What is a transit?
Just like the Moon will sometimes pass between the Earth and the Sun (causing a solar eclipse), so too do the planets on inferior orbits. In other words, Mercury and Venus, which orbit closer than the Earth, will sometimes appear to cross the disk of the Sun. Since the planets are farther away than the Moon and orbits are not perfectly aligned, these events are much rarer than solar eclipses.
|Transits are rare since the orbit planes and planet positions do not always line up.|
Venus transited the Sun back in June of 2004, here's what that looked like:
|The June 2004 transit of Venus|
The cycle continues 8 years later on June 2012. After that, though, you'd have to wait 105.5 years for the next transit: on December 2117, and then again 8 years later on December 2125. After that it's 121.5 years until the next one and then the cycle repeats (121.5, 8, 105.5, 8). Add up the numbers and you'll see this is a 243-year cycle. It is unlikely that anyone reading this blog today in 2012 will be alive on 2117 given the current human lifespan, so you do NOT want to miss this event.
Note that as the planet crosses the Sun it blocks out a tiny portion of the stellar disk. We're close enough to the Sun (93 million miles) that we can see the transit easily. It turns out that we can apply this same technique, though, to far more distant stars. For distant stars we can't see the disk of the star or the planet (it all just looks like a tiny point of light), but we can see how the brightness of the star changes with time. If the planet is big enough, when it crosses the star it will make the star appear just a little bit dimmer. With careful monitoring of a star's light, we can spot this change in brightness and, if periodic, infer that a planet is in the system. This can then be used to determine properties like the orbit of the planet and the planet size. This is called the transit method for extrasolar planet searches and is the way in which the Kepler spacecraft has found over a thousand candidates exoplanets.
|A planet transits in front of a distant star.|
Where can I watch the transit of Venus?
The transit will be visible from the majority of the planet, however the best place to look is in the middle of the Pacific ocean or in East Asia. People in North America can see the start of the event until the Sun sets, people in Europe and the Middle East can see the end of the event after sunrise.
Here in Chile, things don't look to promising but we have a plan (see below).
|June 2012 transit visibility|
Note that it is dangerous to look directly at the Sun! You want to make sure you have proper gear: eclipse viewing glasses, solar filters for telescopes, or project the image of the Sun to a piece of paper. You can create a simple pinhole camera with a few sheets of paper and a pin (to make the hole). Here is a PDF file with some instructions on how to do that. You can find more information on how to safely observe the Sun here.
One of the best ways to experience the transit, though, is with a locally hosted star party so you can share good equipment. Here is a good website to get additional information on the transit. You can also check out this website to find out the exact times for the transit event at any location on Earth.
Why are transits so important?
Since the invention of the telescope only 6 transits of Venus have been observed. A 7th, the first one predicted using the Laws of Gravitation, took place in 1631, but was not visible from Europe and went unobserved. The 2012 transit will be the second in this new era of rapid global communications (the prior one to 2004 was in 1882). Besides the historical importance of this rare event, there's also useful science that can be carried out, such as studying the atmosphere of Venus or characterizing the transit for extrasolar planet searches. One of the coolest (and simplest) things one can do with the transit, however, is calculate the distance between the Sun and the Earth- the Astronomical Unit (AU, see my post on distance here). The precise timing of the transit in various places on the Earth, combined with some very simple geometry, can be used to estimate this distance. The basic idea is that of parallax, just like for the definition of the parsec. Once you have that down, you get the scale of the solar system and can figure out how far away the different planets are.
UPDATE: I describe the basic math and the required measurements here.
Here is a neat video about the transit, determining the distance to the Sun, and how this all relates to the search for extrasolar planets with facilities like the Kepler Space Telescope:
What about Chile?
Our own team of Chilean astronomers will be hosting a viewing event and outreach activities (including star gazing as there's a partial lunar eclipse on June 4). This will be at Easter Island since the Chilean mainland will not be able to see the transit. I encourage you to join a viewing party, either ours in the exotic Easter Island or one close to your home. You do not want to miss this!
Details for the Easter Island event can be found here (Spanish) or here (English).
page) with anyone you know that might be travelling to Easter Island!