Transit of Mercury

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    On May 9 there will be a transit of Mercury, when that planet will pass directly in front of the Sun. The last time that that happened was in 2006, and the next 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 as a black dot silhouetted against the bright surface of the Sun. From the UK the transit begins at 12.12 BST, when the limb of Mercury appears first to touch the limb of the Sun, and ends at 19.42 BST when the limb of the silhouetted planet appears to leave the Sun. Observers in different locations will see the transit taking place at slightly different times. The entire event is visible from most of Western Europe, the western part of North and West Africa, the eastern part of North America and most of South America.
    Mercury completes an orbit around the Sun every 88 days, and passes between the Earth and the Sun every 116 days. As the orbit of Mercury around the Sun is tilted with respect to that of the Earth, the planet usually appears to pass above or below the Sun. A transit can only take place when the Earth, Mercury and the Sun are exactly in line in three dimensions. There are 13 or 14 transits of Mercury each century, so they are fairly rare events, though each one can typically be seen from a large area of the Earth's surface. A transit was first seen in 1631, two decades after the invention of the telescope, by the French astronomer Pierre Gassendi. The most recent transit of Mercury visible in the UK was in 2003 (the 2006 event was visible in the western hemisphere). In transit, Mercury blocks out only a tiny part of the light from the Sun, so the event should NOT be viewed with the unaided eye. Looking at the Sun without appropriate protection, either during the transit or at any other time, can cause serious and permanent damage to the eyes.

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