:''This article is about the astronomical phenomenon. For other meanings, see
Transit of Venus (disambiguation).''
A 'transit of Venus' across the
Sun takes place when the
planet Venus passes directly between the Sun and
Earth, obscuring a small portion of the Sun's 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, but, although the diameter of Venus is almost 4 times that of the Moon, Venus appears much smaller because it is much farther away from Earth. Before the
space age, observations of transits of Venus helped scientists using the
parallax method to calculate the distance between the Sun and the Earth.
Transits of Venus are among the rarest of predictable astronomical phenomena and currently 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. Before 2004, the last pair of transits were in December 1874 and December 1882. The first of a pair of transits of Venus in the beginning of the 21st century took place on
June 8,
2004 (see
Transit of Venus, 2004) and the next will be on
June 6,
2012 (see
Transit of Venus, 2012). After 2012, the next transits of Venus will be in December 2117 and December 2125.
[1]
A transit of Venus can be safely observed by taking the same precautions as when observing the partial phases of a
solar eclipse. Staring at the brilliant disk of the Sun (the
photosphere) with the unprotected eye can quickly cause serious and often permanent eye damage.
[2]
Conjunctions
Diagram of transits of Venus and the angle between the orbital planes of Venus and Earth
Normally when the Earth and Venus are in
conjunction they are not aligned with the Sun. Venus' orbit is inclined by 3.4° to the Earth's so it appears to pass under (or over) the Sun in the sky.
[3]
Transits occur when the two planets happen to be in conjunction at (or very near) the line where their orbital planes cross. Although the inclination is only 3.4°, Venus can be as far as 9.6° from the Sun when viewed from the Earth at
inferior conjunction.
[4] 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.
3
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.
[5]
The pattern of 105.5, 8, 121.5 and 8 years is not the only pattern that is possible within the 243-year cycle, due to 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 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.
5[6]
Ancient history
Ancient
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.
[7] Pythagoras is credited with realizing they were the same planet. In the 4th century BC,
Heraclides Ponticus proposed that both Venus and Mercury orbited the Sun rather than Earth. There is no evidence that any of these cultures knew of the transits.
[8]
Venus was important to
ancient American civilizations, in particular for the
Maya, who called it ''Chak ek'', "''the Great Star''" and considered it possibly even more important than the Sun;
7 they embodied Venus in the form of the god
Kukulkán (Quetzalcoatl) and based their calendar (see
Mayan calendar) largely around the cycles of Venus. In the
Dresden Codex, the Maya chart Venus' full cycle, but despite their precise knowledge of its course, there is no mention of the transit.
[9]
Modern observations
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 size of the
solar system by employing the
parallax method. The technique is to make precise observations of the slight difference in the time of either the start or the end of the transit from widely separated points on the Earth's surface. The distance between the points on the Earth can then be used as to calculate the distance to Venus and the Sun via
triangulation.
[10]
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 calculated.
Johannes Kepler was the first to predict a transit of Venus in 1631, but no one observed it, because Kepler's predictions were not sufficiently accurate to predict that the transit would not be visible in most of Europe.
[11]
Jeremiah Horrocks makes the first observation of the transit of Venus in 1639
The first observation of a transit of Venus was made by
Jeremiah Horrocks from his home in
Much Hoole, near
Preston in
England, on
4 December 1639 (
November 24 under the
Julian calendar then in use in England). His friend,
William Crabtree, also observed this transit from
Salford, near
Manchester. Kepler had predicted transits in 1631 and 1761 and a near miss in 1639. Horrocks corrected
Kepler's calculation for the orbit of Venus and realised that transits of Venus would occur in pairs 8 years apart, and so predicted the transit in 1639. Although he was uncertain of the exact time, 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 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, 0.639
AU) - about half the correct size of 93 million miles, but a more accurate figure than any suggested up to that time. However, Horrocks' observations were not published until 1661, well after his death.
[12]
The 1882 transit of Venus
Based on 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.
