Bucket List for Backyard Stargazers #6: The Transit of Venus

We come to #6 on our “Bucket List for Backyard Stargazers”… the passage of Venus across the solar disk as seen from the Earth, also called the transit of Venus.

While not as striking as a solar eclipse, a transit of Venus is far more rare.   It’s happened just seven times since the invention of the telescope more than 400 years ago.  The next transit in June 2012 will be our last chance to see this remarkable event.  There won’t be another until December 2117.

Here’s what you need to know to cross the transit of Venus off your celestial bucket list…

Like a solar eclipse, a transit occurs when Venus passes between Earth and the Sun.  And like an eclipse, the transit requires careful alignment of the Sun, Earth, and Venus.

As seen from Earth, Venus usually passed over or under the Sun every 584 days, on average.

But the geometry of the orbits of Earth and Venus, and the period of the planets’ orbits cause Venus to pass in front of the Sun at well-defined intervals of 121.5 and 101.5 years, in either June or December.  And the transits occur in pairs separated by eight years.  Right now, we’re between transits.  The last occurred on June 8, 2004.  The next is on June 6, 2012.  The last transits came on December of 1874 and December 1882.

A transit of Venus was once a huge deal for astronomers.  In the early 18th century, Edmond Halley determined a way to measure the distance from the Earth to the Sun by timing the transit of Venus from widely separated parts of the Earth.  Once this distance was known, the distances to other planets could be determined through Kepler’s Laws (which you’ll learn in an upcoming article).  The transits were so important that most advanced nations sent astronomers around to world to measure the events of 1761 and 1769.

(The persistent and unlucky Guillaume le Gentil, who you met in a recent article, was one of hundreds of scientists dispatched to observe the transit of 1761).

The transit of Venus in 1761 yielded few conclusive results from hundreds of attempted measurements.  So the pressure was on for 1769.  And it all worked out… the transit of 1769 was measured precisely by, among others, the team led by one Lieutenant James Cook, RN, who witnessed the event from Tahiti before sailing on to claim Australia for England.  Astronomers used Cook’s measurements to calculate a distance to the Earth of 150 million kilometers, close to the now-accepted value of 149,597,870.7 kilometers.

It’s the history, and the rarity of the event, that makes the transit of Venus such a compelling sight.

And it’s a beautiful sight, too, even for the casual stargazer.

The transit unfolds in four stages.  First, the leading edge of the planet contacts the Sun.  Then the trailing edge, which is hard to time exactly because of the “black drop effect” which bleeds darkness from the limb of the planet as it moves onto the solar disk.  The same two stages reverse themselves as the planet leaves the solar disk.   The whole event takes 3-6 hours, compared to the scant few minutes of a solar eclipse.

During the transit, the black disk of Venus, just 33x smaller than the solar disk, blocks enough light to measurably decrease the Sun’s brightness.  NASA’s Kepler observatory, in fact, uses this same idea… a transiting planet blocking light from its home star… to look for Earth-like planets around nearby stars.  Astronomers will use the 2012 transit of Venus to test new measurement techniques to find extra-solar planets such as those used by Kepler.

The 2012 transit begins at 22:09 UT (GMT) on June 5, and ends at 04:50 UT on June 6.  The western Pacific, including most of Australia and New Zealand can see the entire transit.  Western Africa, Spain and Portugal, and eastern South America will not see the transit because it occurs when the sun is set.  And the rest of the world can see some of the transit after the Sun rises or before it sets.

The maps below shows where the 2012 transit of Venus is visible (click to enlarge)…

To see the transit, you’ll need a safe solar filter.  It’s visible without a telescope, but you’ll get a better view if you watch the event with a scope and a solar filter.

So start planning.  After June 6, 2012, you won’t get another chance to see this rare and beautiful event…

Postscript: Mercury also transits the Sun.  But because it’s closer to the Sun than Venus, Mercury transits much more frequently, about 13-14 times every hundred years.  The planet is also smaller and farther, which makes the transit far harder and less impressive to observe.