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frameless, upright=0.5 A transit of Venus across the Sun takes place when the
planet A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
Venus passes directly between the Sun and a
superior planet In the Solar System, a planet is said to be inferior or interior with respect to another planet if its orbit lies inside the other planet's orbit around the Sun. In this situation, the latter planet is said to be superior to the former. In the ref ...
, becoming visible against (and hence obscuring a small portion of) the solar disk. During a transit, Venus can be seen from
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...
as a small black dot moving across the face of the Sun. The duration of such transits is usually several hours (the transit of 2012 lasted 6 hours and 40 minutes). A transit is similar to a
solar eclipse A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby obscuring the view of the Sun from a small part of the Earth, totally or partially. Such an alignment occurs during an eclipse season, approximately every six mon ...
by the
Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
. While the diameter of Venus is more than three times that of the Moon, Venus appears smaller, and travels more slowly across the face of the Sun, because it is much farther away from Earth. Transits of Venus are among the rarest of predictable astronomical phenomena. They occur in a pattern that generally repeats every 243 years, with pairs of transits eight years apart separated by long gaps of 121.5 years and 105.5 years. The periodicity is a reflection of the fact that the
orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting pla ...
s of Earth and Venus are close to 8:13 and 243:395 commensurabilities. The last transit of Venus was on 5 and 6 June 2012, and was the last Venus transit of the 21st century; the prior transit took place on 8 June 2004. The previous pair of transits were in December 1874 and December 1882. The next transits of Venus will take place on 10–11 December 2117 and 8 December 2125. Venus transits are historically of great scientific importance as they were used to gain the first realistic estimates of the size of the
Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
. Observations of the 1639 transit provided an estimate of both the size of Venus and the distance between the Sun and the Earth that was more accurate than any other up to that time. Observational data from subsequent predicted transits in 1761 and 1769 further improved the accuracy of this initial estimated distance through the use of the principle of parallax. The 2012 transit provided scientists with a number of other research opportunities, particularly in the refinement of techniques to be used in the search for
exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...
s.


Conjunctions

Venus, with an orbit inclined by 3.4° relative to the Earth's, usually appears to pass under (or over) the Sun at inferior conjunction. A transit occurs when Venus reaches conjunction with the Sun at or near one of its nodes—the longitude where Venus passes through the Earth's orbital plane (the ecliptic)—and appears to pass directly across the Sun. Although the inclination between these two orbital planes is only 3.4°, Venus can be as far as 9.6° from the Sun when viewed from the Earth at inferior conjunction. Since the
angular diameter The angular diameter, angular size, apparent diameter, or apparent size is an angular distance describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the visual angle, and in optics, it ...
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. Sequences of transits usually repeat every 243 years. After this period of time Venus and Earth have returned to very nearly the same point in their respective orbits. During the Earth's 243 sidereal orbital periods, which total 88,757.3 days, Venus completes 395 sidereal orbital periods of 224.701 days each, equal to 88,756.9 Earth days. 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, because of the slight mismatch between the times when the Earth and Venus arrive at the point of conjunction. Prior to 1518, the pattern of transits was 8, 113.5 and 121.5 years, and the eight inter-transit gaps before the AD 546 transit were 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. Since the 243:395 Earth:Venus commensurability is only approximate, there are different sequences of transits occurring 243 years apart, each extending for several thousand years, which are eventually replaced by other sequences. For instance, there is a series which ended in 541 BC, and the series which includes 2117 only started in AD 1631.


