ShareThisBy CARRIE PEYTON DAHLBERG
Tuesday, November 21, 2006
When Mercury passes between the Earth and sun on Wednesday, the view in countless telescopes will be more than just the latest slow samba in the dance of planets.
It will offer a hint of history, when the sight of planets moving across the sun helped confirm theories of the solar system and inspired massive expeditions to track it from every land.
Today, planetary "transits" are less epoch-making but still eye-catching.
"It's relatively rare," said Chris Taylor, a physics and astronomy professor at California State University, Sacramento. "What you'll see is a little black dot that creeps across the sun over the course of about five hours."
Essentially, transits are like eclipses; an object is passing between the Earth and the sun. But the planets are so distant that the scale is far different _ a pinprick of darkness in a telescope for Mercury, and a tiny blob to the unaided (but sun-shielded) eye for Venus.
Since Earth is the third planet from the sun, only those two closer in will make a transit, as seen from Earth.
After Wednesday when it will be visible through specially filtered telescopes and only from some locations, there will be only a dozen more chances to see a Mercury transit this century, the next in 2016.
"It's just one of those things that gives you a little perspective. When you look, you will see that Mercury is really, really, really tiny and the sun is really, really, really big," said Claia Bryja, an astronomy instructor at San Francisco City College.
Bryja knows her transits. She flew to Egypt and trekked to a Bedouin encampment in 2004 to see Venus pass before the sun for the first time since the 1880s.
"It was very exciting. I was seeing something that no human alive on Earth had ever seen, because the last time it happened was so long ago," she said.
Mercury's transits are less rare, but played just as important a role in how people began to understand the solar system.
Transits observed in the early 1600s, just as predicted by mathematician and astronomer Johannes Kepler, helped confirm his calculations that planets had elliptical orbits, not round ones.
By the 1700s, astronomers were counting on Venus transits to help solve another puzzle: Just how far is Earth from the sun? Teams were dispatched from Baja to Haiti, Russia to South Africa, to measure the exact second the crossing began and ended as seen from multiple spots around the globe.
Ultimately, the measurements were less precise than hoped, thwarted by the notorious "black drop" effect that made the round planet seem to smear or distend just as it passed the outer edge of the sun.
Still, transits linger in the imagination.
"They were very, very pivotal, greatly significant astronomical observations back in those days," Bryja said. "Now they're just kind of neat to look at."
Transits come in pairs, with a couple of passes a few years apart, then a 100-plus-year wait for Venus, and roughly 10-year waits for Mercury.
How often you see them depends on where on Earth you are.
