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Transit of Venus

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   The 2004 transit of Venus
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   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, 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.

   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.

Conjunctions

   Diagram of transits of Venus and the angle between the orbital planes
   of Venus and Earth
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   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. Transits
   occur when the two planets happen to be in conjunction at (or very
   near) the points 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. 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.

   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.

   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.

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.
   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. 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; they embodied Venus in the form of the god
   Kukulkán (Quetzalcoatl). In the Dresden Codex, the Maya chart Venus'
   full cycle, but despite their precise knowledge of its procession,
   there is no mention of the transit.

Modern observations

   Measuring Venus transit times to determine solar parallax
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   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.

   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.
   Jeremiah Horrocks makes the first observation of the transit of Venus
   in 1639
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   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.
   The 1882 transit of Venus
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   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.

   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), 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. 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. The 1769 transit saw scientists
   travelling 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. 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. 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).
   The "black drop effect" visible during the 2004 transit
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   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.

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

   There was however a good deal of interest in the 2004 transit as
   scientists attempted to measure the pattern of light dimming as Venus
   blocked out some of the Sun's light, in order to refine techniques that
   they hope to use in searching for extrasolar planets. Current methods
   of looking for planets orbiting other stars only work for planets that
   are very large ( Jupiter-like, not Earth-like), whose gravity is strong
   enough to wobble the star sufficiently for us to detect changes in
   proper motion or Doppler shift changes in radial velocity. Measuring
   light intensity during the course of a transit, as the planet blocks
   out some of the light, is potentially much more sensitive, and might be
   used to find smaller planets. However, extremely precise measurement is
   needed: for example, the transit of Venus causes the Sun's light to
   drop by a mere 0.001 magnitude, and the dimming produced by small
   extrasolar planets will be similarly tiny.

Past and future transits

   Transits can currently occur only in June or December (see table).
   These dates are slowly getting later; before 1631, they were in May and
   November. Transits usually occur in pairs, on nearly the same date
   eight years apart. This is because the length of eight Earth years is
   almost the same as 13 years on Venus, so every eight years the planets
   are in roughly the same relative positions. 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
   1153. 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.
   Transits of Venus
   Date of
   mid-transit Time ( UTC) Notes Transit Path
   (HM Nautical
   Almanac Office)
   Start Mid End
   1631 December 7 03:51 05:19 06:47 Predicted by Kepler
   1639 December 4 14:57 18:25 21:54 First transit observed by Horrocks
   and Crabtree
   1761 June 6 02:02 05:19 08:37 Lomonosov observes the atmosphere of
   Venus
   1769 June 3 19:15 22:25 01:35 Captain Cook's voyage to Tahiti
   1874 December 9 01:49 04:07 06:26 Pietro Tacchini leads expedition to
   Muddapur, India.
   1882 December 6 13:57 17:06 20:15 John Phillip Sousa composes a march,
   "The Transit of Venus", in honour of the transit.
   2004 June 8 05:13 08:20 11:26 Various media networks globally broadcast
   live video of the Venus transition.
   2012 June 6 22:09 01:29 04:49 Visible in its entirety from Hawaii,
   Australia, the Pacific and eastern Asia, with the beginning of the
   transit visible from North America.
   2117 December 11 23:58 02:48 05:38 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.
   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.
   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.
   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.
   2360 December 13 22:32 01:44 04:56 Visible in entirety in Australia and
   most of Indonesia. Partly visible in Asia, Africa, and the western half
   of the Americas.
   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.
   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.
   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.

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,
   while the next such transit will occur on 13 December 2611. It is also
   possible that a transit of Venus can be seen in some parts of the world
   as a partial transit, while in others Venus misses the Sun. Such a
   transit last occurred on 19 November, 541 BC, and the next transit of
   this type will occur on 14 December 2854.

   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. 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. The day after the transit
   of Venus on June 3, 1769 there was a total solar eclipse, which was
   visible in Northern America, Europe and Northern Asia almost as partial
   solar eclipse.

Observing

   Eclipse viewing glasses can be used to observe the transit.
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   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 colour 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.

   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:
    1. First contact: Venus is entirely outside the disk of the Sun,
       moving inward
    2. Second contact: Venus is entirely inside the disk of the Sun,
       moving further inward
    3. Third contact: Venus is entirely inside the disk of the Sun, moving
       outward
    4. Fourth contact: Venus is entirely outside the disk of the Sun,
       moving outward.

   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.

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