Watch Venus pass across the sun Tuesday evening or wait 105 years
University of Michigan astronomist Joel N. Bregman expects hundreds of people Tuesday evening to crowd the lawn of Angell Hall and the intersection of Ann and Observatory streets in Ann Arbor.
Why? To catch a glance at a once- or twice-a-life time visual: Planet Earth's neighbor Venus will slowly pass across the sun.
The last visible passes before that were in 1882 and 1874.
And astronomers predict it wont happen again until 2117.
"It's about every hundred years and it occurs in pairs," explained Bregman. "It's a slow thing, it takes hours to go across the sun."
Seven hours, to be exact.
Starting at 6 p.m. Tuesday U-M will have telescopes lining the roof of Angell Hall, as well as the sidewalks near Detroit Observatory at Ann and Observatory streets in Ann Arbor. The telescopes will be set up until 9 p.m., or shortly before the sun sets. U-M will also stream Venus' passing in Angell Hall Auditorium B.
Venus orbits the sun every year. It passes between the Earth and the sun every 564 days; however, Venus' orbit is rarely visible because it doesn't move on the same plane as Earth.
"When you look very closely their orbits are tilted relative to one another," Bregman explained. "If you think of two dinner plates that are tilted to one another, there is a point when the two dinner plates line up."
That point is called a node.
A similar phenomenon occurs during lunar eclipses, when the sun, Earth and moon align briefly and the Earth's shadow blocks out the moon.
But Venus' orbit of the sun takes much longer than the moon's orbit of Earth, and thus Tuesday's glimpse of Venus passing across the sun is more rare than an eclipse.
Bregman recalls watching Venus pass across the sun in 2004.
"I saw it from the roof of Angell Hall. We had a lot of people up there and violated fire code," he said.
From Earth, Venus appears to be roughly one-thirtieth of the sun's apparent width. Its transit will be visible using the naked eye Tuesday, although Bregman suggests looking through smoked glass in order to avoid staring directly at the sun. Venus will look like a small dark disk passing across the sun's surface.
"It's nice, it's so rare and it's of historical interest because people tried to use it in science," Bregman said. Astronomists throughout history have used the orbit of Venus to try to determine the size of the solar system. "You're probaly going to look at it and say 'Well that's pretty interesting,' but you're not going to fall over and weep."