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The transit or passage of a planet across the disk of the Sun may be thought of as a special kind of eclipse. As seen from Earth, only transits of the inner planets Mercury and Venus are possible. Planetary transits are far more rare than eclipses of the Sun by the Moon. On the average, there are 13 transits of Mercury each century. In comparison, transits of Venus usually occur in pairs with 8 years separating the two events. However, more than a century elapses between each transit pair. The first transit ever observed was of the planet Mercury in 1631 by the French astronomer Gassendi. A transit of Venus occurred just one month later but Gassendi's attempt to observe it failed because the transit was not visible from Europe. In 1639, Jerimiah Horrocks and William Crabtree became the first to witness a transit of Venus.

At the present time, all transits of Mercury fall within several days of May 8 and November 10. Since Mercury's orbit is inclined 7 degrees to Earth's, it intersects the ecliptic at two points or nodes which cross the Sun each year on those dates. If Mercury passes through inferior conjunction at that time, a transit will occur. During November transits, Mercury is near perihelion and exhibits a disk only 10 arc-seconds in diameter. By comparison, the planet is near aphelion during May transits and appears 12 arc-seconds across. However, the probability of a May transit is smaller by a factor of almost two. Mercury's slower orbital motion at aphelion makes it less likely to cross the node during the critical period. November transits recur at intervals of 7, 13, or 33 years while May transits recur only over the latter two intervals. The following table lists all transits of Mercury from 1901 through 2050. For a more complete and detailed list, see Seven Century Catalog of Mercury Transits: 1600 CE to 2300 CE.

Transits of Mercury: 1901-2050

To determine whether a transit of Mercury is visible from a specific geographic location, it is simply a matter of calculating the Sun's altitude and azimuth during each phase of the transit using information tabulated in the Seven Century Catalog of Mercury Transit. For the relevant equations and a sample calculation , see Transit Visibility. This web page also has links to several Excel files which perform the calculations automatically when the user inputs the latitude and longitude of any location.

Because Venus's orbit is considerably larger than Mercury's orbit, transits of Venus are much rarer. Indeed, only six such events have occurred since the invention of the telescope (1631,1639, 1761,1769, 1874 and 1882). Transits of Venus are only possible during early December and June when Venus's orbital nodes pass across the Sun. Transits of Venus show a clear pattern of recurrence at intervals of 8, 121.5, 8 and 105.5 years. The following table lists all transits of Venus during the 800 year period from 1601 through 2400. For a more complete and detaled list, see Six Millennium Catalog of Venus Transits: 2000 BCE to 4000 CE.

Transits of Venus: 1601-2400

Date                    Universal        Separation
                             Time              (Sun and Venus)

To determine whether a transit of Venus is visible from a specific geographic location, it is simply a matter of calculating the Sun's altitude and azimuth during each phase of the transit using information tabulated in the Six Millennium Catalog of Venus Transits. For the relevant equations and a sample calculation , see Transit Visibility. This web page also has links to several Excel files which perform the calculations automatically when the user inputs the latitude and longitude of any location.

The 2004 transit of Venus was visible from Europe, Africa and Asia. However, the final stages of the event were also visible from the eastern USA and Canada. Complete details can be found at 2004 Transit of Venus.

The 2012 transit of Venus will be visible from North America, the Pacific, Asia, Australia, eastern Europe, and eastern Africa. Details can be found at 2004 and 2012 Transits of Venus.

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