Yesterday (Monday 9 May 2016) saw a transit of Mercury, a rare celestial event where the planet Mercury passes across the disk of the Sun as seen from Earth. The last one of these to happen was the 8th of November 2006, the next one will be on the 11th of November 2019. Here is a diagram showing where you needed to be to see this transit; it was visible in its entirety from Wales and other parts of western Europe, as well as the eastern United States and western Africa and most of South America.Here is a diagram showing the path that Mercury took across the Sun, showing the timings (in Universal Time) of the beginning, mid-point and end of the transit. Unfortunately, it rained the whole afternoon here in Cardiff. I did not manage to catch single a glimpse of the Sun, let alone the transit itself. Here is a picture which was taken by Wolgang Ellsässer (who obviously had much clearer skies) at 13:52 UTC, about an hour before the transit reached its mid-point. A few sunspots are also visible, in fact some of the sunspots are appear bigger than the disk of Mercury. If, like me, you failed to see this transit then the next one is on the 11th of November 2019. It will start at 12:35 UTC and end at 18:04 UTC, so roughly the same part of the Earth will see it as this 2016 transit. However, being in November those of us in western Europe will not see the end of the transit as the Sun will have already set. After that there are transits in 2032 and 2039, again both in November. The next transit visible in its entirety from western Europe will not be until May 2049!
But, if you think this is a long time, it is an even longer wait until the next transit of Venus. Some of you may remember that I was lucky enough to go to the Gobi desert in Mongolia to see the June 2012 transit of Venus. I had also seen the June 2004 transit of Venus, which was visible in its entirety from Wales. The next transit of Venus will not occur until 2117!
It was seeing a transit of Mercury from the island of Helena in 1677 that led astronomer Edmond Halley to realise that one could use transits of either Mercury or Venus to measure the distance from the Earth to the Sun. He later realised that Mercury was too far from the Earth to be used, but the transits of Venus in 1761 and 1769 were successfully used to calculate the Sun-Earth distance for the first time.