Transit of Venus - AbsoluteAstronomy.com

A transit of Venus across the Sun

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

takes place when the planet

Planet

A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

Venus

Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

passes directly between the Sun and Earth

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, becoming visible against (and hence obscuring a small portion of) the solar disk. During a transit

Astronomical transit

The term transit or astronomical transit has three meanings in astronomy:* A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, hiding a small part of it, as seen by an observer at some particular vantage point...

, Venus can be seen from Earth as a small black disk moving across the face of the Sun. The duration of such transits is usually measured in hours (the transit of 2004 lasted six hours). A transit is similar to a solar eclipse

Solar eclipse

As seen from the Earth, a solar eclipse occurs when the Moon passes between the Sun and the Earth, and the Moon fully or partially blocks the Sun as viewed from a location on Earth. This can happen only during a new moon, when the Sun and the Moon are in conjunction as seen from Earth. At least...

by the Moon

Moon

The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

. While the diameter of Venus is almost four 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. Observations of transits of Venus helped scientists use the principle of parallax

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. The term is derived from the Greek παράλλαξις , meaning "alteration"...

to calculate the distance between the Sun and the Earth.

Transits of Venus are among the rarest of predictable astronomical phenomena. They occur in a pattern that 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 periods of Earth and Venus are close to 8:13 and 243:395 resonances.

Before 2004, the last pair of transits were in December 1874 and December 1882. The first

Transit of Venus, 2004

The most recent transit of Venus when observed from Earth took place on June 8, 2004. The event received significant attention, since it was the first Venus transit to take place after the invention of broadcast media...

of a pair of transits of Venus in the beginning of the 21st century took place on 8 June 2004 and the next

Transit of Venus, 2012

The next transit of Venus will occur on June 5–June 6 in 2012, succeeding the previous transit on June 8, 2004. After 2012, the next transits of Venus will be in December 2117 and December 2125.-Where visible:...

will be on 6 June 2012. After 2012, subsequent transits of Venus will be in December 2117 and December 2125.

A transit of Venus can be safely observed by taking the same precautions used when observing the partial phases of a solar eclipse

Solar eclipse

. Staring at the brilliant disk of the Sun (the photosphere

Photosphere

The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...

) with the unprotected eye can quickly cause serious and often permanent eye damage.

Conjunctions

Venus, with an orbit inclined by 3.4° relative to the Earth's, usually appears to pass under (or over) the Sun in the sky 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, called the ecliptic

Ecliptic

The ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around 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

Angular diameter

The angular diameter or apparent size of an object as seen from a given position is the “visual diameter” of the object measured as an angle. In the vision sciences it is called the visual angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its...

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 occur in a pattern that repeats every 243 years, with transits occurring eight years apart followed by a gap of 121.5 years, then a gap of eight years and then another long gap of 105.5 years. The pattern repeats every 243 years because 243 sidereal orbital periods

Sidereal year

A sidereal year is the time taken by the Earth to orbit the Sun once with respect to the fixed stars. Hence it is also the time taken for the Sun to return to the same position with respect to the fixed stars after apparently travelling once around the ecliptic. It was equal to at noon 1 January...

of the Earth (365.25636 days — slightly longer than the tropical year

Tropical year

A tropical year , for general purposes, is the length of time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice...

) is 88757.3 days, and 395 sidereal orbital periods of Venus (224.701 days) is 88756.9 days. Thus, after this period 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, 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.

Ancient and medieval history

Ancient Greek

Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

, Egyptian

Ancient Egypt

Ancient Egypt was an ancient civilization of Northeastern Africa, concentrated along the lower reaches of the Nile River in what is now the modern country of Egypt. Egyptian civilization coalesced around 3150 BC with the political unification of Upper and Lower Egypt under the first pharaoh...

, Babylonia

Babylonia

Babylonia was an ancient cultural region in central-southern Mesopotamia , with Babylon as its capital. Babylonia emerged as a major power when Hammurabi Babylonia was an ancient cultural region in central-southern Mesopotamia (present-day Iraq), with Babylon as its capital. Babylonia emerged as...

n, and Chinese

China

Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

observers knew of Venus and recorded the planet’s motions. The early Greeks thought that the evening and morning appearances of Venus represented two different objects, Hesperus - the evening star and Phosphorus - the morning star.Pythagoras

Pythagoras

Pythagoras of Samos was an Ionian Greek philosopher, mathematician, and founder of the religious movement called Pythagoreanism. Most of the information about Pythagoras was written down centuries after he lived, so very little reliable information is known about him...

