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Space.com: Transit of Mercury.

the planet Mercury will appear to cross in front of the disk of the Sun on Wednesday, May 7, appearing as a tiny black dot when safely and keenly observed. The event, called a transit of Mercury by astronomers, only occurs 14 times during the 21st Century.

Transits of Mercury and Venus hold an interesting place in astronomical history. The main reason is the slightly different times that the events occur as seen from different locations on the surface of the Earth. The diameter of our planet is appreciable when compared to the distance to Mercury and Venus, allowing astronomers to “triangulate” the planets from various points on Earth when they are seen against the bright background of the Sun’s limb.

Interesting stuff from a historical perspective. Unfortunately the transit is not visible to the unaided eye. A telescope with at least 50 to 100-power must be used. And whatever you do, use a solar filter to avoid eye damage The transit will be between 5:13 GMT and 10:32 GMT.