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Transits - of Mercury and Venus

Normally, when an inferior planet (Mercury or Venus) comes into inferior conjunction with the Sun, it will pass the Sun's disk at some apparent distance. But occasionally, it happens that it passes close enough to the disk's center that it will pass between Earth and Sun. On these occasions, a transit of the dark, small planetary disk can be seen on the bright, large disk of the Sun. Transits occur only if the planet is near (within about 15 arc minutes) of Earth's orbital plane (the ecliptic), i.e., near the ascending or descending node of its orbit. Transits can occur only if the conjunction happens within a short time before or after the planet passes one of these nodes, and Earth happens to be in the same direction as seen from the Sun. This again happens around the same time in the year, or date. In these times (centuries), transits of Mercury can only happen in May or November, Venus transits in June or December.

Mercury Transits

Mercury transits occur every few years, about 13 a century. These events occur when Mercury's inferior conjunction happens to coincide with a crossing of the Earth's orbital plane, the Ecliptic, i.e. when the planet passes its orbit's ascending or descending node. At these times, Earth passes Mercury's orbit each early May (descending node; Mercury passes ecliptic from North to South) and early November (ascending node). If Mercury's inferior conjunction happens to occur at that time, a transit can be seen.

Last two Mercury transits occured on May 7, 2003 between 5:13 and 10:32 UT, with its minimal separation of 708" from the solar disk's apparent center at 7:52, and on November 8, 2006, 19:12 UT to 00:10 UT (Nov 9), with greatest approach of 423" at 21:41.

The next Mercury transits will be on:


Venus Transits

Venus transits are rare, occurring in pairs separated by 8 years, with 105.5 or 121.5 years to the next pair of transits. These events occur when Venus passes between Earth and sun just coincidently with both planets crossing the other's orbital plane (the ascending and descending nodes). These times, Earth always crosses Venus' orbit in early June and early December of each year; if an inferior conjunction happens to take place at these times, a transit can be observed. June events happen when Venus is near the descending node of its orbit, December events if near its ascending node.

Because they are so rare, only six Venus transit have occurred since the invention of the telescope before 2000 AD:

The 20th century has passed without any Venus transit occurring. Most recent Venus transit occured on and next Venus transits will happen on The Venus transits of 1761 and 1769 have been utilized by astronomers to attempt a parallax measure for determining the length of the Astronomical Unit, AU, and thus the linear dimensions of the Solar System.

First photographs had been obtained from the 1882 transit of Venus, but only few are preserved.


Transits on Venus and Mars

Hartmut Frommert [contact]