Transit of Venus
- 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 portion 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. The duration of such transits is usually measured in hours (the transits of 2004 and 2012 last about six hours). 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 currently 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. Before 2004, the last pair of transits of Venus were in December 1874 and December 1882. The first of a pair of transits of Venus in the beginning of the 21st century 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). After 2012, the next transits of Venus will be in 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.
The pattern of 105.5, 8, 121.5 and 8 years is not the only pattern that is possible within the 243-year cycle. Prior to 1518, the pattern of transits was 8, 113.5 and 121.5 years, and prior to 546, transits always took place 121.5 years apart. The current pattern will continue until 2846, when it will be replaced by a pattern of 105.5, 129.5 and 8 years. Thus, the 243-year cycle is relatively stable, but the number of transits and their timing within the cycle will vary over time.
The safest way to observe the event in 2012 would be to project the image of the Sun, as seen through a telescope, onto a screen. Nevertheless, the event can also be seen with free eyes using special stained glasses. Grade 14 of Welder’s glasses should be used. Observing the Sun without filters can cause a temporary or permanent loss of visual function, as it can damage and even destroy retinal cells.
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.
Based on his observation of the transit of Venus of 1761 in Petersburg, Mikhail Lomonosov predicted the existence of an atmosphere on Venus. 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 and then Australia, where his was to be the first expedition to chart the eastern coastline in 1770.
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
Transits can currently occur only in June or December (see table). These dates are slowly getting later; before 1631, they were in May and November. Transits usually occur in pairs, on nearly the same date eight years apart. This is because the length of eight Earth years is almost the same as 13 years on Venus, so every eight years the planets are in roughly the same relative positions. The approximation is usually good enough to provide a pair of transits, but it is not good enough to produce a triplet. The last transit not to be part of a pair was in 1153. The next will be in 3089, though it nearly happens in 2846/2854; in 2854, Venus will just miss the Sun as seen from the centre of the Earth, though a partial transit will be visible from some parts of the southern hemisphere.
|Transits of Venus|
| Date of
|Time (UTC)||Notes|| Transit Path|
|1631 Dec 07||03:51||05:19||06:47||Predicted by Kepler|||
|1639 Dec 04||14:57||18:25||21:54||First transit observed by Horrocks and Crabtree|||
|1761 Jun 06||02:02||05:19||08:37||Lomonosov observes the atmosphere of Venus|||
|1769 Jun 03||19:15||22:25||01:35||Captain Cook's voyage to Tahiti|||
|1874 Dec 09||01:49||04:07||06:26||Pietro Tacchini leads expedition to Muddapur, India.|||
|1882 Dec 06||13:57||17:06||20:15|| John Philip Sousa composes
the Transit of Venus March
|2004 Jun 08||05:13||08:20||11:26||Various media networks globally broadcast live video of the Venus transition.|||
|2012 Jun 06||22:09||01:29||04:49|||
|2117 Dec 11||23:58||02:48||05:38|||
|2125 Dec 08||13:15||16:01||18:48|||
|2247 Jun 11||08:42||11:33||14:25|||
|2255 Jun 09||01:08||04:38||08:08|||
|2360 Dec 13||22:32||01:44||04:56|||
|2368 Dec 10||12:29||14:45||17:01|||
|2490 Jun 12||11:39||14:17||16:55|||
|2498 Jun 10||03:48||07:25||11:02|||
Grazing transits of Venus
Sometimes Venus only grazes the Sun during a transit. In this case it is possible that in some areas of the Earth a full transit can be seen while in other regions there is only a partial transit (no second or fourth contact). The last transit of this type was on December 6th, 1631, while the next such transit will occur on December 13th, 2611.
It is also possible that a transit of Venus can be seen in some parts of the world as a partial transit, while in others Venus misses the Sun. Such a transit last occurred on November 19th, 541BC, and the next transit of this type will occur on December 14th, 2854.
Only 5 hours after the transit of Venus on June 4 1769 there was a total solar eclipse, which was visible in Northern America, Europe and Northern Asia almost as partial solar eclipse. This was the lowest time difference between a transit of a planet and a solar eclipse in the historical past.
In many cultures of the world, the transit of Venus across the Sun has been regarded as a rare and therefore an important event that marks major changes in human history. This is true for the Maya and other ancient American civilizations, for the Indian Vedic traditions and for the Western astrology of Mesopotamian origin. It has been claimed that a transit led to the ancient Greeks celebrating the Ancient Olympics every four years as a form of commemoration.
- Venus Transit FAQ
- 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)
- The 2004 Transit of Venus Halley's method of deriving the AU by a simplified application of 2004 transit data.
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