Garret reminds us that this year you can watch a rare astronomical event on June 5th and 6th: a Venus transit, i. e. the Venus in front of the sun. This only occurs once every 105 to 122 years, in pairs separated by eight years. The last one was on June 8th, 2004, and I watched it with my students at school.
Venus transits were used in the past to measure and calculate the distance from the earth to the sun – which is easy with today’s technology but was not so easy a few centuries ago. They’ve only been observed six times so far: 1639 (Kepler calculated the one in 1631, but it was not visible from Europe), 1761 and 1769, 1874 and 1882. The next one after this year will be in 2117, and I’m not sure how many of us are going to be around then… so if the transit is visible from where you live, make sure to take a look! Sadly I will only be able to see the last part of the transit here in Germany in the early morning.
More information and discussion at this MetaFilter thread: Venus to transit sun in June.
Mehr Informationen auf deutsch gibt es bei Venustransit.de.