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

Maya civilization

The Maya is a Mesoamerican civilization, noted for the only known fully developed written language of the pre-Columbian Americas, as well as for its art, architecture, and mathematical and astronomical systems. Initially established during the Pre-Classic period The Maya is a Mesoamerican...

, who called it Noh Ek, "the Great Star" or Xux Ek, "the Wasp Star"; they embodied Venus in the form of the god Kukulkán

Kukulkan

Kukulkan is the name of a Maya snake deity that also serves to designate historical persons. The depiction of the feathered serpent deity is present in other cultures of Mesoamerica. Kukulkan is closely related to the god Q'uq'umatz of the K'iche' Maya and to Quetzalcoatl of the Aztecs...

(also known as or related to Gukumatz

Gukumatz

Q'uq'umatz was a deity of the Postclassic K'iche Maya. Q'uq'umatz was the feathered serpent god of the Popol Vuh who created humanity together with the god Tepeu...

and Quetzalcoatl

Quetzalcoatl

Quetzalcoatl is a Mesoamerican deity whose name comes from the Nahuatl language and has the meaning of "feathered serpent". The worship of a feathered serpent deity is first documented in Teotihuacan in the first century BCE or first century CE...

in other parts of Mexico). In the Dresden Codex

Dresden Codex

The Dresden Codex, also known as the Codex Dresdensis, is a pre-Columbian Maya book of the eleventh or twelfth century of the Yucatecan Maya in Chichén Itzá. The Maya codex is believed to be a copy of an original text of some three or four hundred years earlier...

, the Maya charted Venus' full cycle, but despite their precise knowledge of its course, there is no mention of the transit.

The transit of Venus was claimed to have been observed by medieval Islamic astronomers. The Persian

Persian people

The Persian people are part of the Iranian peoples who speak the modern Persian language and closely akin Iranian dialects and languages. The origin of the ethnic Iranian/Persian peoples are traced to the Ancient Iranian peoples, who were part of the ancient Indo-Iranians and themselves part of...

polymath Avicenna

Avicenna

Abū ʿAlī al-Ḥusayn ibn ʿAbd Allāh ibn Sīnā , commonly known as Ibn Sīnā or by his Latinized name Avicenna, was a Persian polymath, who wrote almost 450 treatises on a wide range of subjects, of which around 240 have survived...

claimed to have observed the 24 May 1032 Venus transit of the Sun. He used this observation to demonstrate that Venus

Venus

was, at least sometimes, below the Sun in the Ptolemaic cosmology. Soon after, he wrote the Compendium of the Almagest, a commentary on Ptolemy

Ptolemy

Claudius Ptolemy , was a Roman citizen of Egypt who wrote in Greek. He was a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in Egypt under Roman rule, and is believed to have been born in the town of Ptolemais Hermiou in the...

's Almagest
Almagest
The Almagest is a 2nd-century mathematical and astronomical treatise on the apparent motions of the stars and planetary paths. Written in Greek by Claudius Ptolemy, a Roman era scholar of Egypt,...

, in which he concluded that Venus

Venus

is closer to the Earth than the Sun

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

. In the 12th century, the Andalusian

Al-Andalus

Al-Andalus was the Arabic name given to a nation and territorial region also commonly referred to as Moorish Iberia. The name describes parts of the Iberian Peninsula and Septimania governed by Muslims , at various times in the period between 711 and 1492, although the territorial boundaries...

astronomer Ibn Bajjah

Ibn Bajjah

Abū-Bakr Muhammad ibn Yahya ibn al-Sāyigh , known as Ibn Bājjah , was an Andalusian polymath: an astronomer, logician, musician, philosopher, physician, physicist, psychologist, botanist, poet and scientist. He was known in the West by his Latinized name, Avempace...

reported seeing "the two planets as black spots on the face of the Sun." In the 13th century, the Maragha

Maragheh observatory

Maragheh observatory is an astronomical observatory which was established in 1259 CE by Nasir al-Din al-Tusi, an Iranian scientist and astronomer...

astronomer Qotb al-Din Shirazi identified Ibn Bajjah's observation as transits of Venus and Mercury. However, Ibn Bajjah could not observe a transit of Venus, as none occurred in his lifetime.