History of observation


Ancient and medieval history

Ancient
Indian Indian or Indians may refer to: Peoples South Asia * Indian people, people of Indian nationality, or people who have an Indian ancestor ** Non-resident Indian, a citizen of India who has temporarily emigrated to another country * South Asia ...
, Greek, Egyptian,
Babylonia Babylonia (; Akkadian: , ''māt Akkadī'') was an ancient Akkadian-speaking state and cultural area based in the city of Babylon in central-southern Mesopotamia (present-day Iraq and parts of Syria). It emerged as an Amorite-ruled state c ...
n and Chinese observers knew of Venus and recorded the planet's motions. The early Greek astronomers called Venus by two names—''Hesperus'' the evening star and ''Phosphorus'' the morning star.
Pythagoras Pythagoras of Samos ( grc, Πυθαγόρας ὁ Σάμιος, Pythagóras ho Sámios, Pythagoras the Samian, or simply ; in Ionian Greek; ) was an ancient Ionian Greek philosopher and the eponymous founder of Pythagoreanism. His poli ...
is credited with realizing they were the same planet. There is no evidence that any of these cultures knew of the transits. Venus was important to ancient American civilizations, in particular for the Maya, who called it ''Noh Ek'', "the Great Star" or ''Xux Ek'', "the Wasp Star"; they embodied Venus in the form of the god Kukulkán (also known as or related to Gukumatz and
Quetzalcoatl Quetzalcoatl (, ; Spanish: ''Quetzalcóatl'' ; nci-IPA, Quetzalcōātl, ket͡saɬˈkoːaːt͡ɬ (Modern Nahuatl pronunciation), in honorific form: ''Quetzalcōātzin'') is a deity in Aztec culture and literature whose name comes from the Nah ...
in other parts of Mexico). In the
Dresden Codex The ''Dresden Codex'' is a Maya book, which was believed to be the oldest surviving book written in the Americas, dating to the 11th or 12th century. However, in September 2018 it was proven that the Maya Codex of Mexico, previously known as t ...
, the Maya charted Venus's full cycle, but despite their precise knowledge of its course, there is no mention of a transit. However, it has been proposed that frescoes found at
Mayapan Mayapan (Màayapáan in Modern Maya; in Spanish Mayapán) is a Pre-Columbian Maya site a couple of kilometers south of the town of Telchaquillo in Municipality of Tecoh, approximately 40 km south-east of Mérida and 100 km west of ...
may contain a pictorial representation of the 12th or 13th century transits. The Persian polymath Avicenna claimed to have observed Venus as a spot on the Sun. This is possible, as there was a transit on 24 May 1032, but Avicenna did not give the date of his observation, and modern scholars have questioned whether he could have observed the transit from his location at that time; he may have mistaken a sunspot for Venus. He used his transit observation to help establish that Venus was, at least sometimes, below the Sun in Ptolemaic cosmology, i.e. the sphere of Venus comes before the sphere of the Sun when moving out from the Earth in the prevailing geocentric model.


1639first scientific observation

In 1627,
Johannes Kepler Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws ...
became the first person to predict a transit of Venus, by predicting the 1631 event. His methods were not sufficiently accurate to predict that the transit would not be visible in most of Europe, and as a consequence, nobody was able to use his prediction to observe the phenomenon. The first recorded observation of a transit of Venus was made by Jeremiah Horrocks from his home at Carr House in Much Hoole, near
Preston Preston is a place name, surname and given name that may refer to: Places England *Preston, Lancashire, an urban settlement **The City of Preston, Lancashire, a borough and non-metropolitan district which contains the settlement **County Boro ...
in England, on 4 December 1639 (24 November under the
Julian calendar The Julian calendar, proposed by Roman consul Julius Caesar in 46 BC, was a reform of the Roman calendar. It took effect on , by edict. It was designed with the aid of Greek mathematicians and astronomers such as Sosigenes of Alexandri ...
then in use in England). His friend, William Crabtree, also observed this transit from Broughton, near
Manchester Manchester () is a city in Greater Manchester, England. It had a population of 552,000 in 2021. It is bordered by the Cheshire Plain to the south, the Pennines to the north and east, and the neighbouring city of Salford to the west. The ...
. Kepler had predicted transits in 1631 and 1761 and a near miss in 1639. Horrocks corrected Kepler's calculation for the orbit of Venus, realized that transits of Venus would occur in pairs 8 years apart, and so predicted the transit of 1639. Although he was uncertain of the exact time, he calculated that the transit was to begin at approximately 15:00. Horrocks focused the image of the Sun through a simple
telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to obse ...
onto a piece of paper, where the image could be safely observed. After observing for most of the day, he was lucky to see the transit as clouds obscuring the Sun cleared at about 15:15, just half an hour before sunset. Horrocks's observations allowed him to make a well-informed guess as to the size of Venus, as well as to make an estimate of the mean distance between the Earth and the Sunthe
astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits ...
(AU). He estimated that distance to be about two thirds of the actual distance of , but a more accurate figure than any suggested up to that time. The observations were not published until 1661, well after Horrocks's death. Horrocks based his calculation on the (false) presumption that each planet's size was proportional to its rank from the Sun, ''not'' on the parallax effect as used by the 1761 and 1769 and following experiments.


