Astronomy is one of those fields where it just doesn’t pay to procrastinate. The last time Earthlings could spot the planet Venus crossing the yellow disk of the sun was in 2004. But if you don’t take a look this time around, here’s when you’ll get your next opportunity: December 10th of 2117.
One hundred and five years, six months, and a handful of days, says John Gianforte, who teaches astronomy at Granite State College. He says transits come in pairs – two roughly eight years apart, and then, because Earth’s orbit and Venus’s orbit aren’t exactly aligned, well, that’s when you start waiting.
Astronomers didn’t realize there was anything to wait for until Johannes Kepler started writing about transits in the 17th century. But they soon realized the transit could tell them something important.
“Kepler, and a little bit later, Edmund Halley, of Halley’s Comet fame, discovered they could use observations of those transits to calculate the earth-Sun distance," Gianforte explains. That average distance between the earth and the Sun is called the astronomical unit, or AU. That’s kind of like the astronomical yardstick that all astronomers and scientists use when describing the distances within our solar system.”
With each successive transit scientists would fan out across the globe to measure Venus’s movements and refine the Astronomical Unit. By the 19th century transits had become something of a cultural event, too. John Phillips Sousa marked the 1882 transit with – you guessed it – the Transit of Venus March.
Today scientists don’t need transits to calculate distances in the solar system – they use radar and other methods for far more accurate measurements. But planets outside our solar system are too far away for radar. So we follow their transits to learn what these planets are like.
“Scientists using specialized equipment can detect a very slight dimming of the parent star as the planet passes in front of its star," Gianforte says. "We can analyze the light coming from the star and we can determine – we can measure its atmospheric composition, how much of an atmosphere it has, and how big the planet is.”
There are a few scientific projects planned for this transit – you can participate in one through a smartphone app which matches your transit readings with your GPS coordinates and feeds them into a database. But this transit is mostly about observation. And that’s the best part – John Gianforte says, assuming the weather holds, New Hampshire will get a good look at Venus this time around – even better than the view he got in 2004.
“In 2004, we saw a good portion of the transit, but when the sun rose, Venus was close to exiting the sun’s disk. This time we will get to see the beginning of the transit, the most interesting time of the transit.”
Several groups, including the McAuliffe-Shepard Discovery Center and the Astronomical Society of Northern New England, are setting up viewing stations in New Hampshire. Grab your telescopes, your special solar viewing goggles, hope for clear skies and you won’t have to wait for 2117 to get a glimpse of Venus as it cruises across the sun.