Modern observations

Aside 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

Solar System

The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

by employing the parallax

Parallax

method and Kepler's third law. The technique involved making 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's surface. The distance between the points on the Earth was then used as a baseline to calculate the distance to Venus and the Sun via triangulation

Triangulation

In trigonometry and geometry, triangulation is the process of determining the location of a point by measuring angles to it from known points at either end of a fixed baseline, rather than measuring distances to the point directly...

.

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

Astronomical unit

An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

), an accurate absolute value of this distance had not been determined.

In 1627, Johannes Kepler

Johannes Kepler

Johannes Kepler was a German mathematician, astronomer and astrologer. A key figure in the 17th century scientific revolution, he is best known for his eponymous laws of planetary motion, codified by later astronomers, based on his works Astronomia nova, Harmonices Mundi, and Epitome of Copernican...

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 make arrangements to observe the transit.

First European scientific observation

The first European scientific observation of a transit of Venus was made by Jeremiah Horrocks

Jeremiah Horrocks

Jeremiah Horrocks , sometimes given as Jeremiah Horrox , was an English astronomer who was the only person to predict, and one of only two people to observe and record, the transit of Venus of 1639.- Life and work :Horrocks was born in Lower Lodge, in...

from his home at Carr House

Carr House

Carr House, is situated within the Bank Hall Estate, half-way between the villages of Tarleton and Much Hoole at the extreme north-west of the village of Bretherton, Lancashire...

in Much Hoole

Much Hoole

Much Hoole is a village and civil parish in the borough of South Ribble, Lancashire, England. The parish of Much Hoole had a population of 1,851 at the time of the 2001 census.-History:...

, near Preston in England

England

England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

, on 4 December 1639 (24 November under the Julian calendar

Julian calendar

The Julian calendar began in 45 BC as a reform of the Roman calendar by Julius Caesar. It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year .The Julian calendar has a regular year of 365 days divided into 12 months...

then in use in England). His friend, William Crabtree

William Crabtree

William Crabtree was an astronomer, mathematician, and merchant from Broughton, then a township near Manchester, which is now part of Salford, Greater Manchester, England...

, also observed this transit from Salford, near Manchester

Manchester

Manchester is a city and metropolitan borough in Greater Manchester, England. According to the Office for National Statistics, the 2010 mid-year population estimate for Manchester was 498,800. Manchester lies within one of the UK's largest metropolitan areas, the metropolitan county of Greater...

. Kepler had predicted transits in 1631 and 1761 and a near miss in 1639. Horrocks corrected Kepler

Johannes Kepler

's calculation for the orbit of Venus and realized 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, he calculated that the transit was to begin at approximately 3:00 pm. Horrocks focused the image of the Sun through a simple telescope

Telescope

A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

onto a piece of card, 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 3:15 pm, just half an hour before sunset. Horrocks' observations allowed him to make a well-informed guess as to the size of Venus, as well as to make an estimate of the distance between the Earth and the Sun. He estimated the distance of the Sun from the Earth at 59.4 million miles (95.6 Gm

Gigametre

A gigametre is a unit of length in the metric system, equal to one billion meters, the SI base unit of length, hence to 1,000,000 km or approximately 621,370 miles....

, 0.639 AU

Astronomical unit

An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

) – about two thirds the correct distance of 93 million miles (149.6 million km), but a more accurate figure than any suggested up to that time. However, Horrocks' observations were not published until 1661, well after his death.

1761 and 1769

The transit pair of 1761 and 1769 were used to try to determine the precise value of the astronomical unit

Astronomical unit

An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

(AU) using parallax

Parallax

. This method of determining the AU was first described by James Gregory

James Gregory (astronomer and mathematician)

James Gregory FRS was a Scottish mathematician and astronomer. He described an early practical design for the reflecting telescope – the Gregorian telescope – and made advances in trigonometry, discovering infinite series representations for several trigonometric functions.- Biography :The...

in Optica Promota in 1663. Following the proposition put forward by Edmond Halley

Edmond Halley

Edmond Halley FRS was an English astronomer, geophysicist, mathematician, meteorologist, and physicist who is best known for computing the orbit of the eponymous Halley's Comet. He was the second Astronomer Royal in Britain, following in the footsteps of John Flamsteed.-Biography and career:Halley...

