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This article is about the astronomical phenomenon. For other meanings, see Transit of Venus (disambiguation).

A transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun and the Earth, obscuring a small part of the Sun's disc. During a transit, Venus can be seen from the Earth as a small black disc moving across the face of the Sun. A transit is similar to a solar eclipse by the Moon, but, although the diameter of Venus is almost 4 times that of the Moon, Venus appears much smaller because it is much further away from the Earth. Before modern astronomy, observations of transits of Venus helped scientists measure the distance between the Sun and the Earth using the method of parallax.

Transits of Venus are rare and occur in a pattern that repeats every 243 years, with pairs of transits 8 years apart separated by long gaps of 121.5 years and 105.5 years. The first of a pair of transits of Venus took place on June 8, 2004 (see Transit of Venus, 2004) and the next in this pair will be on June 6, 2012 (see Transit of Venus, 2012). Before 2004, the last pair of transits of Venus were in December 1874 and December 1882. After 2012, there will be no more transits of Venus until December 2117 and December 2125.

The pattern repeats every 243 years because 243 sidereal orbital periods of the Earth (365.25636 days, which is slightly different from the tropical year) is 88757.3 days, and 395 sidereal orbital periods of Venus (224.701 days) is 88756.9 days. Thus, after this time both Venus and Earth have returned to very nearly the same point in each of their respective orbits. This period of time corresponds to 152 synodic periods of Venus.

Conjunctions of Venus with the Sun

Normally when the Earth and Venus are in conjunction they are not aligned with the Sun. Venus' orbit is inclined by 3.4° to the Earth's so it appears to pass under (or over) the Sun in the sky. Transits occur when the two planets happen to be in conjunction at (or very near) the points where their orbital planes cross.

Although 3.4° may not seem like much, Venus can be as far as 9.6° from the Sun when viewed from the Earth at inferior conjunction. Since the angular diameter of the Sun is about half a degree, Venus may appear to pass above or below the Sun by more than 18 solar diameters during an ordinary conjunction.

Scientific interest in transits

Apart from its rarity, the original scientific interest in observing a transit of Venus was that it could be used to determine the size of the solar system. The technique is to make precise observations of the slight difference in the time of either the start or the end of the transit from widely separated points on the Earth. The distance between the points on the Earth can then be used as a yard stick for the distance to Venus and the Sun. See "parallax".

Although by the 17th century astronomers could calculate each planet's relative distance from the Sun in terms of the distance of the Earth from the Sun (an astronomical unit), the absolute value (in miles or kilometers) of this distance was not known very precisely.

Johannes Kepler was the first to predict a transit of Venus in 1631, but no one observed it, because Kepler's predictions were not sufficiently accurate to predict the fact that the transit would not be visible in most of Europe.

The first observation of a transit of Venus was made by Jeremiah Horrocks from his home in Much Hoole, near Preston in England, on 4 December 1639 (November 24 under the Julian calendar then in use in England). His friend, William Crabtree, also observed this transit from Salford, near Manchester. Kepler had predicted transits in 1631 and 1761. Horrocks corrected Kepler's calculation for the orbit of Venus and realised that transits of Venus would occur in pairs 8 years apart, and so predicted the transit in 1639, although he was uncertain of the exact time. In the event, after observing for most of the day, he was lucky to see the transit as clouds obscuring the Sun cleared just half an hour before sunset. However Horrocks' observations were not published until 1666, well after his death. In any case his estimate of the size of the solar system was only about half the correct size.

At the suggestion of Edmond Halley, the transit pair of 1761 and 1769 was used to try to determine the precise value of the astronomical unit using parallax. Numerous expeditions were made to various parts of the world in order to observe these transits; in effect this was the first international scientific collaboration. One such expedition was undertaken by the most unfortunate Guillaume Le Gentil, whose unsuccessful journey led to him losing his possessions and wife and being declared legally dead. Another was the first voyage of Captain Cook to observe the 1769 transit from Tahiti, before sailing on to New Zealand.

Unfortunately, it was impossible to time the exact moment of the start and end of the transit due to the phenomenon known as the "black drop effect". The black drop effect was long thought to be due to Venus's thick atmosphere, and initially it was held to be the first real evidence that Venus had an atmosphere; however recent studies demonstrate that it is an optical effect.

In any case, in modern times a precise value for the astronomical unit is known from space probe telemetry and from radar observations of solar system objects, and therefore the 18th-century transit-timing experiments would only be repeated today as a "science project" rather than as serious astronomical research.

There was however a good deal of interest in the 2004 transit as scientists attempted to measure the pattern of light dimming as Venus blocked out some of the Sun's light, in order to refine techniques that they hope to use in searching for extrasolar planets.

Current methods of looking for planets orbiting other stars only work for planets that are very large (Jupiter-like, not Earth-like), whose gravity is strong enough to wobble the star sufficiently for us to detect changes in proper motion or Doppler shift changes in radial velocity. Measuring light intensity during the course of a transit, as the planet blocks out some of the light, is potentially much more sensitive, and might be used to find smaller planets. However, extremely precise measurement is needed: for example, the transit of Venus causes the Sun's light to drop by a mere 0.001 magnitude, and the dimming produced by small extrasolar planets will be similarly tiny.

Past and future transits

See also

• Astronomical transit

• Transit of Mercury

• Transit of Venus from Mars
• Transit of Venus from Jupiter
• Transit of Venus from Saturn

• Transit of Earth from Mars

External links

• Transits of Venus, Six Millennium Catalog: 2000 BCE to 4000 CE
• HM Nautical Almanac Office, Transits of Venus 1000 AD – 2700 AD

• The Economist, May 27th 2004: Transits of Venus
• Reanimating the 1882 Transit of Venus, in Sky and Telescope magazine online

• transit of venus
• Transit of Venus
• Lecture 26: How Far to the Sun? The Venus Transits of 1761 & 1769 (an historical review, with interesting anecdotes)

• Astronomical transits
• Venus
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