|(c) 2004 Fred Espenak|
A planet crossing in front of the star is called a transit. In our Solar System, the only planets that can transit the sun (as seen from Earth) are Venus and Mercury, because they are the only two planets that can come between the Earth and the Sun.
Venus orbits the sun once every 225 days. But since the Earth is moving, too, it takes Venus nearly 19 months to make a complete trip around the sun as seen from the Earth. So, every 19 months, Venus will pass between the Earth and the sun. But Venus's orbit and the Earth's orbit are tilted slightly, and the vast majority of the time, Venus will pass slightly above or slightly below the sun as seen from the Earth. Only when the Earth and Venus are at exactly the right points in their orbits will Venus pass directly in front of the sun. Due to the vagaries of planetary orbits, Venus transits come in a pattern: transit, wait 105.5 years, transit, wait 8 years, transit, wait 121.5 years, transit, wait 8 years, transit, wait 105.5 years, transit, and so on.
The previous transit of Venus was in June 2004. I went to see that transit from my mom's house near Lancaster, PA. The transit was underway at sunrise, and we watched from a Wal-Mart parking lot as the sun rose through fog and clouds - but we saw it.
Many web sites have lots of information on where and how to see the transit of Venus. Rather than reproduce all that information, I'll put a list of links below (including to a Sousa march!). You don't need to go anywhere special, but you do need to be extremely careful. Venus only covers a small part of the sun, and so you must be sure to protect your eyes. Sunglasses (or even multiple pairs of sunglasses) are not safe - infrared radiation can still get through and seriously damage your eyes in seconds, and you won't feel your retina being cooked (your retina has no pain sensors). There will be many groups and schools safely watching the transit, so if you don't have safe equipment, find one of those groups.
I know it is a pain to make a trek, but this is almost certainly the last chance any of us will ever have to see a transit of Venus. (I'm sure a few young'uns may live to see the next transit, but I bet they won't remember this one.)
In the next few days, we'll talk about some of the science behind past and future transits.
Links to Venus transit information and fun stuff:
- Transitofvenus.org This website is very thorough and contains tons of information about where, when, and how to view the transit, plus lots of educational materials, links, history, etc. A very well-done site!
- NASA's 2012 Transit of Venus page Fred Espenak's detailed information about the transit.
- Sky and Telescope's transit information from the editors of the popular astronomical magazine
- Astronomy Magazine's Venus Transit information from the editors of another popular astronomical magazine
- The Transit of Venus phone app (free!) for iPhone and Android. Includes information on the transit as well as a global effort to determine the size of the Solar System from the transit
- The Transit of Venus March by John Phillip Sousa, performed by the Virginia Grand Military Band. This march was inspired by the December 1882 transit of Venus. More information from Wikipedia.
- Captain Cook and the Transit of Venus The story of how the June 1769 transit of Venus launched one of Captain Cook's voyages of exploration.