(who had died almost twenty years earlier), numerous expeditions were made to various parts of the world in order to observe these transits; an early example of international scientific collaboration. In an attempt to observe the first transit of the pair, scientists and explorers from Britain, Austria and France travelled to destinations around the world, including Siberia

Siberia

Siberia is an extensive region constituting almost all of Northern Asia. Comprising the central and eastern portion of the Russian Federation, it was part of the Soviet Union from its beginning, as its predecessor states, the Tsardom of Russia and the Russian Empire, conquered it during the 16th...

, Norway

Norway

Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...

, Newfoundland and Madagascar

Madagascar

The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...

. Most managed to observe at least part of the transit, but excellent readings were made in particular by Jeremiah Dixon

Jeremiah Dixon

Jeremiah Dixon was an English surveyor and astronomer who is perhaps best known for his work with Charles Mason, from 1763 to 1767, in determining what was later called the Mason-Dixon line....

and Charles Mason

Charles Mason

Charles Mason was an English astronomer who made significant contributions to 18th-century science and American history, particularly through his involvement with the survey of the Mason-Dixon line, which came to mark the division between the northern and southern United States...

at the Cape of Good Hope

Cape of Good Hope

The Cape of Good Hope is a rocky headland on the Atlantic coast of the Cape Peninsula, South Africa.There is a misconception that the Cape of Good Hope is the southern tip of Africa, because it was once believed to be the dividing point between the Atlantic and Indian Oceans. In fact, the...

.

On the basis of his observation of the transit of Venus of 1761 from the Petersburg Observatory, Mikhail Lomonosov

Mikhail Lomonosov

Mikhail Vasilyevich Lomonosov was a Russian polymath, scientist and writer, who made important contributions to literature, education, and science. Among his discoveries was the atmosphere of Venus. His spheres of science were natural science, chemistry, physics, mineralogy, history, art,...

predicted the existence of an atmosphere on Venus. Lomonosov detected the refraction of solar rays while observing the transit and inferred that only refraction through an atmosphere could explain the appearance of a light ring around the part of Venus that had not yet come into contact with the Sun's disk during the initial phase of transit.

For the 1769 transit, scientists traveled to Hudson Bay

Hudson Bay

Hudson Bay , sometimes called Hudson's Bay, is a large body of saltwater in northeastern Canada. It drains a very large area, about , that includes parts of Ontario, Quebec, Saskatchewan, Alberta, most of Manitoba, southeastern Nunavut, as well as parts of North Dakota, South Dakota, Minnesota,...

(Canada), San José del Cabo

San José del Cabo

The city of San José del Cabo is located in Baja California Sur Mexico and is the seat of the municipality of Los Cabos at the south end of the Baja California peninsula. In the 2010 census it had a population of 69,788. Together with neighboring Cabo San Lucas it forms a major tourist destination...

(Baja California

Baja California

Baja California officially Estado Libre y Soberano de Baja California is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is both the northernmost and westernmost state of Mexico. Before becoming a state in 1953, the area was known as the North...

, then under Spanish control), and Norway. Observations were also made from Tahiti on the first voyage of Captain Cook, at a location still known as "Point Venus". The Czech astronomer Christian Mayer was invited by Catherine the Great to observe the transit in Saint Petersburg

Saint Petersburg

Saint Petersburg is a city and a federal subject of Russia located on the Neva River at the head of the Gulf of Finland on the Baltic Sea...

with Anders Johan Lexell

Anders Johan Lexell

Anders Johan Lexell was a Swedish-born Russian astronomer, mathematician, and physicist who spent most of his life in Russia where he is known as Andrei Ivanovich Leksel .Lexell made important discoveries in polygonometry and celestial mechanics; the latter led to a comet named in...

, while other members of Russian Academy of Sciences

Russian Academy of Sciences

The Russian Academy of Sciences consists of the national academy of Russia and a network of scientific research institutes from across the Russian Federation as well as auxiliary scientific and social units like libraries, publishers and hospitals....

went to eight other locations in the Russian Empire

Russian Empire

The Russian Empire was a state that existed from 1721 until the Russian Revolution of 1917. It was the successor to the Tsardom of Russia and the predecessor of the Soviet Union...

