This story ran 6/4/2012 on Dane101.com

Tuesday evening, you’ll be able to watch something you’ll never see again in your lifetime: the planet Venus will cross the path between the earth and sun, and make the itty-bittiest, most adorable of shadows on the disk of the sun. And it won’t happen again for more than one hundred years.

This is what scientists call a transit: it’s like an eclipse, but because Venus can’t possibly block enough of the sun to noticeably affect its light, you won’t even see it unless you look for it. Carefully. Like, with eclipse glasses or a solar filter on your camera/telescope.

It won’t look like much: just a black speck slowly crossing the sun, barely visible to the unmagnified eye. But hundreds of years ago, this tiny event led astronomers to an important insight: the transit could tell us the actual distances between objects in the solar system, when before all scientists knew were the relative distances. By placing two observers to view and time the transit from different latitudes, astronomers could use parallax, which is the way objects move against a background depending on where you view them from (you can do this by winking your eyes one at a time)…

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