"A "transit of Venus" happens when the sun, Venus and Earth are in perfect alignment. From Earth, you can observe a small black disc – Venus – slowly wandering over the sun over the course of several hours. This happens in a pattern that repeats every 243 years: there's a gap of 122 years, then a pair of transits spaced eight years apart, then a gap of 105 years, then another pair. On 8 June 2004 the most recent transit happened – the first time any human being then alive could have seen it.
In the 18th and 19th century the phenomenon had huge importance. Scientists would time the passage of Venus from several vantage points on Earth. It was the only way to establish our own exact position in relation to the sun, and hence the universe around us.
Observing the 2004 transit through my telescope, which I still have from my astronomy-obsessed teenage days, had no scientific value, but it was moving to see the mechanics of the sky. To see a planet actually move in front of another gave me a visual sense of my location in space."
-- Wolfgang Tillmans