The 2004 transit of Venus

A transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun and Earth, becoming visible against (and hence obscuring a small portion of) the solar disk. During a transit, 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 by the Moon. 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.

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 commensurabilities.^[1][2]

The next transit of Venus will occur on 5 and 6 June 2012, and will be the last Venus transit this century; the prior transit took place on 8 June 2004. The previous pair of transits were in December 1874 and December 1882. After 2012, the next transits of Venus will be in December 2117 and December 2125.^[3][4]

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. Observations of the 1639 transit, combined with the principle of parallax, provided an estimate of the distance between the Sun and the Earth that was more accurate than any other up to that time. In addition, the June 2012 transit will provide scientists with a number of other research opportunities, particularly the refinement of techniques to be used in the search for exoplanets.

A transit of Venus can be safely observed by taking the same precautions used to observe the partial phases of a solar eclipse. Staring at the Sun without appropriate eye protection can quickly cause serious and often permanent eye damage.^[5]

Conjunctions[link]

Diagram of transits of Venus and the angle between the orbital planes of Venus and Earth

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.^[6] 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). 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.^[7] Since the angular diameter of the Sun is about half a degree, Venus may appear to pass above or below the Sun by more than 18 solar diameters during an ordinary conjunction.^[6]

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 of the Earth (365.25636 days—slightly longer than the tropical year) 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.^[8]

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.^[8][9]

Ancient and medieval history[link]

"Venus Tablet of Ammisaduqa", a cuneiform clay tablet of astrological forecasts from the Neo-Assyrian period. Library of Ashurbanipal. Ref:K.160 .

Ancient Indian, Greek, Egyptian, Babylonian and Chinese 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.^[10] Pythagoras 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";^[11] they embodied Venus in the form of the god Kukulkán (also known as or related to Gukumatz and Quetzalcoatl in other parts of Mexico). In the Dresden Codex, the Maya charted Venus' full cycle, but despite their precise knowledge of its course, there is no mention of the transit.^[12]

Modern observations[link]

Measuring Venus transit times to determine solar parallax

Aside from its rarity, the original scientific interest in observing a transit of Venus was that it could be used to determine the distance from the Earth to the Sun, and from this the size of the Solar System, by employing the 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.^[13]

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

In 1627, Johannes Kepler 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 phenomenon.^[14]

1639 - first scientific observation[link]

Jeremiah Horrocks makes the first observation of the transit of Venus in 1639.

The first known observation of a transit of Venus was made by Jeremiah Horrocks from his home at Carr House in Much Hoole, near Preston in England, on 4 December 1639 (24 November under the Julian calendar then in use in England). His friend, William Crabtree, also observed this transit from Broughton, near Manchester.^[15] 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 in 1639.^[16] 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 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 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 that distance to be 59.4 million miles (95.6 Gm, 0.639 AU) – about two thirds of the actual distance of 93 million miles (149.6 million km) but a more accurate figure than any suggested up to that time. The observations were not published until 1661, well after Horrock's death.^[16]

1761 and 1769[link]

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 and hence the astronomical unit. Aware of this, a young Edmund Halley made observations of such a transit in 1676 from St Helena, but was disappointed to find that there had been only one other observation of the event and was not satisfied that the resulting calculation of the solar parallax at 45" was accurate. In 1678 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. 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.^[17] 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, Norway, Newfoundland and Madagascar.^[18] Most managed to observe at least part of the transit, but successful observations were made in particular by Jeremiah Dixon and Charles Mason at the Cape of Good Hope.^[19]

On the basis of his observation of the transit of Venus of 1761 from the Petersburg Observatory, Mikhail Lomonosov 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.^[20]

For the 1769 transit, scientists traveled to Hudson Bay (Canada), San José del Cabo (Baja California, then under Spanish control), and Norway. Observations were also made from Tahiti on the first voyage of Captain Cook,^[21] at a location still known as "Point Venus".^[22] The Czech astronomer Christian Mayer was invited by Catherine the Great to observe the transit in Saint Petersburg with Anders Johan Lexell, while other members of Russian Academy of Sciences went to eight other locations in the Russian Empire.^[23] In Philadelphia, the American Philosophical Society erected three temporary observatories and appointed a committee, of which David Rittenhouse was the head. The results of these observations were printed in the first volume of the Society's Transactions, published in 1771.^[24]

The "black drop effect" visible during the 2004 transit

The 1882 transit of Venus.

Transit of Venus from Degania A, Israel, 2004

The unfortunate Guillaume Le Gentil 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 by Maureen Hunter).^[18]

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". This 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.^[25][26]

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 hoped-for because of the black drop effect, but still a considerable improvement on Horrocks' calculations.^[18]

1874 and 1882[link]

Transit observations in 1874 and 1882 allowed this value to be refined further. Several expeditions 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 from space probes, and of radar measurements of the distances to planets and asteroids in the Solar System, have allowed a reasonably accurate value for the astronomical unit (AU) to be calculated to a precision of about ±30 meters. Hence the need for parallax calculations has been superseded.^[18][26]

2004[link]

For details, see Transit of Venus, 2004

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.^[26][27] Current methods of looking for planets orbiting other stars only work for a few cases—planets that are very large (Jupiter-like, not Earth-like), whose gravity is strong enough to wobble the star sufficiently for us to detect changes in proper motion or Doppler shift changes in radial velocity, Jupiter or Neptune sized planets very close to their parent star, or through gravitational microlensing by planets which pass in front of background stars with the planet-parent star separation comparable to the Einstein ring.^[28] 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.^[26] However, extremely precise measurement is needed: for example, the transit of Venus causes the Sun's light to drop by a mere 0.001 magnitude, and the dimming produced by small extrasolar planets will be similarly tiny.^[29]

