Fig. 1. The 2004 Transit of Venus Simulation © 2003 HL Cohen (Click to play a larger avi movie)

IN 2004 and 2012 Venus Will Cross the Sun's Face

THESE EVENTS are among the rarest of all planetary alignments. In fact, the last such occurrence was in the year 1882. (See Fig. 1 on left for simulation of the 2004 transit.)

More than one century will elapse before these transits occur again. Indeed, few humans have ever seen Venus cross the Sun's disk.

Transits of Venus are easily seen only with telescopes and only six transits of Venus have occurred since the telescope was invented in the early seventeenth century: 1631, 1639, 1761, 1769, 1874, and 1882. (Notice transits now occur in pairs with eight year separations between the two transits in the pair. Intervals of about 105 or 122 years elapse before a pair of transits occurs again.)

Fig. 2. Seen from our planet only objects with orbits smaller than Earth's can transit the Sun

Nevertheless, transits of Mercury are common (thirteen to fourteen per century) because the orbit of Venus is larger than the orbit of Mercury. (Mercury orbits the Sun every 88 days; Venus requires 225 days.) So, transits of Venus happen infrequently and most people never see this remarkable event. (Also, the disk of Venus appears about five times larger than the disk of Mercury making transits of this planet far more spectacular and special than for Mercury.)