There’s a little black spot on the Sun today, 9th May 2016, as Mercury, the innermost planet, transits across the face of our star as seen from Earth. These transits are relatively rare (although not as rare as transits of Venus!), with 14 transits occurring during the 21st century. We’ve already observed two in 2003 and 2006 and this will be the third. Nearly all of the world except Australia and New Zealand will get a glimpse of the 2016 event.
For a transit to occur, Mercury has to be crossing the Earth’s orbital plane (the point which is known as a node) at the same time as it reaches inferior conjunction (when is directly between the Earth and the Sun. There are only two times of the year when this may occur, and that is within a few day of May 8th, and a few days of November 10th. And then to make it even more fun to work out, the transits happen in overlapping cycles of 7, 13 or 33 years depending on which node is being considered.
If you take a look at the last two to occur and the two this decade – 2003, 2006, 2016, 2019 – you might go “hey, that doesn’t add up! There is only three years between them.” But if you consider that 2003 and 2016 are both May node events with 13 years between them, and 2006 and 2019 are November node events with 13 years between them, it fits just right.
Want to know more? Fred Espanek at EclipseWise.com has summarised the info on transits nicely on this page here. He’s considered an expert in all things to do with eclipses and transits so I recommend having a good read of all the pages on his site if this subject intrigues you.
And you can sing along with The Police as you do so if you like.