Wednesday, June 6, 2012

The Transit of Venus in history


Watched it on the web. Many crashes. Slightly disappointing. No, it's the history that excites me. There's a heap of stuff on the web about how the Transit of Venus was used for measuring the heavens in the 18th century.  Here's the best I found.  First,  Stuart Clark on the Guardian science blog.  He says that in scale and ambition, plans to record the transit of Venus were the 18th century equivalent of the Large Hadron Collider.

There's a book just brought out by the historian Andrea Wulf: Chasing Venus: The Race to Measure the Heavens, which documents the collaboration required to observe and track the 1761 and 1769 transits.

She discussed the book on the Guardian Science Weekly podcast of 25th May.

And a few days ago she gave a lecture to the Royal Society. This link allows you to see the video, or download the audio.

The maths: how the Transit of Venus tells us our distance from the Sun

Imagine two different people, one on each pole of the Earth, viewing the transit of Venus. The person on the North pole sees Venus following one path across the Sun. The person on the South pole sees Venus follow a slightly higher path, one that's shifted a little to the north. Because we see the Sun as a circle, these two different paths will have different lengths. Halley proposed that an easy way to measure the difference between the lengths of these two paths would be to time the transits, using the four phases of the transit — the first, second, third, and fourth contacts — as indicators. With the two different paths known, the distance between the Earth and the Sun can be pretty easily calculated using trigonometry and Kepler's third law of planetary motion.

If you want a bit more, including just a few equations,  this explainer is for you, it's from the Exploratorium museum in San Francisco.

Guardian Science blog: Transit of Venus as it happened

The Guardian, as we used to say, and I still do, is definitely where it's at. Trawl through their live blog, dead now of course, for snippets of history, images of the transit, and comments as it occurred.

Edmond Halley, portrait by Thomas Murray, c. 1687 (detail)
This is Edmond Halley, the English astronomer and geophysicist.  A true giant. From beyond the grave, he it was who launched the world expeditions to view the transit. In 1716, he instructed the next generation of astronomers: This is what you have to do. Fifty years from now, in 1761, there will be a Transit of Venus, and you all have to work together. It's all in the Guardian blog, scroll down for this image.

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