Transit of Venus, 2012

Today, starting at 6:04 pm, you will have a chance to witness an event that will not happen again for more than a century. Venus will pass directly between us and the Sun, an event called a “transit.” If you can safely view the Sun (more on that in a minute), you will see a tiny black dot slowly move across its face. It will continue to transit well after the Sun has set. Alaska and Hawaii are the only U.S. states that will witness the entire event.

Time lapse photographs of the June 8, 2004 transit taken from a solar observatory in the Canary Islands. The black spots are Venus as it transited the Sun. Courtesy, the National Solar Observatory. 

Even though Venus is in an orbit closer to the Sun than the Earth’s, its orbit is tilted ever so slightly so that it’s usually never directly between us and the Sun. But every century or so, our orbital planes line up and we have a chance to witness two transits, eight years apart. Then we have to wait another century or so to see it again. The last Venus transit happened in 2004, and could be seen in progress at sunrise. I was lucky enough to see it. If you missed it, you have another chance today.

You might think that this event is interesting but otherwise useless. Until this century, though, countries spent enormous amounts of money and risked ships and lives to see and precisely record the transit. Why? Because it enabled us to measure the distance of the Earth to the Sun, what astronomers call the Astronomical Unit (A.U.). This distance, 93 million miles (150 million kilometers), is the ruler we use to measure all other distances in the solar system. Without it, we couldn’t send astronauts to the Moon or rovers to Mars.

Johannes Kepler first predicted that a transit would occur in 1631, but it wasn’t until the second 1639 event (eight years later) that Jeremiah Horrocks and William Crabtree actually witnessed one. Edmund Halley, of Halley’s Comet fame, suggested that by placing observers at different points on the Earth and carefully timing the event, one could triangulate and calculate the distance to Venus and the Sun. In response, all of the major European powers sent observers to the far corners of the Earth to record the 1761 and 1769 events and later, the 1874 and 1882 events. The travels and trials of these scientists as they went off to Siberia, Newfoundland, Madagascar, and other exotic locations are the stuff of adventure novels. You may recognize at least two names in the annals of this history – Captain James Cook observed the 1769 transit from Tahiti, and Charles Mason and Jeremiah Dixon (who later surveyed PA’s southern border, the Mason-Dixon Line) went to Cape Town, South Africa for the 1761 transit. At least one play, appropriately titled “The Transit of Venus” by Maureen Hunter, loosely documents the decade-long adventure of Guillaume Le Gentil, who lost his wife and home and was declared dead before returning from his expedition!

 Jeremiah Horrocks making the first observation of a transit of Venus in 1639. By Eyre Crowe. Courtesy Wikipedia.

How can you see this marvelous event? The safest way is to watch a broadcast from a solar telescope. There is an excellent educational website, http://www.transitofvenus.org/, that has loads of interesting information about the event and will broadcast live pictures. IF it’s clear, a 50-50 prospect around here in June, there are a few safe ways to view the event and I will again point you to that website for the best and safest ways. You must remember though, accidents while attempting to view the Sun with telescopes or binoculars often result in permanent damage or blindness. You cannot blink fast enough to protect your eyes. Sunglasses will not protect you. Black film (negatives) will not protect you. Even welding glass may not protect you because some wavelengths you cannot see may penetrate and still cause damage. Your retina has no pain sensors, so you wouldn’t know it was burning it until it was too late.

So be safe and enjoy the show! The next feature won’t start until December, 2117.


Originally published in the Bloomsburg Press Enterprise on June 5, 2012.