Transit of Planets Across the Sun's Disk

the astronomical phenomenon observed at certain inferior conjunctions of Mercury and Venus to the sun in which the planet, visible as a small, dark circle, traverses the brilliant solar disk. Because of the inclination of the orbits of Mercury and Venus to the plane of the ecliptic, not every inferior conjunction results in an observable transit (seeCONFIGURATIONS); a transit is observed only when the conjunction is near one of the orbital nodes of the planet.

Transits of Mercury occur in cycles, with intervals between transits of 13, 7, 9.5, 3.5, 9.5, and 3.5 years; the transits have a duration of not more than 8 hours. The nearest transits of Mercury will be observed November 1986, November 1993, and May 2003.

Transits of Venus occur in cycles, with intervals between transits of 8,105.5, 8, and 121.5 years; the transits have a duration of up to 6.5 hours. Transits of Venus were observed on June 6, 1761, June 3, 1769, Dec. 9, 1874, and Dec. 6, 1882. The next transits of Venus will occur on June 8, 2004 and June 6, 2012. During Venus’ transit of 1761, M. V. Lomonosov discovered that the planet has an atmosphere. Transits of Venus were observed during the 18th and 19th centuries in order to determine the solar parallax.