. In Philadelphia, the American Philosophical Society

American Philosophical Society

The American Philosophical Society, founded in 1743, and located in Philadelphia, Pa., is an eminent scholarly organization of international reputation, that promotes useful knowledge in the sciences and humanities through excellence in scholarly research, professional meetings, publications,...

erected three temporary observatories and appointed a committee, of which David Rittenhouse

David Rittenhouse

David Rittenhouse was a renowned American astronomer, inventor, clockmaker, mathematician, surveyor, scientific instrument craftsman and public official...

was the head. The results of these observations were printed in the first volume of the Society's Transactions, published in 1771.

The unfortunate Guillaume Le Gentil

Guillaume Le Gentil

Guillaume Joseph Hyacinthe Jean-Baptiste Le Gentil de la Galaisière was a French astronomer.-Biography:...

spent 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
Transit of Venus (play)
Transit of Venus is a play by Canadian playwright Maureen Hunter. It was first produced at the Manitoba Theatre Centre in November 1992.The play is based on the life of Guillaume Le Gentil , a gentleman astronomer....

by Maureen Hunter

Maureen Hunter

Maureen Hunter is a Canadian playwright who lives in Winnipeg, Manitoba.She was born in Indian Head, Saskatchewan and graduated from the University of Saskatchewan. Transit of Venus was performed by the Royal Shakespeare Company and recorded by the BBC...

).

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

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.Just after second contact, and again just before third contact during the transit, a small black "teardrop" appears to connect Venus' disk to the limb of the Sun, making it...

". The black drop effect was long thought to be due to Venus' 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.

In 1771, using the combined 1761 and 1769 transit data, the French astronomer Jérôme Lalande

Jérôme Lalande

Joseph Jérôme Lefrançois de Lalande was a French astronomer and writer.-Biography:Lalande was born at Bourg-en-Bresse...

calculated the astronomical unit to have a value of 153 million kilometers (±1 million km). The precision was less than hoped-for because of the black drop effect, but still a considerable improvement on Horrocks' calculations.

1874 and 1882

Transit observations in 1874 and 1882 allowed this value to be refined further. Several expeditions were sent to the Kerguelen Archipelago

Kerguelen Islands

The Kerguelen Islands , also known as the Desolation Islands, are a group of islands in the southern Indian Ocean constituting the emerged part of the otherwise submerged Kerguelen Plateau. The islands, along with Adélie Land, the Crozet Islands and the Amsterdam and Saint Paul Islands are part of...

for the 1874 observations. The American astronomer Simon Newcomb

Simon Newcomb

Simon Newcomb was a Canadian-American astronomer and mathematician. Though he had little conventional schooling, he made important contributions to timekeeping as well as writing on economics and statistics and authoring a science fiction novel.-Early life:Simon Newcomb was born in the town of...

combined the data from the last four transits and derived a value of 149.59 million kilometers (±0.31 million km). Modern techniques, such as space probe

Space probe

A robotic spacecraft is a spacecraft with no humans on board, that is usually under telerobotic control. A robotic spacecraft designed to make scientific research measurements is often called a space probe. Many space missions are more suited to telerobotic rather than crewed operation, due to...

telemetry

Telemetry

Telemetry is a technology that allows measurements to be made at a distance, usually via radio wave transmission and reception of the information. The word is derived from Greek roots: tele = remote, and metron = measure...

and radar

Radar

Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

observations of solar system

Solar System

objects, have allowed a precise value for the astronomical unit to be calculated (to an accuracy of ±30 m), and so negated the need for parallax calculations.

2004

for details, see Transit of Venus, 2004
Transit of Venus, 2004
The most recent transit of Venus when observed from Earth took place on June 8, 2004. The event received significant attention, since it was the first Venus transit to take place after the invention of broadcast media...

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 planet

Extrasolar planet

An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...

s. Current methods of looking for planets orbiting other star

Star

A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s only work for a few cases: planets that are very large (Jupiter

Jupiter

Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

-like, not Earth-like), whose gravity is strong enough to wobble the star sufficiently for us to detect changes in proper motion

Proper motion

The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...

or Doppler shift changes in radial velocity

Radial velocity

Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...