[ Mikhail Lomonosov and the discovery of the atmosphere of Venus during the 1761 transit, Mikhail Ya. Marov, , , Proceedings of the International Astronomical Union, ]
The transit pair of 1761 and 1769 were used to try to determine the precise value of the
astronomical unit (AU) using
parallax. This method of determining the AU was first described by
James Gregory in ''
Optica Promota'' in 1663. Following the proposition put forward by
Edmond Halley (who had died almost twenty years earlier),
10 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,
Norway,
Newfoundland and
Madagascar.
[13] Most managed to observe at least part of the transit, but excellent readings were made in particular by
Jeremiah Dixon and
Charles Mason at the
Cape of Good Hope.
[14] For the 1769 transit scientists travelled to
Hudson Bay,
Baja California (then under Spanish control) and Norway, as well as the first voyage of
Captain Cook in order to observe the transit from
Tahiti.
[15] The Czech astronomer
Christian Mayer was invited by
Catherine the Great to observe the transit in
Saint Petersburg, but his observations were mostly obscured by clouds.
[16] 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).
13
The "black drop effect" visible during the 2004 transit
Unfortunately, it was impossible to time the exact moment of the start and end of the transit due to the phenomenon known as the "
black drop effect". The black drop effect was long thought to be due to Venus' 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.
[17][18]
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 kilometres(±1 million km). The precision was less than hoped-for because of the black drop effect, but still a considerable improvement on Horrocks' calculations.
13 Transit observations in 1874 and 1882 allowed this value to be refined further. The American astronomer,
Simon Newcomb, combined the data from the last four transits and derived a value of 149.59 million kilometres (±0.31 million km). Modern techniques, such as
space probe telemetry and
radar observations of
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.
1318
There was however a good deal of interest in the 2004 transit as scientists attempted to measure the pattern of light dimming as Venus blocked out some of the Sun's light, in order to refine techniques that they hope to use in searching for
extrasolar planets.
18[19] Current methods of looking for planets orbiting other
stars only work for planets that are very large (
Jupiter-like, not Earth-like), whose
gravity is strong enough to wobble the star sufficiently for us to detect changes in
proper motion or
Doppler shift changes in
radial velocity. Measuring light intensity during the course of a transit, as the planet blocks out some of the light, is potentially much more sensitive, and might be used to find smaller planets.
18 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.
[20]
Past and future transits
Transits can currently occur only in June or December (see table). These dates are slowly getting later; before 1631, they were in May and November.
5 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.
5 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 centre of the Earth, a partial transit will be visible from some parts of the southern hemisphere.
[21]
Future Transits of Venus| Transits of Venus |
|---|
Date(s) of
transit | Time (UTC) | Notes | Transit Path
(HM Nautical
Almanac Office) | ||
|---|---|---|---|---|---|
| Start | Mid | End |
|---|
| 2012 June 5-6 | 22:09
June 5 | 01:29
June 6 | 04:49
June 6 | Visible in its entirety from Hawaii, Australia, the Pacific and eastern Asia, with the beginning of the transit visible from North America. | [11] |
| 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. | [12] |
| 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. | [13] |
| 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. | [14] |
| 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. | [15] |
| 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. | [16] |
| 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. | [17] |
| 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. | [18] |
| 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. | [19] |
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.
5 It is also possible that a transit of Venus can be seen in some parts of the world as a partial transit, while in others Venus misses the Sun. Such a transit last occurred on November 19, 541 BC, and the next transit of this type will occur on
14 December,
2854.
5
The simultaneous occurrence of a
transit of Mercury and a transit of Venus is possible, but only in the distant future. Such an event will next occur on
26 July,
69163, and again in
224508.
[22][23] 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,
15232.
22 The day after the transit of Venus on
June 3 1769 there was a total solar eclipse,
[24]
which was visible in Northern America, Europe and Northern Asia.
Observing
Eclipse viewing glasses can be used to observe the transit.
The safest way to observe a transit is to project the image of the Sun through a
telescope,
binoculars, 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 layer of
chromium, eclipse viewing glasses, or Grade 14
welder's glass. An earlier method of using exposed black-and-white
film 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
retinal cells.