2012[link]

For details, see Transit of Venus, 2012

The full 2012 transit will be visible from the Pacific Ocean—including Hawai'i, northwestern North America, northern Asia, Japan, Korea, New Zealand, eastern Australia and the Pacific islands. Most of North America and northwestern South America will be able to see the start of the transit before sunset, while southern Asia, the Middle East, eastern Africa, western Australia, and most of Europe will be able to observe the end of the transit as the Sun rises.^[29]

As with the 2004 transit, the 2012 transit will provide scientists numerous research opportunities, in particular in regards to the study of exoplanets. Research of the 2012 Venus transit will include:^[30][31][32]

• 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 occurs 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.

• Measurement 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 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 climatology of the planet.

• Spectrographic study of the well-known atmosphere of Venus will be compared to studies of exoplanets whose atmospheres are thus far unknown.

• The Hubble Space Telescope will use the Moon as a mirror to study the light that bounces off Venus to determine the makeup of its atmosphere. This will be a technique that astronomers could also use to study exoplanets.

Past and future transits[link]

William Crabtree observing the transit of Venus, from The Manchester Murals by Ford Madox Brown

• 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). The time of these events is slowly becoming later in the year; before 1631, transits occurred in May and November.^[8] 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.^[8] 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.^[33]

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[link]

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.^[8] 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.^[8] 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 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 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 again on 29 March 224,508.^[34][35] The simultaneous occurrence of a 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.^[34] The last time a solar eclipse occurred during a transit of Venus was on 1 November 15,607 BC.^[36] It could be noticed that the day after the Venerean transit of 3 June 1769 there was a total solar eclipse,^[37] which was visible in Northern America, Europe and Northern Asia.

Observing[link]

Safety precautions[link]

The safest way to watch a transit is to observe an image of the Sun projected onto a screen through a telescope, binoculars, pinhole^[38] or reflected pinhole.^[39] The event can be viewed without magnification using filters specifically designed for this purpose, such as an astronomical solar filter or eclipse viewing glasses coated with a vacuum-deposited layer of chromium. The once-recommended method of using exposed black-and-white film as a filter is not now considered safe, as small imperfections or gaps in the film may permit harmful UV rays to pass through. Observing the Sun directly without appropriate protection can damage or destroy retinal cells, causing temporary or permanent blindness.^[5][40][41]

Contacts[link]

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

1. First contact (external ingress): Venus is entirely outside the disk of the Sun, moving inward
2. Second contact (internal ingress): Venus is entirely inside the disk of the Sun, moving further inward
3. Third contact (internal egress): Venus is entirely inside the disk of the Sun, moving outward
4. Fourth contact (external egress): Venus is entirely outside the disk of the Sun, moving outward.^[5]

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.^[5]

See also[link]

• Transit of Mercury

Notes[link]

Further reading[link]

• Maor, Eli (2000). Venus in Transit. Princeton: Princeton University Press. ISBN 0-691-11589-3. http://www.amazon.com/dp/0691115893. 
• Maunder, Michael; Moore, Patrick (2000). Transit: When Planets Cross the Sun. London: Springer-Verlaf. ISBN 1-85233-621-8. http://www.amazon.co.uk/dp/1852336218. 
• Sellers, David (2001). The Transit of Venus: The Quest to Find the True Distance of the Sun. Leeds, UK: Magavelda Press. ISBN 0-9541013-0-8. http://www.amazon.co.uk/dp/0954101308. 
• Sheehan, William; Westfall, John (2004). The Transits of Venus. Amherst, New York: Prometheus Books. ISBN 1-59102-175-8. http://www.amazon.com/dp/1591021758. 
• Lomb, Nick (2011). Transit of Venus: 1631 to the Present. Sydney, Australia: NewSouth Publishing. ISBN 978-1-74223-269-0. OCLC 717231977. http://www.amazon.com/dp/1615190554. 
• Wulf, Andrea (2012). Chasing Venus: The Race to Measure the Heavens. New York: Knopf. ISBN 978-0-307-70017-9. http://www.amazon.com/dp/0307700178. 
• Anderson, Mark (2012). The Day the World Discovered the Sun: An Extraordinary Story of Scientific Adventure and the Race to Track the Transit of Venus. Boston: Da Capo Press. ISBN 978-0-306-82038-0. http://www.amazon.com/dp/0306820382. 

External links[link]

General

• Transit of Venus – article posts from various authors, historians, astronomers
• Venus Transits: Measuring the Solar System
• Historical observations of the transit of Venus
• The transit of Venus across the Sun (PDF)

June 2012 transit

• Advice from professional Astronomers about observing the transit
• NASA - 2012 Transit of Venus Live Webcast and Celebration
• Transit of Venus from transitofvenus.org
• 2012 Transit of Venus – International Astronomical Union
• Watch Transit of Venus 2012 LIVE - Many Live Webcasts / Broadcasts
• TRANSIT OF VENUS 2012: Live Webcast (Multipoint) – by SWAN-India
• National Solar Observatory – Transit of Venus June 5–6 2012
• NightSkyInfo.com: The Transit of Venus, June 2012
• 2012 Transit of Venus Live Webcast and chatroom with SEMS at UND live From Alaska

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