June 5, 2012 -- Look -- up in the sky. If you have clear weather and a safe way to look, the small black dot on the face of the sun is a transit of Venus -- the heavens aligning so that the planet Venus marches slowly across the disc of the sun.
The transit began at 6:09 p.m. EDT (times vary by a few minutes depending on your location) and lasted about six hours and 40 minutes (times vary by a few minutes depending on your location).
It was a last-time-in-a-lifetime spectacle, one that only happens when Venus, the second planet in the solar system, comes directly between Earth and the sun. Because of the complexities of orbital mechanics, there are two such alignments in a period of eight years -- the last was on June 8, 2004 -- followed by a break of more than a century. If it was cloudy where you were today, you'll have to hang on until the next transit on December 10, 2117.
So do look if you get a chance -- but please don't look directly. If you haven't read enough warnings not to look directly into the sun, here's one more.
So how to watch safely? Some tips:
One way is by briefly projecting an image onto a flat surface, perhaps through a small telescope or a pair of binoculars. Many science museums and planetariums are having viewing parties, with properly-filtered telescopes set up. NASA put together a map on which you may be able to find a viewing party near you.
If you happen to be a welder, you're in luck -- No. 14 welder's glass or darker is considered safe. Sunglasses, no matter how dark, simply are not enough to protect your eyes.
The best viewing will be from the Pacific, including Hawaii, Alaska, Japan and easternmost Asia. Alaska and Hawaii are the only U.S. states that get to see the whole thing; everywhere else, the transit is still in progress at sunset.
One very simple viewer you can make is called a pinhole projector. Prick a tiny hole (several are better) in a piece of thick paper or cardboard, and let the sunlight passing through it fall on a second sheet. You'll find that the pinhole acts as a tiny lens, projecting a just-big-enough image of the sun to show the tiny dot of Venus.
"Don't let the requisite warnings scare you away from witnessing this singular spectacle!" says NASA's public outreach team. "You can experience the transit of Venus safely, but it is vital that you protect your eyes at all times with the proper solar filters."