Telescopes around the world — and in space — will be aimed at the Sun today to catch the rare astronomical event of a transit of Venus. Our planetary neighbour crosses in front of the Sun (from the viewpoint of Earth, that is) only twice every 120 years or so. The last time this happened was in 2004; it won’t happen again until 2117.
This has professional and amateur astronomers alike jumping up and down with excitement, getting out their welding goggles and pinhole cameras to watch the event from their back yards. For tips on when and how you might be able to spot the transit, see TransitofVenus.org.
If you are trapped beneath clouds or in a part of the world that won’t be able to see it, there are live webcam streams being hosted by NASA starting at about 2:45 p.m. Pacific time (5:45 Eastern time), and by the Slooh telescope network, with ‘pre-game’ commentary starting at 2:30 p.m. Pacific time (5:30 p.m. Eastern time). The entire transit will take about seven hours.
Although much of the excitement around this transit is aimed at public-outreach efforts aiming to get the public interested in astronomy, there is also some real science to be done. Astronomer and transit enthusiast Jay Pasachoff of Williams College in Williamstown, Massachusetts, wrote in Nature a few weeks ago about the science goals. These include trying to ‘ground truth’ detections of exoplanets circling far-off stars, and getting a once-in-a-lifetime measure of the climate of Venus. Although there’s a probe in orbit around Venus, this gets a very limited view of the planet’s climate: it can basically see the atmosphere in only one spot at a time, making it impossible to know whether the changes it sees are due to changes in space or changes in time. A simultaneous measure of the climate along an entire pole-to-pole line is measurable only today.
Check back here for updates.
UPDATE 1 (4:30pm Pacific time)
As predicted, Venus started its path across the Sun at around 3 p.m. Pacific time. Some fantastic photos have been released already, including this spectacular one from the space-bound Solar Dynamic Observatory (SDO) of Venus approaching the Sun.
SDO’s observations are going to be used to calibrate a couple of instruments on the observatory, including confirming where exactly the ‘north pole’ of the Sun lies in its view, and measuring the ‘point spread function’ of its telescope — how much light leaks from one pixel into others around it. SDO should also be able to take its own measure of how much oxygen is in Venus’s atmosphere.
The weather is looking perfect for observations from Hawaii — one of the best spots for viewing this transit and the source of NASA’s main webcast.
UPDATE 2: 6 June
Hawaii didn’t just have clear skies for the transit — it had the perfect conditions, known as ‘coronal skies’. “I held my thumb up to block out the Sun, in the traditional test carried out by solar astronomers, and the sky was the same blue right up to the edge of my finger. That shows how low the scattering in the sky was,” says Pasachoff. It was, however, very windy, he adds, so they took short exposures to lock in details before pictures were blurred by motion, and they will have a lot of post-processing to do.
Positive reports of good measurements have already come in from Arizona, Udaipur, Japan and, for the last half of the transit, from Australia. The site in the Marquesas in the South Seas had instrument problems, says Pasachoff. He has had no report yet from New Mexico, where astronomers were using a new carbon dioxide filter to study Venus’s atmosphere.
Perhaps one of the most novel observations comes from the International Space Station, where astronaut Don Pettit snapped a photo of the transit from the window (see picture).
The Hubble space telescope, which, like our own eyes was too sensitive to be pointed directly at the Sun, was aiming to gather information about Venus from the sunlight reflected from Tycho crater on the Moon. Likewise, the European Space Agency’s Very Large Telescope in Chile was aimed at the Moon to catch these reflections. There are no reports from either as yet. David Ehrenreich of French National Centre for Scientific Research in Grenoble says that they will get the Hubble data by the end of the week, though analysis will take months.
All in all, “we had a wonderful day, even better than expected or hoped for,” says Pasachoff. “At this point, it is not possible to say if anything surprising turned up.”
“We are exhausted after spending almost seven hours outside, but we feel great, and we can’t wait to look at our observations in detail.”