, Jupiter or Neptune sized planets

Hot Jupiter

Hot Jupiters are a class of extrasolar planet whose mass is close to or exceeds that of Jupiter...

very close to their parent star, or through gravitational microlensing

Gravitational microlensing

Gravitational microlensing is an astronomical phenomenon due to the gravitational lens effect. It can be used to detect objects ranging from the mass of a planet to the mass of a star, regardless of the light they emit. Typically, astronomers can only detect bright objects that emit lots of light ...

by planets which pass in front of background stars with the planet-parent star separation comparable to the Einstein ring

Einstein ring

In observational astronomy an Einstein ring is the deformation of the light from a source into a ring through gravitational lensing of the source's light by an object with an extremely large mass . This occurs when the source, lens and observer are all aligned...

. 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

Apparent magnitude

The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

, and the dimming produced by small extrasolar planets will be similarly tiny.

2012

for details, see Transit of Venus, 2012
Transit of Venus, 2012
The next transit of Venus will occur on June 5–June 6 in 2012, succeeding the previous transit on June 8, 2004. After 2012, the next transits of Venus will be in December 2117 and December 2125.-Where visible:...

The best place to see the 2012 transit is in the Pacific Ocean

Pacific Ocean

The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...

- including Hawai'i, Alaska

Alaska

Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...

, and central Pacific islands. Some organizations have begun to prepare for the 2012 transit well in advance, including a group organizing festivities and observation in Tahiti.

Past and future transits

For a complete list see NASA's Six Millennium Catalog of Venus Transits: 2000 BCE to 4000 CE

Transits can currently occur only in June or December (see table). These dates are slowly becoming later in the year; before 1631, transits occurred 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. 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.

Past Transits of Venus
Transits of Venus
Date(s) of transit	Time (UTC)			Notes	Transit Path (HM Nautical Almanac Office)
Date(s) of transit	Start	Mid	End	Notes	Transit Path (HM Nautical Almanac Office)
1032 May 24	15:19	19:04	22:48	Observed by the Persian astronomer and polymath Avicenna Avicenna Abū ʿAlī al-Ḥusayn ibn ʿAbd Allāh ibn Sīnā , commonly known as Ibn Sīnā or by his Latinized name Avicenna, was a Persian polymath, who wrote almost 450 treatises on a wide range of subjects, of which around 240 have survived... .	http://www.hmnao.com/nao/transit/V_1032/
1396 November 23	15:45	19:27	23:09	Last transit not part of a pair. Some believe Aztec Aztec The Aztec people were certain ethnic groups of central Mexico, particularly those groups who spoke the Nahuatl language and who dominated large parts of Mesoamerica in the 14th, 15th and 16th centuries, a period referred to as the late post-classic period in Mesoamerican chronology.Aztec is the... astronomers may have seen this transit.	http://www.hmnao.com/nao/transit/V_1396/
1518 May 25–26	22:46 May 25	01:56 May 26	05:07 May 26		http://www.hmnao.com/nao/transit/V_1518/
1526 May 23	16:12	19:35	21:48	Last transit before invention of telescope Telescope A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...	http://www.hmnao.com/nao/transit/V_1526/
1631 December 7	03:51	05:19	06:47	Predicted by Kepler Johannes Kepler Johannes Kepler was a German mathematician, astronomer and astrologer. A key figure in the 17th century scientific revolution, he is best known for his eponymous laws of planetary motion, codified by later astronomers, based on his works Astronomia nova, Harmonices Mundi, and Epitome of Copernican...	http://www.hmnao.com/nao/transit/V_1631/
1639 December 4	14:57	18:25	21:54	First transit observed by Horrocks Jeremiah Horrocks Jeremiah Horrocks , sometimes given as Jeremiah Horrox , was an English astronomer who was the only person to predict, and one of only two people to observe and record, the transit of Venus of 1639.- Life and work :Horrocks was born in Lower Lodge, in... and Crabtree William Crabtree William Crabtree was an astronomer, mathematician, and merchant from Broughton, then a township near Manchester, which is now part of Salford, Greater Manchester, England...	http://www.hmnao.com/nao/transit/V_1639/
1761 June 6	02:02	05:19	08:37	Lomonosov Mikhail Lomonosov Mikhail Vasilyevich Lomonosov was a Russian polymath, scientist and writer, who made important contributions to literature, education, and science. Among his discoveries was the atmosphere of Venus. His spheres of science were natural science, chemistry, physics, mineralogy, history, art,... observes the atmosphere of Venus Atmosphere of Venus The atmosphere of Venus is much denser and hotter than that of Earth. The temperature at the surface is 740 K , while the pressure is 93 bar. The Venusian atmosphere supports opaque clouds made of sulfuric acid, making optical Earth-based and orbital observation of the surface impossible...	http://www.hmnao.com/nao/transit/V_1761/
1769 June 3–4	19:15 June 3	22:25 June 3	01:35 June 4	Cook James Cook Captain James Cook, FRS, RN was a British explorer, navigator and cartographer who ultimately rose to the rank of captain in the Royal Navy... 's voyage to Tahiti First voyage of James Cook The first voyage of James Cook was a combined Royal Navy and Royal Society expedition to the south Pacific ocean aboard HMS Endeavour, from 1768 to 1771...	http://www.hmnao.com/nao/transit/V_1769/
1874 December 9	01:49	04:07	06:26	Pietro Tacchini Pietro Tacchini Pietro Tacchini was an Italian astronomer.He was born and raised in Modena, Italy. He studied engineering at the University of Padova. At the age of 21, he was appointed the director of a small observatory in Modena. By 1863 he became the Primo Astronomo Aggiunto, or director of the observatory,... leads expedition to Muddapur, India India India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world... . A French expedition goes to New Zealand's Campbell Island Campbell Island, New Zealand Campbell Island is a remote, subantarctic island of New Zealand and the main island of the Campbell Island group. It covers of the group's , and is surrounded by numerous stacks, rocks and islets like Dent Island, Folly Island , Isle de Jeanette Marie, and Jacquemart Island, the latter being the...	http://www.hmnao.com/nao/transit/V_1874/
1882 December 6	13:57	17:06	20:15	John Phillip Sousa composes a march, "The Transit of Venus Transit of Venus March The Transit of Venus March is a march scored for military brass band written by John Philip Sousa in 1883 to celebrate the 1882 Transit of Venus and published by the J.W. Pepper Company. The work was erroneously thought to be lost for over 100 years when a piano transcriptionThe front cover is... ", in honor of the transit.	http://www.hmnao.com/nao/transit/V_1882/
2004 June 8	05:13	08:20	11:26	Various media networks globally broadcast live video of the Venus transition.	http://www.hmnao.com/nao/transit/V_2004/