1761 and 1769

In 1663 Scottish mathematician James Gregory had suggested in his ''Optica Promota'' that observations of a transit of the planet Mercury, at widely spaced points on the surface of the Earth, could be used to calculate the
solar parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby object ...
and hence the astronomical unit using triangulation. Aware of this, a young Edmond Halley made observations of such a transit on 28 October O.S. 1677 from Saint Helena but was disappointed to find that only
Richard Towneley Richard Towneley (10 October 1629 – 22 January 1707) was an English mathematician, natural philosopher and astronomer, resident at Towneley Hall, near Burnley in Lancashire. His uncle was the antiquarian and mathematician Christopher Town ...
in Burnley, Lancashire had made another accurate observation of the event whilst Gallet, at Avignon, simply recorded that it had occurred. Halley was not satisfied that the resulting calculation of the solar parallax at 45" was accurate. In a paper published in 1691, and a more refined one in 1716, he proposed that more accurate calculations could be made using measurements of a transit of Venus, although the next such event was not due until 1761 (6 June N.S., 26 May O.S.). Halley died in 1742, but in 1761 numerous expeditions were made to various parts of the world so that precise observations of the transit could be made in order to make the calculations as described by Halley—an early example of international scientific collaboration. This collaboration was, however, underpinned by competition, the British, for example, being spurred to action only after they heard of French plans from Joseph-Nicolas Delisle. In an attempt to observe the first transit of the pair, astronomers from Britain ( William Wales and Captain James Cook), Austria ( Maximilian Hell) and France ( Jean-Baptiste Chappe d'Auteroche and Guillaume Le Gentil) traveled to destinations around the world, including Siberia, Newfoundland and Madagascar. Most managed to observe at least part of the transit, particularly successful observations were made by Jeremiah Dixon and Charles Mason at the Cape of Good Hope. Less successful, at Saint Helena, were Nevil Maskelyne and Robert Waddington, although they put the voyage to good use by trialling the lunar-distance method of finding longitude. That Venus might have an atmosphere was widely expected (because of the plurality of worlds belief) even before the transit of 1761. However, few if any seem to have predicted that it might be possible to actually detect it during the transit. The actual discovery of the atmosphere on Venus has long been attributed to the Russian Academician Mikhail Lomonosov on the basis of his observation of the transit of Venus of 1761 from the Imperial Academy of Sciences of St. Petersburg. At least in the English-speaking world, this attribution seems to have been owing to comments from the multi-lingual popular astronomy writer Willy Ley (1966), who consulted sources in both Russian and German, and wrote that Lomonosov observed a luminous ring (this was Ley's interpretation and was not indicated in quotes) and inferred from it the existence of an atmosphere "maybe greater than that of the Earth" (which was in quotes). Because many modern transit observers have also seen a threadlike arc produced by refraction of sunlight in the atmosphere of Venus when the planet has progressed off the limb of the Sun, it has generally, if rather uncritically, been assumed that this was the same thing that Lomonosov saw. Indeed, the term "Lomonosov’s arc" has frequently been used in the literature. In 2012, Pasachoff and Sheehan consulted original sources, and questioned the basis for the claim that Lomonosov observed the thin arc produced by the atmosphere of Venus. A reference to the paper was even picked up by the Russian state-controlled media group RIA Novosti on 31 January 2013, under the headline "Astronomical Battle in US Over Lomonosov’s discovery." An interesting attempt was made by a group of researchers to experimentally reconstruct Lomonosov's observation using antique telescopes during the transit of Venus on 5–6 June 2012. One of them, Y. Petrunin, suggested that the telescope Lomonosov actually used was probably a 50 mm Dolland with a magnifying power of 40x. It was preserved at Pulkova Observatory but destroyed when the Germans bombed the observatory during World War II. Thus, Lomonosov's actual telescope was not available, but other presumably similar instruments were employed on this occasion, and led the researchers to affirm their belief that Lomonosov's telescope would have been adequate to the task of detecting the arc. Thus A. Koukarine, using a 67 mm Dollond on Mt. Hamilton, where seeing was likely much better than Lomonosov enjoyed at St. Petersburg, clearly observed the spiderweb-thin arc known to be due to refraction in the atmosphere of Venus. However, Koukarine's sketches do not really resemble the diagram published by Lomonosov. On the other hand, Koukarine's colleague V. Shiltsev, who more nearly observed under the same conditions as Lomonosov (using a 40 mm Dollond at Batavia, Illinois), did produce a close duplicate of Lomonosov's