![Venustransit 2004-06-08 07-44](https://upload.wikimedia.org/wikipedia/commons/3/30/Venustransit_2004-06-08_07-44.jpg)
The black drop effect is an optical phenomenon visible during a
transit of Venus
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A transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun and a superior planet, becoming visible against (and hence obscuring a small portion of) the solar disk. During a trans ...
and, to a lesser extent, a
transit of Mercury
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A transit of Mercury across the Sun takes place when the planet Mercury passes directly between the Sun and a superior planet. During a transit, Mercury appears as a tiny black dot moving across the Sun as the planet obs ...
.
Description
Just after
second contact, and again just before
third contact during the transit, a small black "teardrop" appears to connect
Venus
Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...
's disc to the
limb
Limb may refer to:
Science and technology
*Limb (anatomy), an appendage of a human or animal
*Limb, a large or main branch of a tree
*Limb, in astronomy, the curved edge of the apparent disk of a celestial body, e.g. lunar limb
*Limb, in botany, ...
of the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
, making it impossible to time the exact moment of second or third contact accurately. This led to the failure of the attempts during the 18th century transits of Venus to establish a truly precise value for the
astronomical unit
The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits t ...
.
The black drop effect was long thought to be due to Venus's thick atmosphere, and indeed it was held to be the first real evidence that Venus had an atmosphere. However, it is now thought by many to be an optical effect caused by the combination of the extreme darkening of the Sun's disk near its apparent edge and the intrinsic imperfection of the viewing apparatus.
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Observing Mercury simultaneously during its transit in May 1832 with different instruments,
Bessel and
Argelander noticed a black drop effect (though the term had not been coined yet) with that instrument of less resolution. With precise measurements, a black drop effect was observed from outside the Earth's atmosphere during the 1999 and 2003
transits of Mercury
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A transit of Mercury across the Sun takes place when the planet Mercury passes directly between the Sun and a superior planet. During a transit, Mercury appears as a tiny black dot moving across the Sun as the planet obsc ...
, although
Mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
has no significant atmosphere.
[
PrePrint or article in Icarus 168, 249–256. Retrieved 2015-05-16.]
transit of Venus, many observers reported that they did not see the black drop effect, or at least that it was much less pronounced than had been reported in earlier centuries' transits.
Larger telescopes, better optics, and
limb darkening
Limb darkening is an optical effect seen in stars (including the Sun), where the central part of the disk appears brighter than the edge, or ''limb''. Its understanding offered early solar astronomers an opportunity to construct models with such ...
may have been factors.
1832 Mercury transit
The
Shuckburgh telescope
The Shuckburgh telescope or Shuckburgh equatorial refracting telescope was a diameter aperture telescope on an equatorial mount completed in 1791 for Sir George Shuckburgh (1751–1804) in Warwickshire, England, and built by British instrument ...
of the
Royal Observatory, Greenwich
The Royal Observatory, Greenwich (ROG; known as the Old Royal Observatory from 1957 to 1998, when the working Royal Greenwich Observatory, RGO, temporarily moved south from Greenwich to Herstmonceux) is an observatory situated on a hill in ...
in London was used for the 1832
transit of Mercury
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A transit of Mercury across the Sun takes place when the planet Mercury passes directly between the Sun and a superior planet. During a transit, Mercury appears as a tiny black dot moving across the Sun as the planet obs ...
.
It was equipped with a micrometer by Dollond and was used for a report of the events as seen through the small refractor.
By observing the transit in combination with timing it and taking measures, a diameter for the planet was taken.
They also reported the peculiar effects that they compared to pressing a coin into the Sun.
The observer remarked:
Gallery
File:Venus Black Drop effect.png, The black drop effect as depicted by Torbern Bergman
Torbern Olaf (Olof) Bergman (''KVO'') (20 March 17358 July 1784) was a Swedish chemist and mineralogist noted for his 1775 ''Dissertation on Elective Attractions'', containing the largest chemical affinity tables ever published. Bergman was the ...
in 1761.
File:Venus Drawing.jpg, The black drop effect as observed by Captain James Cook
James Cook (7 November 1728 Old Style date: 27 October – 14 February 1779) was a British explorer, navigator, cartographer, and captain in the British Royal Navy, famous for his three voyages between 1768 and 1779 in the Pacific Ocean an ...
and Charles Green in 1771.
File:BlackDrop-Venus-Transit.jpg, The black drop effect in 2004, in moments of "bad" seeing (left) and "good" seeing (right).
See Also
*
Shadow blister effect
The shadow blister effect is a visual phenomenon in which a shadow bulges (or blisters) as it approaches another.
The effect takes place when two objects are at varying distances between a non-point light source and a background upon which the ...
References
External links
The black drop effectThe "Black Drop" effect€”explanation at th
TransitOfVenus.orgwebsite
Video Simulating Black Drop effect using your hand
{{DEFAULTSORT:Black Drop Effect
Astronomical transits
Optical phenomena