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A transit of Venus across the Sun takes place when the planet Venus comes between the Sun and the Earth, and Venus is seen as a small black dot moving across the face of the Sun. It is similar to a solar eclipse when the Moon comes between the Sun and the Earth, but because the angular size of Venus seen from the Earth is very much smaller it hardly obscures the Sun at all.

Transits of Venus with respect to Earth are rare; they occur in pairs 8 years apart, which are separated by more than a century. Transits occurred in 1874 and 1882; the next will be on June 8, 2004 and June 6, 2012; and then not until 2117 and 2125.

The June 8 2004 transit of Venus

The next transit of Venus will occur on June 8 2004. It will be best seen from Europe, Asia and Africa, although eastern North America will catch the end of it. Western North America will not see it at all, nor will Hawaii or New Zealand. The regions from which the transit will be visible are shown on the map. DO NOT LOOK DIRECTLY AT THE SUN. One safe method is to use two paper plates, with one plate having a pin hole in it to project an image onto the other plate. File:Viewing the sun safely.png	June 8 2004 Time (UTC) Times are given for a hypothetical observer at the center of the Earth. Due to parallax, actual observed times may differ by up to ±7 minutes at different observation points on Earth.
	I	II	Mid	III	IV
	05:13:29	05:32:55	08:19:44	11:06:33	11:25:59

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 conjuction at (or very near) the points where their orbital planes cross.

Scientific interest in transits

Apart from its rarity, the original scientific interest in observing a transit of Venus is that it can be used to determine the size of the solar system. In the 17th century, though European astronomers had fairly detailed observations of the orbits of the major planets and could calculate each planet's relative distance from the Sun, the absolute distance of the Earth from the Sun (an astronomical unit) was unknown. Worse, the prevailing model of the solar system was still the geocentric Ptolemaic system as the Catholic church had suppressed publication of the Copernicus' heliocentric system.

Johannes Kepler was the first to predict a transit of Venus in 1631, but no one observed it, possibly because Kepler's predictions were not sufficiently accurate.

The first observation of a transit of Venus was made by Jeremiah Horrocks from his home in Much Hoole in England on 24 November 1639. Horrocks predicted the transit by correcting Kepler's calculation for the orbit of Venus, although he was uncertain of the exact time. In the event 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. 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".

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, his wife, and his 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 indeed it was held to be the first real evidence that Venus had an atmosphere; however recent studies claim 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 is however a good deal of interest in the present transit because scientists will be able to measure the pattern of light dimming as Venus blocks out some of the Sun's light. It is hoped that by studying this, new smaller planets orbiting other stars may be discovered. Current methods of looking for "wobbles" only work for planets that are very large (their gravity is strong enough to wobble the star sufficiently for us to detect). Measuring light intensity of the course of a transit as the planet blocks out some of the light is potentially much more sensitive.

Past and future transits

See also

• Astronomical transit

External links

• Predictions for the 2004 Transit of Venus
• Transits of Venus, Six Millennium Catalog: 2000 BCE to 4000 CE
• The Transit of Venus: Where to See It
• transit of venus