diagram; however, the rather large wing of light shown next to the black disk of Venus in his drawing (and Lomonosov's) is too coarse to have been the arc. Instead it appears to be a complicated manifestation of the celebrated optical effect known as the " black drop". (It should be kept in mind that, as stated in Sheehan and Westfall, "optical distortions at the interface between Venus and the Sun during transits are impressively large, and any inferences from them are fraught with peril". Again, the actual words used by Lomonosov do not refer to an "arc" at all. In the Russian version, he described a brief brightening lasting a second or so, just before third contact, which appeared to Pasachoff and Sheehan to most probably indicate a last fleeting glimpse of the photosphere. As a check against this, Lomonosov's German version (he had learned to speak and write German fluently as a student at Marburg) was also consulted; he describes seeing "ein ganz helles Licht, wie ein Haar breit"=”a very bright light, as wide as a hair". Here, the adverb "ganz" in connection with "helles" (bright) could mean "as bright as possible" or "completely bright"), i.e., as bright as the surface brightness of the solar disk, which is even stronger evidence that this can't be Venus's atmosphere, which always appears much fainter. Lomonosov's original sketches, if they existed, do not appear to have survived, Modern observations made during the nineteenth century transits and especially those of 2004 and 2012 suggest that what Lomonosov saw was not the arc associated with the atmosphere of Venus at all but the bright flash of the solar photosphere before third contact. The first observers to record the actual arc associated with the atmosphere of Venus, in a form comporting with modern observations, appear to have been Chappe, Rittenhouse, Wayles and Dymond and several others at the transit in June 1769. For the 1769 transit (taking place on 3–4 June N.S., 23 May O.S.), scientists traveled to Tahiti, Norway, and locations in North America including Canada, New England, and San José del Cabo (
Baja California Baja California (; 'Lower California'), officially the Free and Sovereign State of Baja California ( es, Estado Libre y Soberano de Baja California), is a state in Mexico. It is the northernmost and westernmost of the 32 federal entities of Mex ...
, then under Spanish control). The Czech astronomer Christian Mayer was invited by
Catherine the Great , en, Catherine Alexeievna Romanova, link=yes , house = , father = Christian August, Prince of Anhalt-Zerbst , mother = Joanna Elisabeth of Holstein-Gottorp , birth_date = , birth_name = Princess Sophie of Anha ...
to observe the transit in
Saint Petersburg Saint Petersburg ( rus, links=no, Санкт-Петербург, a=Ru-Sankt Peterburg Leningrad Petrograd Piter.ogg, r=Sankt-Peterburg, p=ˈsankt pʲɪtʲɪrˈburk), formerly known as Petrograd (1914–1924) and later Leningrad (1924–1991), i ...
with Anders Johan Lexell, while other members of the
Russian Academy of Sciences The Russian Academy of Sciences (RAS; russian: Росси́йская акаде́мия нау́к (РАН) ''Rossíyskaya akadémiya naúk'') consists of the national academy of Russia; a network of scientific research institutes from across t ...
went to eight other locations in the
Russian Empire The Russian Empire was an empire and the final period of the Russian monarchy from 1721 to 1917, ruling across large parts of Eurasia. It succeeded the Tsardom of Russia following the Treaty of Nystad, which ended the Great Northern War ...
, under the general coordination of Stepan Rumovsky.
George III of the United Kingdom George III (George William Frederick; 4 June 173829 January 1820) was King of Great Britain and of Ireland from 25 October 1760 until the union of the two kingdoms on 1 January 1801, after which he was King of the United Kingdom of Great Br ...
had the King's Observatory built near his summer residence at
Richmond Lodge Richmond Lodge was a historic property located near the River Thames in Richmond, London, in what is now known as Old Deer Park, lands belonging to the historic Richmond Palace. It was located close to the King's Observatory. It should not be con ...
for him and his royal astronomer Stephen Demainbray to observe the transit. The Hungarian astronomer Maximilian Hell and his assistant János Sajnovics traveled to
Vardø ( fi, Vuoreija, fkv, Vuorea, se, Várggát) is a municipality in Troms og Finnmark county in the extreme northeastern part of Norway. Vardø is the easternmost town in Norway, more to the east than Saint Petersburg or Istanbul. The admi ...
, Norway, delegated by Christian VII of Denmark. William Wales and Joseph Dymond made their observation in
Hudson Bay Hudson Bay ( crj, text=ᐐᓂᐯᒄ, translit=Wînipekw; crl, text=ᐐᓂᐹᒄ, translit=Wînipâkw; iu, text=ᑲᖏᖅᓱᐊᓗᒃ ᐃᓗᐊ, translit=Kangiqsualuk ilua or iu, text=ᑕᓯᐅᔭᕐᔪᐊᖅ, translit=Tasiujarjuaq; french: b ...
, Canada, for the
Royal Society The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society and the United Kingdom's national academy of sciences. The society fulfils a number of roles: promoting science and its benefits, re ...
. Observations were made by a number of groups in the British colonies in America. In
