The Transits Of Venus

Looking up "Venus" in a gazetteer of world place names, one would find there are towns with that name in Florida, Pennsylvania, and Texas, a Venus Bay near Melbourne in Australia, and a Point Venus (actually Pointe Vénus in the local French) in Tahiti. It was from there that James Cook and his companions observed the transit of Venus in 1769. Why they traveled so far and at such expense to witness this celestial event is a matter of some significance. As we will see, Cook's expedition had enormous ramifications for the European settlement of the Pacific.

As it passes across the face of the Sun, Venus appears about one-sixtieth of a degree wide, equivalent to one part in 30 of the solar diameter. This means that it could be observed with the naked eye using a suitable filter, but it is better and safer to use a telescope projecting an image onto a screen. Venus then would look like a circular sunspot taking typically six hours to cross the face of the Sun, depending on which path across the disk (which chord) it follows. No living person has seen such a thing, because none has occurred since 1882.

In principle Venus could have been seen in transit before the invention of the telescope early in the seventeenth century, but no such observation prior to 1600 has been identified. This is hardly surprising since a transit is such an infrequent event. At about the same time as the first telescopes were being turned to the skies by Galileo and his followers, Kepler was evincing the laws of plan etary motion. These laws enabled him, in 1629, to predict that transits of both Mercury and Venus would occur in 1631. It happened that he died in 1630. Even if he had lived, Kepler knew he would not see the Venusian transit, as it was only visible from much further west than Europe, from the Americas and the Pacific. But his prophecy of its occurrence was in itself a triumph. Most European astronomers had no doubt that Kepler was correct about that, despite the lack of visual confirmation, because the predicted transit of Mercury was seen, from Paris in particular, in November 1631.

Kepler did get something wrong though. He thought there would be no more Venusian transits until 1761, whereas in England Jeremiah Horrocks realized that Kepler was mistaken, just in time for the 1639 transit. In principle this event was visible over a wide area, but Horrocks only managed to alert one other observer to his calculations. Between clouds and between church services— it was a Sunday and he was the curate at a small village just north of Liverpool—by projecting an image onto a screen using a small telescope Horrocks glimpsed Venus creeping over the face of the Sun.

0 most gratifying spectacle! The object of so many earnest wishes,

1 perceived a new spot of unusual magnitude, and of a perfectly round form, that had just wholly entered upon the left limb of the Sun, so that the margin of the Sun and spot coincided with each other, forming the angle of contact.

Horrocks was able to monitor the transit for only half an hour before sunset, but his observation of the planet starkly and sedately moving over the disk of the Sun was confirmed by his friend William Crabtree, who lived about 30 miles away, near Manchester.

We saw earlier that total solar eclipse tracks crossed Britain in 1715 and 1724, followed by a hiatus of two centuries. This was just due to chance, in essence. Were the two transits of Venus in 1631 and 1639, followed by a gap of over a century, similar chance occurrences?

The answer is no. Transits of Venus occur as regular as clockwork, following a simple cycle. The transits always occur in pairs separated by 8 years. The Venusian orbit lasts for 8 parts in 13 of a year, an example of a resonance (the technical term is a "commen-surability") in the Solar System. This means that after eight of our orbitsVenus has circuited the Sun 13 times, and returns to more or less the same position relative to us. Due to the precessional movements of both planets the alignment does not repeat precisely. If in one nodal passage Venus happens to be near conjunction, resulting in a transit, eight years later it has shifted such that its apparent path has moved, but it is still within the ecliptic limit and a transit recurs, following a different chord across the Sun. Another eight years later the node has moved beyond the ecliptic limit, and no transit can take place. There is then another century or so of nodal movement before an alignment can occur again.

The clockwork of the heavens is such that transits of Venus occur with separations of 8.0, 121.5, 8.0, and then 105.5 years. Two transits occur spaced by 8 years; then there is a 121.5-year gap before there is another pair at a time of year 6 months away from the first pair; then another 105.5-year gap producing a pair again in the original month. This is because the nodes of the orbit of Venus pass across the Sun in early June at the descending node, and early December at the ascending node. (Note that taking 105.5 away from 121.5 you get 16, which is twice 8, showing the clockwork in action again.)

Including 1639, only five transits of Venus have ever been observed, in December of that year by Horrocks and Crabtree, in June of 1761 and 1769, and in December of 1874 and 1882. None occurred during the twentieth century. Without too much mental exhaustion you should be able to see that we are due to be treated to a repeat performance soon: the first transit of Venus for more than 120 years is scheduled for June 8, 2004.

This forthcoming transit is centered on about 08:20, Universal Time (UT: the standard time for the prime meridian passing through the Greenwich Observatory in London, England). If you plan to be in London on that day, the time on your watch would be an hour later, because Britain will be using summer time (clocks moved forward an hour) in June. To save confusion I will use UT for all times here.

The transit begins when Venus first appears to make contact with the solar disk at 05:15, about an hour after sunrise in Britain, and continues until 11:28, so that the event lasts for more than six hours in all. In Continental Europe, and further to the east, the Sun will have risen earlier and consequently be higher in the sky. One could argue that the optimum location from which to view the transit would be where the Sun is close to overhead at mid-transit, and that would indicate somewhere in the Middle East, such as Saudi Arabia. There is also a higher chance of the sky being clear there than in London. Much further east, such as in Japan, only the onset will be visible, the Sun setting before the transit ends.

For American viewers, the advice must be to head east. If you are enthusiastic (and wealthy) enough, head for Europe or beyond. If you stay in North America, you need to be close to the Atlantic seaboard. For example, Venus will be near mid-transit when the

Sun rises as seen from Boston. As far west as the Mississippi the end of the show will be visible, as the planet slips off the Sun's face soon after it rises over the eastern horizon.

But what if it's cloudy? At least there is not another century to wait. On June 6, 2012 another transit ofVenus will occur. This time you would need to travel to eastern Asia or Australia to get the best view. After that, Venus does not align again with the Sun until December 11, 2117 and December 8, 2125.

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