Transit of Mercury

A transit of Mercury across the Sun takes place when the planet Mercury comes between the Sun and the Earth, and Mercury is seen as a small black dot moving across the face of the Sun.

Transits of Mercury with respect to Earth are much more frequent than transits of Venus, with about 13 or 14 per century, in part because Mercury is closer to the Sun and orbits it faster.

Transits of Mercury can happen in May or November. November transits occur at intervals of 7, 13, or 33 years; May transits only occur at intervals of 13 or 33 years. The last two transits were in 1999 and 2003; the next two will occur in 2006 and 2016.

During a May transit, Mercury is near aphelion and has an angular diameter of 12"; during a November transit, it is near perihelion and has an angular diameter of 10".

The first observation of a transit of Mercury was on November 7 1631 by Pierre Gassendi. He unsuccessfully attempted to observe the transit of Venus just one month later, but due to inaccurate astronomical tables he did not realize that it was not visible from most of Europe, including Paris. A transit of Venus was not observed until 1639, by Jeremiah Horrocks.

Grazing Transits of Mercury

Sometimes Mercury only grazes the Sun during a transit. In this case it is possible that in some areas of the Earth a full transit can be seen while in other regions there is only a partial transit (no second or fourth contact). The transit of November 15 1999 was such a transit, and the previous one before that was on October 28 743. The next such transit will occur on May 11 2391.

It is also possible that a transit of Mercury can seen in some parts of the world as a partial transit, while in others Mercury misses the Sun. Such a transit last occurred on May 11 1937, and the previous one was on October 21 1342. The next such transit will occur on May 13 2608.

Simultaneous Transits

The simultaneous occurrence of a transit of Mercury and a transit of Venus is extremely rare, and will next occur only in the years 69163 and 224508.

The simultaneous occurrence of a solar eclipse and a transit of Mercury is very rare. The next solar eclipse occurring during a transit of Mercury will be on July 5 6757, and will be visible in Eastern Siberia.