On Monday, Nov. 11, a most unusual event will take place: the transit (passage) of the planet Mercury across the sun's disk. Such a sight is a relatively rare occurrence as seen from Earth. From our perspective, only transits of Mercury and Venus are possible. This event will be the fourth of 14 transits of Mercury that will occur during the 21st century. In contrast, transits of Venus occur in pairs, with more than a century separating each pair. Mercury will take nearly 5.5 hours to cross in front of the sun. The transit will be widely visible from most of the Earth, including the Americas, the Atlantic and Pacific oceans, New Zealand, Europe, Africa and western Asia. It will not be visible from central and eastern Asia, Japan, Indonesia and Australia.