Future Transits of Venus
Transits of Venus
Date(s) of transit	Time (UTC)			Notes	Transit Path (HM Nautical Almanac Office)
Date(s) of transit	Start	Mid	End	Notes	Transit Path (HM Nautical Almanac Office)
2012 June 5–6	22:09 June 5	01:29 June 6	04:49 June 6	Visible in its entirety from Hawaii, Alaska, Australia, the Pacific and eastern Asia, with the beginning of the transit visible from North America.	http://www.hmnao.com/nao/transit/V_2012/
2117 December 10–11	23:58 December 10	02:48 December 11	05:38 December 11	Visible in entirety in eastern China, Japan, Taiwan, Indonesia, and Australia. Partly visible on extreme U.S. West Coast, and in India, most of Africa, and the Middle East.	http://www.hmnao.com/nao/transit/V_2117/
2125 December 8	13:15	16:01	18:48	Visible in entirety in South America and the eastern U.S. Partly visible in Western U.S., Europe, and Africa.	http://www.hmnao.com/nao/transit/V_2125/
2247 June 11	08:42	11:33	14:25	Visible in entirety in Africa, Europe, and the Middle East. Partly visible in East Asia and Indonesia, and in North and South America.	http://www.hmnao.com/nao/transit/V_2247/
2255 June 9	01:08	04:38	08:08	Visible in entirety in Russia, India, China, and western Australia. Partly visible in Africa, Europe, and the western U.S.	http://www.hmnao.com/nao/transit/V_2255/
2360 December 12–13	22:32 December 12	01:44 December 13	04:56 December 13	Visible in entirety in Australia and most of Indonesia. Partly visible in Asia, Africa, and the western half of the Americas.	http://www.hmnao.com/nao/transit/V_2360/
2368 December 10	12:29	14:45	17:01	Visible in entirety in South America, western Africa, and the U.S. East Coast. Partly visible in Europe, the western U.S., and the Middle East.	http://www.hmnao.com/nao/transit/V_2368/
2490 June 12	11:39	14:17	16:55	Visible in entirety through most of the Americas, western Africa, and Europe. Partly visible in eastern Africa, the Middle East, and Asia.	http://www.hmnao.com/nao/transit/V_2490/
2498 June 10	03:48	07:25	11:02	Visible in entirety through most of Europe, Asia, the Middle East, and eastern Africa. Partly visible in eastern Americas, Indonesia, and Australia.	http://www.hmnao.com/nao/transit/V_2498/

