—Image courtesy NASA
If you've been following the exploits of NASA's Kepler spacecraft, you probably already know that the mission finds new planets using what's called the transit method.
In short, Kepler stares at a bunch of stars and records when there's a periodic dip in a star's light caused by an object passing in front. With enough data and some careful followup work, scientists can tell whether the passing object is a planet orbiting the star.
So far, Kepler has confirmed 15 new planets using transits, and an additional 1,200 planetary candidates were recently announced.
And next year, people around the world will be able to watch a transit of an Earth-size planet with their own eyes.
OK, fine, I admit—the planet in question is our own Venus. But that's still pretty cool, because Venus transits are exceedingly rare.
Due to its slightly tilted orbit with respect to Earth's, Venus crosses between Earth and the sun on an oddly spaced cycle.
Transits come in pairs spaced eight years apart, but the time between pairs is 122 years, then 105 years. Due to this pattern, only six Venus transits have been seen since the invention of the telescope.
A Venus transit in 2004 was the first of a pair. Before that, the most recent transit had been in 1882—which means no living people on Earth had seen a Venus transit when the 2004 event occurred.
The next one will happen in June 2012, and it'll be visible from only certain parts of the globe (see map). When it ends, there won't be another Venus transit until 2117.
What follows is an edited transcript of a conversation I had last week with National Geographic grantee Jay Pasachoff, an astronomer at Williams College in Massachusetts and an expert on eclipses and transits.
So what does a Venus transit look like from Earth?
The great [German] astronomer Johannes Kepler in the 17th century predicted that there'd be a transit of Venus in 1631, but nobody had any idea at the time how big Venus would look, because they didn't know how big it was or how far it was away. In fact, the transit in 1631 wasn't visible from Europe, and we didn't have telescopes at that time in California, so it took until 1639 before anybody saw a transit of Venus. When it was seen, it was just a huge surprise that there was a black dot about 1/30 the diameter of the sun that moved across the surface of the sun.
Venus can look about the same size as a big sunspot, but it looks perfectly round and regular, whereas a sunspot has dark inner regions and lighter outer regions, the umbra and penumbra. In fact, one can also see transits of Mercury across the face of sun. But Mercury is both smaller than Venus and farther away from Earth—it's only 1/30 the area of Venus—and it looks less impressive, unless you're looking with a really good telescope or spacecraft.
When I saw a transit of Venus in 2004, I used my 500mm telephoto lens to take pictures that show a beautiful black dot, whereas in 2006 we went to Hawaii to observe a transit of Mercury, and when I took a picture with the same lens, the dot of Mercury was barely visible and was much smaller than a sunspot that was on the sun at the time.
Why study Venus transits?
In the original studies, Edmond Halley [of Halley's comet fame] figured out a way of calculating how far the sun is away from Earth, and therefore how big the solar system is, by studying a transit of Venus. Measuring the size of the solar system used to be the most important activity in astronomy in the 18th and 19th centuries, so hundreds of expeditions went all over the world to make observations of Venus transits.
[Editor's note: Capt. James Cook was funded by England's Royal Society to sail to Tahiti and observe a Venus transit in 1769, collecting valuable data for astronomers back home, who were not in the viewing path. The 2012 transit will also be visible in the South Pacific, and travel agencies are already pitching trips to Tahiti's black-sand beaches to "follow in Cook's footsteps" during the celestial event ... I think a field trip is in order!
Now we know the distance to the sun through other methods, so that's no longer an important reason to study transits. But there are some contemporary uses ... including calibrating the spacecraft that are observing the sun, because here's a perfectly black disk outside Earth's atmosphere silhouetted against the sun.
|Capt. Cook's drawings of a Venus transit.|
—Image courtesy NASA
It's also helpful for the [Kepler] people doing those exoplanet transit measurements to see the real situation close up and see the details of what's happening.
Where do you plan to watch the 2012 Venus transit?
We plan to observe from the solar observatory at Haleakala on Maui at 10,000 feet [3,054 meters] and also from the Sacramento Peak observatory in New Mexico at an altitude of 9,000 feet [2,743 meters]. I myself plan to be in Hawaii.
We have some new scientific capabilities since 2004 for measuring the atmosphere in different spectral bands, which could be ways to help people who are studying exoplanet transits to look for signs of life. Overall, we're trying to get as complete a set of data as possible from the year 2012.
There are also some new spacecraft aloft that will help us be more precise than we were in 2004. The Solar and Heliospheric Observatory is not in the path, but the Solar Dynamics Observatory will be, as well as a Belgian craft called PROBA2 and the Japanese Hinode spacecraft.
How can amateur astronomers get involved in the 2012 event?
In 2004 the European Southern Observatory coordinated a big push all over the world to collect citizen science observations and use Halley's old method from almost 200 years earlier to measure Earth's distance from the sun, and they got a pretty good value. I'm sure something like that will be done in 2012, but the organization is still under way.
What's the best way to watch the transit safely?
First you get one of these solar filters that you can get for about a dollar [from optical distributors]. ... You can get little bits of special mylar that filter the sun properly and get them put into glasses. Then if you look through it just with your eye ... you can actually see a black dot on the sun with your own eye, with no other aid. You can see a transit of Venus just by filtering down the sun to a safe brightness level.
If you have binoculars or a telescope, get some of the filter material or use welder's glass #13 and put that over the front of the binoculars—or get a solar filter for a small telescope—and you can see more detail in the shape of Venus. Or you can use binoculars or a telescope to project an image onto the ground or on a piece of cardboard and look down at the image rather than up at the sun ... and you'll be able to see the silhouette of Venus go across the sun. It doesn't happen very rapidly, the transit takes about six hours.
I do like to look myself, see it with my own eyes, and just this morning I was talking with a colleague about getting some old telescopes from the 18th and 19th centuries and looking through them, so we're planning on having a few of those set up outside the big telescope dome on Haleakala.