2[25]
For the amateur astronomer there are four "contacts" of interest during the transit - moments when the circumference of Venus touches the circumference of the Sun at a single point:
#First contact: Venus is entirely outside the disk of the Sun, moving inward
#Second contact: Venus is entirely inside the disk of the Sun, moving further inward
#Third contact: Venus is entirely inside the disk of the Sun, moving outward
#Fourth contact: Venus is entirely outside the disk of the Sun, moving outward.
2
A fifth point is that of greatest transit, when Venus is at the middle of its path across the disk of the Sun and which marks the halfway point in the timing of the transit.
2
See also
★
Astronomical transit
Notes
1. June 8, 2004: The Transit of Venus John E. Westfall
2. Transit of Venus - Safety
3. Venus compared to Earth
4. Transit Motion Applet Juergen Giesen
5. Transits of Venus, Six Millennium Catalog: 2000 BCE to 4000 CE Fred Espenak
6. Transits of Venus from Earth John Walker
7. Planet Venus: Earth's 'evil twin' Paul Rincon
8. Heraclides of Pontus J.J. O'Connor and E.F. Robertson
9. The Dresden Codex — the Book of Mayan Astronomy Bohumil Böhm and Vladimir Böhm
10. A New Method of Determining the Parallax of the Sun, or His Distance from the Earth, Sec. R. S., N0 348, Dr. Edmund Halley, , , , ,
11. 1631 Transit of Venus HM Nautical Almanac Office
12. Jeremiah Horrocks - young genius and first Venus transit observer, Paul Marston, , , University of Central Lancashire, ,
13. How far to the sun? The Venus Transits of 1761 & 1769 Prof. Richard Pogge
14. National Dictionary of Biography - Biography of Jeremiah Dixon
15. The Voyages of Captain Cook, , , , Wordsworth Editions Ltd, , ISBN 1-84022-100-3
16. An Account of the Transit of Venus: In a Letter to Charles Morton, M. D. Secret. R. S. from Christian Mayer, S. J. Translated from the Latin by James Parsons, M. D., Christian Mayer, , , Royal society (GB). Philosophical transactions,
17. Explanation of the Black-Drop Effect at Transits of Mercury and the Forthcoming Transit of Venus
18. Transits of Venus - Kiss of the goddess
19. Extrasolar planet hunters eye Venus transit Maggie McKee
20. 2004 and 2012 Transits of Venus Fred Espenak
21. Transits of Venus 1000 AD – 2700 AD Steve Bell
22. "Hobby Q&A", ''Sky&Telescope'', August 2004, p. 138.
23. Transits of Mercury, Seven Century Catalog: 1601 CE to 2300 CE Fred Espenak
24. Observations of the Transit of Venus on June 3, 1769, and the Eclipse of the Sun on the Following Day, Made at Paris, and Other Places. Extracted from Letters Addressed from M. De la Lande, of the Royal Academy of Sciences at Paris, and F. R. S. to the Astronomer Royal; And from a Letter Addressed from M. Messier to Mr. Magalhaens, de La Lande, M.; Messier, M., , , Philosophical Transactions (1683–1775), 1769
25. Eye Safety During Solar Eclipses (Adapted from NASA RP 1383 Total Solar Eclipse of 1998 February 26, April 1996, p. 17.) Fred Espenak
External links
★ ''
Chasing Venus, Observing the Transits of Venus'' Smithsonian Institution Libraries
★
Predictions for the 2012 transit of Venus
★
Venus Transit FAQ
★
The 2004 Transit of Venus Time Series for Ingress, Maximum and Egress.
★
''Reanimating the 1882 Transit of Venus'', in ''Sky and Telescope'' magazine online
★
Transit of Venus
★
Transit of Venus
★
The History of the Transits of Venus at h2g2
★
Historical observations of the transit of Venus
★
Two exoplanets discovered by transit light dimming method
★ Venustransit from 1000 to 10000 see you http://www.marco-peuschel.de
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