Philadelphia Philadelphia, often called Philly, is the largest city in the Commonwealth of Pennsylvania, the sixth-largest city in the U.S., the second-largest city in both the Northeast megalopolis and Mid-Atlantic regions after New York City. Since ...
, the
American Philosophical Society The American Philosophical Society (APS), founded in 1743 in Philadelphia, is a scholarly organization that promotes knowledge in the sciences and humanities through research, professional meetings, publications, library resources, and communit ...
erected three temporary observatories and appointed a committee, of which
David Rittenhouse David Rittenhouse (April 8, 1732 – June 26, 1796) was an American astronomer, inventor, clockmaker, mathematician, surveyor, scientific instrument craftsman, and public official. Rittenhouse was a member of the American Philosophical Society a ...
was the head. Observations were made by a group led by Dr. Benjamin West in
Providence, Rhode Island Providence is the capital and most populous city of the U.S. state of Rhode Island. One of the oldest cities in New England, it was founded in 1636 by Roger Williams, a Reformed Baptist theologian and religious exile from the Massachusetts ...
, and published in 1769. The results of the various observations in the American colonies were printed in the first volume of the American Philosophical Society's ''Transactions'', published in 1771. Comparing the North American observations, William Smith published in 1771 a best value of the solar parallax of 8.48 to 8.49 arc-seconds, which corresponds to an Earth-Sun distance of 24,000 times the Earth's radius, about 3% different from the correct value. Observations were also made from Tahiti by James Cook and Charles Green at a location still known as Point Venus. This occurred on the first voyage of James Cook, after which Cook explored
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and
Australia Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...
. This was one of five expeditions organised by the Royal Society and the Astronomer Royal Nevil Maskelyne. Jean-Baptiste Chappe d'Auteroche went to San José del Cabo in what was then
New Spain New Spain, officially the Viceroyalty of New Spain ( es, Virreinato de Nueva España, ), or Kingdom of New Spain, was an integral territorial entity of the Spanish Empire, established by Habsburg Spain during the Spanish colonization of the A ...
to observe the transit with two Spanish astronomers (Vicente de Doz and Salvador de Medina). For his trouble he died in an epidemic of
yellow fever Yellow fever is a viral disease of typically short duration. In most cases, symptoms include fever, chills, loss of appetite, nausea, muscle pains – particularly in the back – and headaches. Symptoms typically improve within five days. ...
there shortly after completing his observations. Only 9 of 28 in the entire party returned home alive. The unfortunate Guillaume Le Gentil spent over eight years travelling in an attempt to observe either of the transits. His unsuccessful journey led to him losing his wife and possessions and being declared dead (his efforts became the basis of the play '' Transit of Venus'' by
Maureen Hunter Maureen Hunter (born 1948)Hunter, Maureen
in the Canadian Theatre Encyclopedia. is a
) and a subsequent opera, though eventually he regained his seat in the French Academy and had a long marriage. Under the influence of the Royal Society
Ruđer Bošković Roger Joseph Boscovich ( hr, Ruđer Josip Bošković; ; it, Ruggiero Giuseppe Boscovich; la, Rogerius (Iosephus) Boscovicius; sr, Руђер Јосип Бошковић; 18 May 1711 – 13 February 1787) was a physicist, astronomer, ...
travelled to
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, but arrived too late. Unfortunately, it was impossible to time the exact moment of the start and end of the transit because of the phenomenon known as the "
black drop effect The black drop effect is an optical phenomenon visible during a transit of Venus and, to a lesser extent, a transit of Mercury. Description Just after second contact, and again just before third contact during the transit, a small black "tear ...
". This 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 caused by the smearing of the image of Venus by turbulence in the Earth's atmosphere or imperfections in the viewing apparatus along with the extreme brightness variation at the edge (limb) of the Sun as the line-of-sight from Earth goes from opaque to transparent in a small angle. In 1771, using the combined 1761 and 1769 transit data, the French astronomer Jérôme Lalande calculated the astronomical unit to have a value of 153 million kilometers (±1 million km). The precision was less than had been hoped for because of the black drop effect, but still a considerable improvement on Horrocks's calculations. Maximilian Hell published the results of his expedition in 1770, in Copenhagen. Based on the results of his own expedition, and of Wales and Cook, in 1772 he presented another calculation of the astronomical unit: 151.7 million kilometers. Lalande queried the accuracy and authenticity of the Hell expedition, but later he retreated in an article of '' Journal des sçavans'', in 1778.


1874 and 1882

Transit observations in 1874 and 1882 allowed this value to be refined further. Three expeditions—from Germany, the United Kingdom and the United States—were sent to the Kerguelen Archipelago for the 1874 observations. The American astronomer Simon Newcomb combined the data from the last four transits, and he arrived at a value of about 149.59 million kilometers (±0.31 million kilometers). Modern techniques, such as the use of radio
telemetry Telemetry is the in situ collection of measurements or other data at remote points and their automatic transmission to receiving equipment (telecommunication) for monitoring. The word is derived from the Greek roots ''tele'', "remote", an ...
from
space probe A space probe is an artificial satellite that travels through space to collect scientific data. A space probe may orbit Earth; approach the Moon; travel through interplanetary space; flyby, orbit, or land or fly on other planetary bodies; o ...
s, and of
radar Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, Marine radar, ships, spacecraft, guided missiles, motor v ...
measurements of the distances to planets and asteroids in the
Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
, have allowed a reasonably accurate value for the
astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits ...
(AU) to be calculated to a precision of about ±30 meters. As a result, the need for parallax calculations has been superseded.


2004 and 2012

A number of scientific organizations headed by the European Southern Observatory (ESO) organized a network of amateur astronomers and students to measure Earth's distance from the Sun during the transit. The participants' observations allowed a calculation of the astronomical unit (AU) of 149,608,708 km ± 11,835 km which had only a 0.007% difference to the accepted value. There was 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 a few cases: planets that are very large (
Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousand ...
-like, not Earth-like), whose
gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
is strong enough to wobble the star sufficiently for us to detect changes in proper motion or Doppler shift changes in radial velocity; Jupiter or Neptune sized planets very close to their parent star whose transit causes changes in the luminosity of the star; or planets which pass in front of background stars with the planet-parent star separation comparable to the Einstein ring and cause gravitational microlensing. 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 amount of light received from the Sun to drop by a fraction of 0.001 (that is, to 99.9% of its nominal value), and the dimming produced by small extrasolar planets will be similarly tiny. The 2012 transit provided scientists numerous research opportunities as well, in particular in regard to the study of exoplanets. Research of the 2012 Venus transit includes: * Measuring dips in a star's brightness caused by a known planet transiting the Sun will help astronomers find exoplanets. Unlike the 2004 Venus transit, the 2012 transit occurred during an active phase of the 11-year activity cycle of the Sun, and it is likely to give astronomers practice in picking up a planet's signal around a "spotty" variable star. * Measurements made of the apparent diameter of Venus during the transit, and comparison with its known diameter, will give scientists an idea of how to estimate exoplanet sizes. * Observation made of the atmosphere of Venus simultaneously from Earth-based telescopes and from the Venus Express gives scientists a better opportunity to understand the intermediate level of Venus's atmosphere than is possible from either viewpoint alone. This will provide new information about the
climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...
of the planet. * Spectrographic data taken of the well-known atmosphere of Venus will be compared to studies of exoplanets whose atmospheres are thus far unknown. * The
Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most vers ...
, which cannot be pointed directly at the Sun, used the
Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
as a mirror to study the light that had passed through the atmosphere of Venus in order to determine its composition. This will help to show whether a similar technique could be used to study exoplanets.


