Frosty Drew Observatory Presents: The Transit of Venus 2012

Frosty Drew Observatory Presents: The Transit of Venus 2012

On June 5th, 2012, an astronomical phenomenon that will never again be seen by anyone alive today will occur. This transit involves the planet Venus appearing to cross the face of the Sun. Although in the past a Venus transit was a very important occurrence for astronomical measurement, in 2012 it will be an event to be observed, the result of the Earth, Venus and the Sun being in a perfect alignment. This will not occur again until December 11th, 2117.

The cycle of Venus transits is one of the most unusual in Nature: Two occur within the span of eight years; then there is a wait of 105 ½ years. Two again occur within eight years, and there is then a wait of 121 ½ years. Once again, two happen within eight years. . . and the cycle continues. Every 243 years, four transits of Venus are visible. This does defy logic, as it would seem they should happen every time the planet Venus passes between the Earth and the Sun, but the alignment is so special that that doesn’t happen. It is like the idea that a solar eclipse should be visible with every New Moon, each time the Moon is in the same direction as the Sun, but we are all aware that that is not the case.

The first person to predict a transit of Venus was Johannes Kepler, a mathematician who noted in 1627 that Venus would appear to pass the face of the Sun in 1631; unfortunately, no one was able to view it. Kepler’s mathematics didn’t predict a follow-up transit in 1639, but it was noted and observed by two people, Jeremiah Horrox and his friend William Crabtree.

In the late 17th century, Edmund Halley, of comet fame, upon witnessing the planet Mercury undergo a transit, realized that this occurrence could in fact create the geometry that would allow scientists to determine the distance to the Earth and the Sun. This distance, a yardstick for understanding the distances to all the known planets at that time, Mercury through Saturn, is referred to as the astronomical unit. Johannes Kepler had created laws noting the distances of the planets to the Sun were all proportional to the time they took to revolve around the Sun; unfortunately, he had no good way to determine the actual number for the astronomical unit. Edmund Halley realized watching a planet cross the face of the Sun was a possibility in doing so. Using Mercury, however, would be difficult, Halley believed, because Mercury is such a small planet, and is quite far from us, so its size as seen from here would be difficult to track and time. Venus, however, is over twice the size of Mercury, and about twice as close to us, so its observed ball would be a bit easier to observe. He also noted that two Venus transits were coming in the 18th century, in 1761 and 1769.

To that end, expeditions went all over the world, to places that would give the best views possible, in order to determine the exact figure for the astronomical unit. Unfortunately, the results were not as accurate as hoped; but, luckily, there was another set of transits in the 19th century, in 1874 and 1882. Again, no accurate results were recorded.

Today, the transit of Venus is merely a curiosity, to be observed only because it can never again be seen by anyone living today.

- Francine Jackson

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