall the ’73 event in the late afternoon from my parent’s house in Cincinnati. Not at all spectacular or dramatic like solar eclipses, a telescope—equipped for safe solar viewing—only shows the planet’s disk as a small dot slowly tracing a line across the Sun. Venus’ transits are far rarer events: the last was in 1882, and following a gap of 105 or 122 years, two transits occur 8 years apart. The next two will be in 2004 and 2012.

Great scientific significance was attached to transits in the 18th century. In 1716, Edmund Halley realized that a transit could determine the “solar parallax” and thus the scale of the solar system. Confusingly, “solar parallax” refers to the size of the Earth as seen from the Sun, or more precisely, the arc subtended by the Earth’s equatorial radius. The geometrical argument is complex and rarely explained in texts, plus you need to invoke Kepler's third law to get the relative distances. Just imagine two observers separated on Earth (say, at the north and south poles): they see different apparent paths of the planet across the Sun, with their transits lasting differing durations. Essentially, these observers project a ruler of known size against the Sun. Multiplying this ruler literally measures the Sun, and knowing its angular size, the distance is readily computed. Which times or angles are measured, as well as the observers’ exact locations, the Earth’s rotation and motion about the Sun, all complicate matters in practice. It was widely recognized that Venus’ transits should be preferable to Mercury’s because the angles measured were larger, and the timings of the ingress and egress would be more precise. First put to the test for the 1761 transit, those results were judged unsatisfactory due to the unexpected “black-drop” effect.

In 1769, the Royal Society mounted the largest scientific expedition of its day in sending Captain James Cook and the HMS Endeavour to Tahiti to observe the July transit of Venus. Successful in these observations, Cook’s orders then instructed him to sail southwest, reaching New Zealand in October. Cook anchored in a sheltered bay on the Coromandel Peninsula to observe a transit of Mercury. This event played a pivotal role in the country’s history: it literally put New Zealand on the map. The longitude was determined from transit timings, the latitude from the Sun’s altitude: the first scientific astronomical observations on New Zealand soil were made at what is now Cooks Beach. Upon departure, Cook claimed the bay for England, naming it Mercury Bay.

Cooks Beach (background) on Mercury Bay, near Whitianga, New Zealand. At 7 o'clock in the morning on Novemeber 9th, Cook and his astronomer Charles Green set up their telescopes adjacent to the ships anchorage 300 m west of the mouth of the river (just visible at far left in the background). Green recorded Mercury's ingress while Cook determined the Sun's  altitude. Just after noon, both timed the egress succussfully. They determined the spot to be 36° 48' 5.2" South, 176° 03.5' East. The picture was taken from the moument where Queen Elizabeth II celebrated the 200th anniversary in 1969.

Will the 16th’s transit be as memorable? Doubtful: since the astronomical unit, the fundamental scale of the solar system and beyond, is measured by far more accurate means now, transits are of no scientific value. And too much is going on around us, with the All Blacks in rugby and the America’s Cup 2000 underway in Auckland. But this will be the last transit of the millennium for sure, with the next not until 2003. Anyway, I hope it’s clear: the show will surely be worth the price of admission and an interesting opportunity to recollect this intersection of astronomical and colonial history.

Moondark is written by Doug Miller and published on the web, in the Delmarva Star Gazers'Star Gazer News and in the Delaware Astronomical Society's FOCUS. Please address comments and suggestions to dmiller@udel.edu. This document was last revised on 24 October '99. All text and images copyright © 1999 Douglas C. Miller, All Rights Reserved. This material may not be reproduced in any form without prior permission.