On the evening of June 5th in North America the planets will align and Venus will be visible as a shadow on the disc of the Sun. This rare event happens in pairs spaced eight years apart with over a century between pairs, making this event a once-in-a-lifetime opportunity for anyone who (like me) missed the 2004 transit. NASA has a handy diagram showing when and where this will be visible.
As interesting as it is to look at something passing between the Sun and Earth, as the past weekend's hoopla over the eclipse of the Sun by Earth's moon, transits (the passing directly between Earth and the Sun) of Venus have a greater historical significance - until the late 20th century they were the best chance humanity had to accurately measure the distance between our planet and its star, the Astronomical Unit (AU). Because of the large distances in the solar system, the AU is the most practical unit to describe the distances between planets.
How does it work? Much in the same way that the separation between the eyes allows humans to perceive distance, separation between observation points on Earth's surface allow the distance to stellar objects to be calculated by careful timing of the beginning and end of a transit as realized by Edmond Halley (of comet fame) in the 17th century. Teams of scientists spread to points around the globe a hundred years after his 1677 discovery in an attempt to time the transits of 1761 and 1769, braving seas, mountains, and wildlife in the name of scientific discovery. Not all of them made it back to their homes with their findings.
Unfortunately for the 18th-century scientists, an odd effect of Earth's atmosphere made precise measurements of the transit impossible: As the black shadow of Venus neared the apparent edge of the Sun the darkness of space seemed to reach into the bright disc to grab the planet. We now know that this is caused by the distortion of light as it reaches Earth's turbulent atmosphere.
Undeterred, scientists the world over once again set out in 1874 and 1882, the last pair of transits, to once again attempt accurate measurements of the AU. Once again this "black drop" effect stymied their efforts.
While the recent advent of radar has allowed scientists to very accurately measure distances in the solar system, there is once again an effort to make the measurements using accurate timing of Venus' transit. This time around astronomers are enlisting the help of the public and their camera-bearing smartphones to collect as many timings as possible in the hopes of overcoming the black drop and realizing Halley's discovery.
If you decide to take advantage of this opportunity to view another planet passing between our star and Earth, make sure to take some simple safety precautions - looking directly at the sun can cause permanent eye damage and there are very simple methods for viewing the transit with common items. Here are a few proven methods.
References:
1) Backhaus, U. The Transit of Venus. American Journal of Physics 2012, Vol. 80 Issue 5, p. 361.
2) NASA page explaining Venus transits and their recorded history: http://eclipse.gsfc.nasa.gov/OH/transit12.html (accessed 22 May 2012)
3) Obligatory Wikipedia reference: http://en.wikipedia.org/wiki/Transit_of_Venus
4) Want to help out? http://www.transitofvenus.org has everything you'll need to know.
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