This evening you can watch a cosmic spectacle that most of us won't see again.
The planet Venus will pass directly between the Earth and the sun - a movement that will look like a black dot inching its way across the sun. The movement, known to astronomers as the Transit of Venus, will be visible in the skies above Minnesota from about 5 p.m. to sunset.
It's a rare event that won't happen again until 2117. The spectacle has sparked Venus viewing parties at museums and schools around the world.
There's also a cosmic party planned for the astronauts aboard the International Space Station.
MPR's Cathy Wurzer spoke with University of Minnesota astronomy professor Terry Jones about the history of the Venus transit and how you can prepare to view it.
Cathy Wurzer: Why is this so rare?
Terry Jones: It's rare because Venus and the earth don't have orbits exactly in the same plane. So it's only every now and then that the two of them can line up with one another and the sun.
Wurzer: Take us back. When scientists first realized this was happening, did the public flock outside to look?
Jones: No, it was really something mostly internal to astronomers and the educated in Europe at the time. The reason why the transit of Venus historically is important is that we knew the relative distances of planets in the solar system from the sun. For instance, we knew Mars was one-and-a-half times further away from the sun than the earth, but we didn't know what that distance actually was in miles.
So the transit of Venus gives us a chance to actually measure what is the distance from the earth to the sun in units, like miles, and that's why it was important, and that was predicted by several astronomers.
But Edmond Halley, the astronomer who is known as being famous for having figured out Comet Halley, he's the one who did the calculations and figured out when it was going to happen and what to do. He knew that it wouldn't take place until after he died, but there was quite a bit of activity in 1761 and 1769 to observe transit of Venus, and that's how we figured out how big the solar system is.
Wurzer: So if people go outside this evening, what will it actually look like?
Jones: Well, first of all, if you go out, you have to have the right equipment. Under no circumstances should you observe the sun straight with your eye or look through a telescope that isn't properly guarded for that.
Then you will see a little black dot on the surface of the sun, and it isn't going to wow you or anything, but you have to think in your mind that you're looking toward the sun and there's a planet between us and the sun blocking some of the light.
Wurzer: I wonder what people back in the eighteenth century, the early days, thought when they observed (it)? Maybe they didn't really even know about it much.
Jones: The people in the eighteenth century who were observing the transit of Venus knew exactly what they were doing, and they were timing the length of the transit. That's how they figured out the distance from the Earth to the sun. They all had to have accurate clocks, and they wanted them accurate to about two seconds over seven hours, because that's about how long typically it would take the transit.
Wurzer: Did you have any historic background in this? As we've gone along in years, were there such viewing parties as we're having today?
Jones: Unlikely, because there wasn't equipment available for everybody. I mean anybody can get a telescope today with a solar filter on it. There's the Minnesota Astronomical Society. There's the University. There's various other groups. There's NASA. We didn't have any of those back in 1884 when the last eclipse took place.
Wurzer: Where do you recommend people go see this eclipse?
Jones: They can come to the University of Minnesota. On the top of the physics building, we'll have a viewing available, and we'll have a little lecture starting at 4 (p.m.) down in room 166 where we'll talk about the history of the transit. There's some fabulous history involved and truly dramatic events that took place in people's lives trying to observe the transit back in the past. And the Minnesota Astronomical Society has (a viewing) ... and in Hopkins, Bell Museum will have a party there.
Wurzer: Are you excited about this?
Jones: It's good to contact the public and have the public interested in this, but scientifically it's a historical interest, mostly.
(Interview edited and transcribed by MPR reporter Madeleine Baran)