Note: DO NOT LOOK AT THE SUN DURING A TRANSIT.
THIS CAN CAUSE BLINDNESS IN ONLY A FEW SECONDS.

What is a transit?
A transit is when a smaller body passes in front of a larger one (for example, when an object like Venus passes between the Earth and the Sun). During a transit, the object (Venus in this case) appears as a small, dark object crossing the larger object (the Sun in this case). A transit is just like a special type of solar eclipse, but the Sun does not go dark; only a tiny bit of the Sun is obscured.

The only planets that ever pass between the Earth and the Sun are Mercury and Venus (since they are the only planets closer to the Sun than the Earth is). Therefore, the only planets that can produce a transit are Mercury and Venus. Transits are extremely rare events. Since Venus' orbit is much larger than Mercury's orbit, transits of Venus are even rarer than those of Mercury.

A transit of Venus occurs only when two things happen simultaneously:

• Venus is in inferior conjunction with the Sun (this is when Venus is located directly between the Earth and the Sun).
• Venus is crossing the through Earth's orbital plane (the ecliptic). Venus' orbital plane is tilted by 3.39 degrees with respect to the ecliptic.

The June 2004 transit of Venus lasted over 5 hours, starting at 5:13 Universal Time (UT) and ending at 11:25 UT.

How often does a transit of Venus occur?
A transit of Venus is very rare. On average, there are two transits of Venus every 125 years. Only six transits of Venus have been seen by human beings. Before the transit of Venus in June 2012, previously observed ones happened on December 4, 1639, June 5, 1761, June 3, 1769, December 8, 1874, and December 6, 1882, June 8, 2004. There have been only 52 transits of Venus across the Sun between 2000 B.C and A.D.1882. The next transit of Venus will occcur in 2117.

What can a transit of Venus tell us?
In 1716, Edmund Halley published a paper describing how a planetary transit (a transit of Venus or Mercury) could be used to measure our distance from the Sun, called, "A new Method of determining the Parallax of the Sun." Expeditions were sent out in 1761 and 1769 to observe transits of Venus (to estimate the Earth's distance from the Sun).

James Cook (October 27, 1728 - February 14, 1779) was a British explorer and astronomer who sailed to Tahiti in 1769 on the H.M.S. Endeavor in order to observe a transit of Venus; he was also searching for a large, southern continent that was thought to exist (but does not). After sailing for 8 months, he stayed in Tahiti for many months preparing for the transit. Astronomers later combined his data with those of other widely-separated observers to estimate the distance from the Earth to the Sun (1 astronomical unit).

Glossary of Terms:

ASTRONOMICAL UNIT An astronomical unit, or AU, is equal to the mean (average) distance from the Earth to the Sun, about 93 million miles (150 million km). It takes a beam of light about 8.3 minutes to travel 1 AU.

BLACK DROP EFFECT The black drop effect is an optical effect that happens early in a transit of Venus, when Venus just travels in front of the solar disk. The light from the Sun is bent around Venus (it is refracted by Venus' dense atmosphere) making Venus look stretched-out.

CONJUNCTION Conjunction is the apparent close approach of a planet to the Sun (or another planet), from the point of view of an observer on the Earth. A planet is in conjunction when the Sun is exactly between that planet and the Earth or, for Mercury and Venus (the two inferior planets), when that planet, the Sun, and the Earth are lined up. Mercury and Venus have two positions of conjunction: when either planet is directly between the Earth and the Sun, it is in inferior conjunction; when either planet is on the far side of the Sun from the earth, it is in superior conjunction. During conjunction, a planet cannot be seen from Earth (unless it is in transit); it is either behind the Sun or is lost in the glare of the Sun.

ECLIPTIC The ecliptic is the plane defined by the Earth's orbit around the Sun; conversely, in the course of a year, the sun traces a path in the sky along the ecliptic. Most of the planets in our solar system appear close to the ecliptic plane from Earth. The Earth's axis is tilted at a 23.5° from the ecliptic (which causes the seasons).

INFERIOR PLANETS Mercury and Venus are called inferior planets because they are closer to the Sun than Earth. [Planets that are farther from the Sun than Earth are called superior planets.]

SYNODIC Synodic means pertaining to two successive conjunctions of celestial bodies. For example, a synodic month is the time between successive new moons (29.531 days). The synodic month is is slightly longer than a sidereal month.

TRANSIT 1. Transit is when a smaller body passes in front of a larger one (for example, when an object passes between the Sun and the Earth). During this time, the object seems to be crossing the disk of the Sun. 2. Transit is the passage of a moon in front of its primary. 3. Transit is the passage of an object across an observer's meridian.

UNIVERSAL TIME Universal time (abbreviated UT) is the same as Greenwich Mean Time (abbreviated GMT); it is the time zone of Greenwich, England (longitude zero).