By Carol Van Strum
On Tuesday, June 5, will occur one of the rarest of astronomical events: a transit of the planet Venus across the face of the Sun. For people living today, June 5 is the last Venus transit in our lifetime; the next will not happen until December 11, 2117 and December 8, 2125. Weather permitting, people in North America should be able to see at least part of Venus’s passage across the solar disk on June 5. This is an event not to miss!
Two excellent, complementary books together provide a roadmap to the history and significance of this rare occasion:
Transit of Venus: 1631 to the Present, by Nick Lomb, 2011. Powerhouse Museum, paperback, $24.95
The Day the World Discovered the Sun: An Extraordinary Story of Scientific Adventure and the Race to Track the Transit of Venus, by Mark Anderson, 2012. Da Capo Press, hardcover, $26.00
A transit occurs when a planet passes between Earth and the Sun across the Earth’s orbital plane (ecliptic). Because Venus and Mercury are the only planets with orbits closer to the Sun than Earth’s, they are the only planets that ever pass between us and the Sun. Mercury’s orbit being smaller and closer to the Sun, the innermost planet transits more frequently than Venus, but is harder to see. Venus transits are much rarer and usually occur in pairs eight years apart, separated by roughly 122 years. The June 5 transit is the second of this century’s pair (the first occurred on June 8, 2004).
Venus transits occur in eight-year pairs because for every eight orbits of Earth, Venus takes thirteen circuits around the Sun before crossing Earth’s orbit at the right moment again. (Just to confuse things, since Venus actually reaches the eight-year rendezvous 2.45 days earlier than Earth, occasionally Venus will arrive too early for conjunction and make only one transit in a century.) Calculating planetary orbits and the transit times of Venus was one of the great achievements of 17th Century science.