“So, what’s the big deal with the Venus transit?”

As I write this, an astronomical event is occurring which will not be visible from the Earth again for another 105 years: the transit of the Sun by Venus. One might wonder what the hubbub is about. After all, what is the big deal about a tiny little dot crawling across the Sun?

Well, for astronomers, it is a very big deal, because, at this very moment, it is providing them with valuable data which will improve their ability to detect planets orbiting other stars.  Observing the brief dimming of light from distant stars as planets transit them is one of the principal tools for detecting extrasolar planets, and astronomers are getting a front-seat view of such an event right at their doorstep. The data collected regarding the dimming, as well as the spectroscopic data of the sunlight passing through the upper reaches of the Venusian atmosphere, gives astronomers a baseline for evaluating observations of such events around other stars.

Such Venusian transits also played a valuable role to astronomers in the 18th and 19th centuries. Why? Because it provided a valuable yardstick for measuring the dimensions of the solar system. By collecting detailed measurements of the timing of the start and end of the transit at multiple points around the world (including an expedition to Tahiti by Captain Cook), astronomers could use basic trigonometry to calculate the distances between Earth, Venus, and the Sun. These global expeditions were the first multinational “big science” projects, the precursors to the International Space Station and the Large Hadron Collider.

By way of illustration of how such measurements could help astronomers determine the dimensions of the solar system, I should relate something that happened earlier this evening during the early stages of the current transit. I was watching a video stream of a Google+ Hangout, in which several astronomers (including Phil Plait) and science bloggers participated. Several of them provided video feeds of their own observations of the transit, so I was able to simultaneously see views of the transit from locations all over North America (and later, Australia). In another window, I could observe a NASA video feed from an observatory in Hawaii. At the point at which the disc of Venus had half-way entered the Sun’s disk as seen in, say, Tennessee, only about a quarter of Venus had encroached upon the sun a seen from the West coast, and far less than that in Hawaii. This was parallax in action, and it was this phenomenon which enabled astronomers in the 18th and 19th century to figure out the distances involved.

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About Glen Mark Martin

MCSE-Messaging. Exchange Administrator at the University of Texas at Austin. Unrepentant armchair physicist.
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