Transit of Venus
A transit of Venus across the
Sun takes place when the
planet Venus passes directly between the Sun and the
Earth, obscuring a small portion of the Sun's disc. During a
transit, Venus can be seen from the Earth as a small black disc moving across the face of the Sun. The duration of such transits is usually measured in 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 further away from the Earth. Before modern
astronomy, observations of transits of Venus helped scientists using the
parallax method calculate the distance between the Sun and the Earth.
Encyclopedia
A
transit of Venus across the
Sun takes place when the
planet Venus passes directly between the Sun and the
Earth, obscuring a small portion of the Sun's disc. During a
transit, Venus can be seen from the Earth as a small black disc moving across the face of the Sun. The duration of such transits is usually measured in 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 further away from the Earth. Before modern
astronomy, observations of transits of Venus helped scientists using the
parallax method calculate the distance between the Sun and the Earth.
Transits of Venus are one of 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 and the next in this pair will be on June 6, 2012 . After 2012, the next transits of Venus will be in December 2117 and December 2125.
A transit of Venus should be observed only with proper precautions - the same measures should be taken as when observing the partial phases of a
solar eclipse. Staring at the brilliant disk of the Sun with the unprotected eye can quickly cause serious and often permanent eye damage.
Conjunctions
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 the Sun in the sky.
Transits occur when the two planets happen to be in conjunction at 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.ences of transits occur in a pattern that repeats every 243 years, with transits occuring 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 is 88757.3 days, and 395 sidereal orbital periods of Venus 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. Prior to 1518, the pattern of transits was 8, 113.5 and 121.5 years, and prior to 546, transits always took place 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.
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 . 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 - 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.
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 using
parallax. Following the proposition put forward by
Edmond Halley , The unfortunate Guillaume Le Gentil spent eight years travelling in an attempt observe either of the transits; his unsuccessful journey led to him losing his wife and possessions and being declared dead .age:Venustransit 2004-06-08 07-44.jpg|thumb|200px|left|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. The precision was less than hoped-for because of the black drop effect, but still a considablr improvement on Horrocks' calculations.e 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.st and future transits
Transits can currently occur only in June or December . These dates are slowly getting later; before 1631, they were in May and November.order=1 style="border-collapse:collapse" cellspacing=0 cellpadding=5 align="center"
!colspan=6 bgcolor="#cccccc" |Transits of Venus
|-
!rowspan=2| Date of
mid-transit
!colspan=3| Time
!rowspan=2| Notes
!rowspan=2| Transit Path
|-
! Start !! Mid !! End
|-
| 1631 Dec 07
| 03:51 || 05:19 || 06:47
| Predicted by Kepler
|
|-
| 1639 Dec 04
| 14:57 || 18:25 || 21:54
| First transit observed by Horrocks and Crabtree
|
|-
| 1761 Jun 06
| 02:02 || 05:19 || 08:37
| Lomonosov observes the atmosphere of Venus
|
|-
| 1769 Jun 03
| 19:15 || 22:25 || 01:35
| Captain Cook's voyage to Tahiti
|
|-
| 1874 Dec 09
| 01:49 || 04:07 || 06:26
| Pietro Tacchini leads expedition to Muddapur, India.
|
|-
| 1882 Dec 06
| 13:57 || 17:06 || 20:15
| John Phillip Sousa composes a march, "The Transit of Venus", in honor of the transit.tp://www.nao.rl.ac.uk/nao/transit/V_1882/]
|- bgcolor="#ffffcc"
! 2004 Jun 08
! 05:13 !! 08:20 !! 11:26
| Various media networks globally broadcast live video of the Venus transition.
|
|-
| 2012 Jun 06
| 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 Dec 11
| 23:58 || 02:48 || 05:38
| Visible in entirety in eastern China, Japan, Indonesia, and Australia. Partly visible on extreme U.S. West Coast, and in India, most of Africa, and the Middle East.
|
|-
| 2125 Dec 08
| 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 Jun 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 Jun 09
| 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 Dec 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 Dec 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 Jun 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 Jun 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.
|
|}
Observing
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. Observing the Sun directly without filters can cause a temporary or permanent loss of visual function, as it can damage or destroy retinal cells.lear="all" />
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.fth contact 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.azing 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 . The last transit of this type was on December 6th, 1631, while the next such transit will occur on December 13th, 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 November 19th, 541BC, and the next transit of this type will occur on December 14th, 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 July 26, 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 April 5, 15232.ncient history
Thousands of years ago, early 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;ee also
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Notes
External links
- Time Series for Ingress, Maximum and Egress.
