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The black drop effect is an optical phenomenon visible during a
File:Venus Black Drop effect.png, The black drop effect as depicted by Torbern Bergman in 1761.
File:Venus Drawing.jpg, The black drop effect as observed by Captain
The black drop effect
The "Black Drop" effect
€”explanation at th
TransitOfVenus.org
website
Video Simulating Black Drop effect using your hand
{{DEFAULTSORT:Black Drop Effect Astronomical transits Optical phenomena
The black drop effect is an optical phenomenon visible during a transit of Venus
A transit of Venus takes place when Venus passes directly between the Sun and the Earth (or any other superior planet), becoming visible against (and hence obscuring a small portion of) the solar disk. During a transit, Venus is visible as ...
and, to a lesser extent, a transit of Mercury
file:Mercury transit symbol.svg, frameless, upright=0.5
A transit of Mercury across the Sun takes place when the planet Mercury (planet), Mercury passes directly between the Sun and a superior planet. During a Astronomical transit, transit, Merc ...
.
Description
Just after second contact, and again just before third contact during the transit, a small black "teardrop" appears to connectVenus
Venus is the second planet from the Sun. It is often called Earth's "twin" or "sister" planet for having almost the same size and mass, and the closest orbit to Earth's. While both are rocky planets, Venus has an atmosphere much thicker ...
's disc to the limb of the Sun
The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
, 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 AU) is a unit of length defined to be exactly equal to . Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth's aphelion and perihelion), before its m ...
.
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.
Observing Mercury simultaneously during its transit in May 1832 with different instruments, the German astronomers Friedrich Wilhelm Bessel
Friedrich Wilhelm Bessel (; 22 July 1784 – 17 March 1846) was a German astronomer, mathematician, physicist, and geodesist. He was the first astronomer who determined reliable values for the distance from the Sun to another star by the method ...
and Friedrich Wilhelm 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, although Mercury 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 may have been factors.
1832 Mercury transit
The Shuckburgh telescope of theRoyal 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 Gre ...
in London was used for the 1832 transit of Mercury
file:Mercury transit symbol.svg, frameless, upright=0.5
A transit of Mercury across the Sun takes place when the planet Mercury (planet), Mercury passes directly between the Sun and a superior planet. During a Astronomical transit, transit, Merc ...
. 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
James Cook
Captain (Royal Navy), Captain James Cook (7 November 1728 – 14 February 1779) was a British Royal Navy officer, explorer, and cartographer famous for his three voyages of exploration to the Pacific and Southern Oceans, conducted between 176 ...
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 th ...
References
External links
The black drop effect
The "Black Drop" effect
€”explanation at th
TransitOfVenus.org
website
Video Simulating Black Drop effect using your hand
{{DEFAULTSORT:Black Drop Effect Astronomical transits Optical phenomena