A transit of Venus, essentially an eclipse of the sun by that planet, is rare for two principal reasons. One is that the planet’s orbit around the sun is tilted several degrees with respect to Earth’s orbit, so most of the time Venus does not get in the way of our view of the sun. On top of that, Venus orbits the sun 13 times for every eight orbits by Earth. In short, it is unusual for everything to line up perfectly.
On the fifth-floor roof deck of the Franklin Institute, dozens of people looked hopefully at the skies before the event was to begin, at 6:03 p.m. The museum staff had set up telescopes masked with special filters so people could look at the sun safely, should it appear.
Queen Village resident Daniel Weil, with his son and two nephews, came prepared with his own eye-protection equipment. Weil bought a welder’s mask online two weeks ago, and nephew Andrew Bowers demonstrated how its dark glass could be raised and lowered.
Paul Sank of Maple Shade brought a pair of binoculars, which he planned to use to focus an image of the sun on a piece of white paper. He was optimistic that the skies would clear.
"I’ve got a $50- bet with my wife," he said.
Alas, by 7 p.m. he was driven inside by rain.
Before the transit began, two geologists spoke in the Fels Planetarium about research on the second planet from the sun.
Tracy K.P. Gregg, an associate professor at the University at Buffalo, said the planet’s surface was hard to photograph because of its dense cloud cover.
Radar images, however, reveal a rich topography with such features as volcanoes and coronae — caused by blobs of magma that push up against the planet’s outer surface but don’t quite erupt.
"They’re like volcano wannabes," Gregg said.
The average temperature on the planet’s surface is nearly 900 degrees Fahrenheit, and its atmosphere is nearly 100 times denser than Earth’s.
Its mountaintops appear to be covered with a sparkly surface, which is thought to be iron pyrite, commonly called fool’s gold.
"If you were to land there, it would sparkle like fool’s gold, and then you would die," Gregg said. "But it would be lovely."
Much safer to watch the planet from Earth. Even if you have to use the Internet.
Contact Tom Avril at 215-854-2430 or tavril@phillynews.com.
