The two inner planets of the solar system, Mercury and Venus, have transits across the face of the Sun. These transits have an important history in astronomical metrology, having been used in the last century for the determination of the solar horizontal parallax, and, consequently, the distance between the Earth and the Sun. (For more details, click here.)

Transits of Venus occur when the planet is within 1°45' of the nodes. If Venus in transit passes more than 12' from the Sun's centre, it will transit again 8 years later, while transits nearer the center are "solitary". Approximately 113 years later, another pair of transits will occur, this time at the other node of the transit.

The first predictions of the transits of Mercury and Venus were made at the beginning of the 17th century when astronomers began to gain a good understanding of the orbital motions of the inner planets. Johannes Kepler predicted the transit of Mercury of 7 November 1631, and the transit of Venus of 7 December 1631. The first record of a transit of Venus is the one of 1639. Since then, there have been pairs of transits in 1761 and 1769 and in 1874 and 1882. The first of another pair of transits occurred on 8 June 2004 and the second will occur on 6 June 2012.

Such relatively rare astronomical events have inspired numerous expeditions from around the world. The transit of 1761 was observed in Asia, mainly in the Indian region. That of 1769 was observed in Central America, Tahiti and the north of Europe. The transit of 1874 was observed in the south of China and Japan, and in the north-east of Asia. The transit of 1882 was visible from South America.

The June 2004 transit of Venus was observable in Europe and over much of Africa, Asia and Australasia (see visibility chart). The duration of the transit observed from Paris was 5h 24m 32s. The following table gives, in UTC, the instants of the four contacts of Venus with the solar disk. The maximum approach of the centre of Venus to the solar centre was at 8h 22m 53s UTC, when the distance was 10' 41".

			Place: Paris (latitude: +48° 52'; longitude: 02° 20' W) UTC times of contact 1st 05h 20m 06s 2nd 05 39 48 3rd 11 04 20 4th 11 23 39 maximum 08 22 53

The transit on 6 June 2012 will not be fully visible from Europe; it will start during the night in Paris, at 01h 29 m 35s UTC. Parisians will be able to observe the latter part of the phenomenon during the morning. After that, the next transit of Venus will not be until 11 December 2117.