Over longer periods of time, new series of transits will start and old series will end. Unlike the saros series 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

27th century

The 27th century of the anno Domini era will span from January 1, 2601–December 31, 2700 of the Gregorian calendar.-List of the long total solar eclipses:* August 28, 2603 : Solar eclipse, , of saros 164....

. 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

29th century

The 29th century of the anno Domini era will span from January 1, 2801–December 31, 2900 of the Gregorian calendar.-List of the long total solar eclipses:* May 21, 2813 : Solar eclipse, , of saros 170....

. These effects occur due to parallax

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 you open the other eye and close the first, the finger will no longer be in front of the object.

The simultaneous occurrence of a transit of Mercury

Transit of Mercury

A transit of Mercury across the Sun takes place when the planet Mercury comes between the Sun and the Earth, and Mercury is seen as a small black dot moving across the face of the Sun....

and a transit of Venus is possible, but only in the distant future. Such an event last occurred on 22 September 373,173 BC

Paleolithic

The Paleolithic Age, Era or Period, is a prehistoric period of human history distinguished by the development of the most primitive stone tools discovered , and covers roughly 99% of human technological prehistory...

and will next occur on 26 July 69,163, and again on 29 March 224,508. The simultaneous occurrence of a solar eclipse

Solar eclipse

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.
It could be noticed that the day after the Venusian transit of 3 June 1769 there was a total solar eclipse,
which was visible in Northern America, Europe and Northern Asia.

Observing

The safest way to observe a transit is to project the image of the Sun through a telescope

Telescope

, binoculars

Binoculars

Binoculars, field glasses or binocular telescopes are a pair of identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, allowing the viewer to use both eyes when viewing distant objects...

, or pinhole onto a screen, but the event can be viewed with the naked eye using filters specifically designed for this purpose, such as an astronomical solar filter with a vacuum-deposited

Vacuum deposition

Vacuum deposition is a family of processes used to deposit layers atom-by-atom or molecule-by-molecule at sub-atmospheric pressure on a solid surface. The layers may be as thin as one atom to millimeters thick . There may be multiple layers of different materials...

layer of chromium

Chromium

Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...

, eclipse viewing glasses, or Grade 14 welder's glass

Eye protection

Eye protection is protective gear for the eyes, which comes in many types depending upon the threat that is to be reduced. The threats can be particles, light, wind blast, heat, sea spray or some type of ball or puck used in sports...

. An earlier method of using exposed black-and-white film

Film

A film, also called a movie or motion picture, is a series of still or moving images. It is produced by recording photographic images with cameras, or by creating images using animation techniques or visual effects...

as a filter is no longer regarded as safe, as small imperfections or gaps in the film may permit damaging UV rays to pass through. Also, processed color film (unlike black-and-white film) does not contain silver, and is transparent to infra-red. This may result in burns to the retina. Observing the Sun directly without filters can cause a temporary or permanent loss of visual function, as it can damage or destroy retina

Retina

The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

l cells.

There are four named "contacts" during a transit — moments when the circumference of Venus touches the circumference of the Sun at a single point:

First contact (external ingress): Venus is entirely outside the disk of the Sun, moving inward
Second contact (internal ingress): Venus is entirely inside the disk of the Sun, moving further inward
Third contact (internal egress): Venus is entirely inside the disk of the Sun, moving outward
Fourth contact (external egress): Venus is entirely outside the disk of the Sun, moving outward.

A fifth named point is that of greatest transit, when Venus is at the middle of its path across the solar disk and which marks the halfway point in the timing of the transit.

External links

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Conjunctions

Ancient and medieval history

Modern observations

First European scientific observation

1761 and 1769

1874 and 1882

2004

2012

Past and future transits

Grazing and simultaneous transits

Observing

See also

External links