Past and future transits

NASA maintains a catalog of Venus transits covering the period 2000 BC to 4000 AD. Currently, transits occur only in June or December (see table) and the occurrence of these events slowly drifts, becoming later in the year by about two days every 243-year cycle. 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. This approximate conjunction usually results in a pair of transits, but it is not precise enough to produce a triplet, since Venus arrives 22 hours earlier each time. The last transit not to be part of a pair was in 1396. The next will be in 3089; in 2854 (the second of the 2846/2854 pair), although Venus will just miss the Sun as seen from the Earth's equator, a partial transit will be visible from some parts of the southern hemisphere. Thus after 243 years the transits of Venus return. The 1874 transit is a member of the 243-year cycle #1. The 1882 transit is a member of #2. The 2004 transit is a member of #3 and the 2012 transit is a member of #4. The 2117 transit is a member of #1 and so on. However, the ascending node (December transits) of the orbit of Venus moves backwards after each 243 years so the transit of 2854 is the last member of series #3 instead of series #1. The descending node (June transits) moves forwards, so the transit of 3705 is the last member of #2. From −125,000 till +125,000 there are only about ten 243-year series at both nodes regarding all the transits of Venus in this very long time-span, because both nodes of the orbit of Venus move back and forward in time as seen from the Earth. Over longer periods of time, new series of transits will start and old series will end. Unlike the
saros series The saros () is a period of exactly 223 synodic months, approximately 6585.3211 days, or 18 years, 10, 11, or 12 days (depending on the number of leap years), and 8 hours, that can be used to predict eclipses of the Sun and Moon. One saros period a ...
for lunar eclipses, it is possible for a transit series to restart after a hiatus. The transit series also vary much more in length than the saros series.


Grazing and simultaneous transits

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 third contact). The last transit of this type was on 6 December 1631, and the next such transit will occur on 13 December 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 19 November 541 BC, and the next transit of this type will occur on 14 December 2854. These effects occur due to parallax, since the size of the Earth affords different points of view with slightly different lines of sight to Venus and the Sun. It can be demonstrated by closing an eye and holding a finger in front of a smaller more distant object; when the viewer opens the other eye and closes the first, the finger will no longer be in front of the object. The simultaneous occurrence of a transit of Mercury and a transit of Venus does occur, but extremely infrequently. Such an event last occurred on 22 September 373,173 BC and will next occur on 26 July 69,163, and—given unlikely assumptions on the constancy of Earth's rotation—again on 29 March 224,508."Hobby Q&A", ''Sky&Telescope'', August 2004, p. 138 The simultaneous occurrence of a
solar eclipse A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby obscuring the view of the Sun from a small part of the Earth, totally or partially. Such an alignment occurs during an eclipse season, approximately every six mon ...
and a transit of Venus is currently possible, but very rare. The next solar eclipse occurring during a transit of Venus will be on 5 April 15,232. The last time a solar eclipse occurred during a transit of Venus was on 1 November 15,607 BC. A few hours after the transit of 3–4 June 1769 there was a total solar eclipse, which was visible in
Northern America Northern America is the northernmost subregion of North America. The boundaries may be drawn slightly differently. In one definition, it lies directly north of Middle America (including the Caribbean and Central America).Gonzalez, Joseph. 20 ...
, Europe, and Northern Asia.


See also

*
Aspects of Venus In astrology, an aspect is an angle a planet makes to another planet or point of astrological interest. As the second-brightest object in the night sky after the Moon, often prominent during the morning or evening, Venus has aspects that are rea ...
(astrology) *
Geodynamics of Venus NASA's Magellan spacecraft mission discovered that Venus has a geologically young surface with a relatively uniform age of 500±200 Ma (million years). The age of Venus was revealed by the observation of over 900 impact craters on the surface of ...
*
Outline of Venus The following outline is provided as an overview of and topical guide to Venus: Venus – second planet from the Sun, orbiting it every 224.7 Earth days. It has the longest rotation period (243 days) of any planet in the Solar System and ro ...
* Venus zone


References


Further reading

* * * * * * * * * * *


External links


General


Venus Transits: Measuring the Solar System
describing how transits could be used to measure the Sun's distance, translated from Latin.

*


June 2012 transit



* ttps://web.archive.org/web/20120518015045/http://venustransit.nso.edu/ National Solar Observatory – Transit of Venus 5–6 June 2012br>2012 Transit of Venus Live Webcast and chatroom with SEMS at UND live From Alaska
{{DEFAULTSORT:Transit of Venus Venus Stellar occultation Articles containing video clips