Transit of Venus

During a transit of Venus, Venus passes directly between the Earth and the Sun, and Venus appears as a small, dark dot crossing the bright disk of the Sun. Transits of Venus are very rare - two transits of Venus occur every 125 years. Transits of Venus occur on June 8, 2004 and on June 5 or 6, 2012 (depending on your time zone).

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.

How long does a transit of Venus last?

A transit of Venus lasts from 3 to 7 hours; the exact time depends on the path taken across the Sun. The June 2004 and June 2012 transits of Venus lasted over 5 hours.

The black drop effect

At the beginning of a transit of Venus, when Venus has just entered the Sun’s disk, an odd optical phenomenon occurs; it is called the black drop effect. The light from the Sun is bent around Venus (it is refracted by Venus’ dense atmosphere), making Venus look stretched-out. This phenomenon also occurs towards the end of a transit of Venus, just before Venus leaves the solar disk.

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).