On Tuesday afternoon, June 5th, everyone in the United States will have a chance to witness one of the rarest celestial phenomena known: a "transit of Venus."
Such an event occurs when the planet Venus passes almost exactly between the Earth and the Sun, and they are incredibly rare. The United States Naval Observatory provides the following fascinating historical story about Venus transits.
Since first predicted by the German mathematician and astronomer Johannes Kepler in the 17th century, only six transits of Venus have been observed. Weather permitting, this will be the seventh.
Transits of Venus occur at regular intervals that repeat over a 243-year period. Intervals between successive transits are 8 years, 105.5 years, 8 years, and 120.5 years. The next transit of Venus won’t occur until December 11, 2117, and it will not be visible from most of the U.S.
Kepler predicted the transit of December 7, 1631, but died before the event occurred. The next transit, on December 4, 1639, was observed by only two individuals, Jeremiah Horrocks and William Crabtree, from England.
In 1677 Edmond Halley (of comet fame) observed a transit of Mercury from St. Helena Island and realized that such events, if observed from many widely-spaced sites, could provide a geometric measure of the scale of the solar system. His work led to several far-flung expeditions to observe the Venus transits of June 6, 1761 and June 3, 1769. One of the British expeditions to the latter transit was led by Captain James Cook. Results from these expeditions were mixed, but enough experience was gained to attempt observations of the next series in the 19th century.
The transits of December 9, 1874, and December 6, 1882, were met with an armada of scientific expeditions equipped with state-of-the-art astronomical instruments. The U.S. Congress funded and outfitted eight separate expeditions for each event and placed overall scientific direction of these teams under the command of the U.S. Naval Observatory (USNO). Once again the results were inconclusive, but many of the instruments from these expeditions are still in the observatory’s possession.
The 20th century saw no transits of Venus; the next one occurred on June 8, 2004. By this time the size of the solar system had been well-established, so observing the transit became more of an historical event than a scientific one.
This year’s transit will begin about three hours before sunset here in Detroit, at 6:04 p.m. Eastern Daylight Time. It will occur earlier in the day and at a higher altitude as one moves farther west, but no place in the “lower 48” will see the event in its entirety. Residents of Alaska, Hawai’i, and the U.S. Pacific Territories are the only Americans who will see the complete event.
Observing the transit will not require a telescope; the disc of Venus is large enough to be seen with the unaided eye. However, just as with a solar eclipse, extreme precaution must be taken when observing the event, or permanent eye damage and/or blindness will occur. You can try to see the transit using the same pinhole projection method used with solar eclipses: punch a small hole with smooth edges in one piece of paper, stand with the sun at your back and project the sunlight through the hole onto the second piece of paper. You can "focus" the projection of the sun by moving the pieces of paper closer or farther away. What will you see? Venus transiting the sun will look like a "dot" slowly passing across the solar disk.
You can also observe the transit with some of the local science centers, planetariums, or amateur astronomy clubs, as they will have the proper equipment to enjoy this rare event (check ahead to make sure they are having transit observing parties). Or, better yet, several organizations will be live streaming the transit.
You can try these links to see it online as it happens: