My friends talk about any wonderful coincidence as being due to “an alignment of the planets” – a joking reference to astrology. Yet, though astronomers may not hold astrology in high repute, we know that on occasion planets do align.

A transit happens when a planet such as Venus is lined up exactly between us and the Sun. In June 2004 I wrote here about seeing the transit of Venus; I also got to see it again in 2012. I’m not likely to see any more of them – the next won’t occur until 2117. Observing the transit of Venus from different spots on Earth, as was first possible in the eighteenth century, finally gave astronomers the ability to calculate the size of the solar system.

Sending observers around the world for this event motivated Captain Cook’s voyages and the visit of Mason and Dixon to America. We have more accurate measurements today, but nonetheless the 2004 and 2012 transits were widely observed just for the spectacle of seeing the large spot of Venus’ shadow move across the Sun.