For six hours and 40 minutes Tuesday night, the world looked skyward to observe a once-in-a-lifetime celestial crossing. Across the globe, people squinted through telescopes and solar filters and homemade paper pinhole contraptions to watch tiny Venus cross directly between the Sun and Earth, a tiny dot on disk of orange.
A transit of Venus occurs twice every 243 years, paired eight years apart--one of the rarest of predictable astronomical events. Tuesday's event was the second in the pair--the first was on June 8, 2004--so if you missed it, you're out of luck until December 2117. Not many people alive today will likely be around to watch that one, in 105 years.
Tuesday's transit, unusually, was observable in large areas of all six inhabited continents, and even slivers of Antarctica.
While NASA has a catalogue of transits going back 4,000 years, and looking forward another 2,000, humans have only observed them since 1639, making Tuesday's transit only the 7th ever watched.
The historical significance of the Venus transits goes beyond the cool factor, too--the first observation helped scientists calculate the distance between the Sun and Earth for the first time.