A very rare event is due to occur this spring! Venus will pass directly between the Earth and Sun: For several hours, the planet will appear silhouetted against our daytime star. The next transit of Venus won't occur for another 105 years.
A rare event ...
Venus is the second planet from the Sun and its orbit is, therefore, smaller than Earth's: While our planet circles the Sun in one year, Venus completes its orbit in only 225 days.
Every 584 days, Venus catches up to our planet and passes between the Earth and Sun: This is known as inferior conjunction. Normally though, Venus’ orbit, which is tilted 3.4° with respect to that of Earth, carries it either above or below the Sun’s disk. But on rare occasions, when the inferior conjunction occurs in early June or early December, Venus’ alignment carries it directly in front of the Sun. This is called a transit.
Transits of Venus usually occur in pairs, eight years apart: Each pair is separated from the next by either 105 ½ or 121 ½ years, in alternating sequence. The June, 2012 transit is the second of such a pair – in fact, you may have seen the 2004 transit of Venus. But who can claim today with absolute certainty that they'll be around to witness the next transit, in December, 2117?