On 9 May there will be a rare transit of Mercury, when the smallest planet in our solar system will pass directly between the Earth and the Sun. The last time this happened was in 2006, and the next two occasions will be in 2019 and 2032. During the transit, which takes place in the afternoon and early evening in the UK, Mercury will appear (in a suitably filtered telescope: see below) as a dark silhouetted disc against the bright surface of the Sun.
From the British Isles, the transit begins at 11:12 GMT (12:12pm BST), when the limb of Mercury appears to touch the limb of the Sun, and ends at 18:42 GMT (7:42pm BST) when the limb of the silhouetted planet appears to leave the Sun. Observers in different locations will see the transit taking place at a slightly different time, as the planet will appear to take a slightly different path across the Sun.