For a few hours Wednesday and Thursday, astronomers in North America, East Asia, and Oceania have a rare opportunity to see Mercury pass in a direct line across the Sun. Some regions see just some of the transit - before sunset on Wednesday in the rest of the Americas, and after sunrise on Thursday in East Asia and the rest of Australia.
While Mercury orbits the sun in 88 days, its tilting orbit means that we can only view its transit of the sun 13 times per century. 

More (via BBC)