Mercury scoots round the sun in only 88 days, overtaking the more sedately moving Earth every three or four months. Because Mercury’s orbit is tilted at about seven degrees with respect to the Earth’s, it passes directly between us and the sun (a transit) only when both it and the Earth are close to the points where their orbital planes intersect. This can happen only in early May or early November.
In every century there are only 13 or 14 transits of Mercury and you have to be on the right part of the globe if you want to watch a particular transit from beginning to end, which usually lasts for several hours. The May 9 afternoon transit is perfectly timed for viewing the entire event from Europe and most of the Americas.
The May 9 transit will begin at 12:12 BST and end at 19:42 BST, which could hardly be more convenient for viewing from western Europe. Those in India will be able to watch for an hour or two before the sun sets whereas people on the east coast of North America will have to rise early to catch the start. However, people living in Japan and Australia will miss the whole thing.
The next transit of Mercury after this will be 12:35 to 18:04 GMT on November 11, 2019, but in the UK sunset happens well over an hour before the end. After that there’s a long wait until November 2032.
Unlike Venus, Mercury is too small to see against the sun without magnification, and it can be dangerous to try due to the sun’s glare. So my advice is to go to an organised transit viewing event – many astronomy clubs and universities are organising these. Another option is to view it online. The European Space Agency will be webstreaming live images from space (no clouds in the way) and from solar telescopes in Spain and Chile.
David Rothery is Professor of Planetary Geosciences at The Open University
Picture: Mercury transiting across the face of the Sun on 7 May 2003 (SSPL/Getty Images)