View source for Black drop effect

{{Short description|Visual distortion of Venus when it transits the sun}}
{{distinguish|One-drop rule}}
{{Use dmy dates|date=June 2022}}
[[File:Venustransit 2004-06-08 07-44.jpg|thumb|250px|Venus transits the face of the [[Sun]] on 8 June 2004. Here, the black drop effect is visible.]]
The '''black drop effect''' is an optical phenomenon visible during a [[transit of Venus]] and, to a lesser extent, a [[transit of Mercury]].

==Description==
Just after [[astronomical transit#Contacts|second contact]], and again just before [[astronomical transit#Contacts|third contact]] during the transit, a small black "teardrop" appears to connect [[Venus]]'s disc to the [[wikt:limb#Etymology 2|limb]] of the [[Sun]], 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 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.<ref name="aas">
{{cite conference
 |title= Explanation of the Black-Drop Effect at Transits of Mercury and the Forthcoming Transit of Venus by Pasachoff/Schneider/Golub
|conference=American Astronomical Society Meeting 203
|journal=Bulletin of the American Astronomical Society
|volume=35
|page=1202
|publisher=AAS
|bibcode=2003AAS...203.0104P
 |date=December 2003
}}</ref><ref name="economist">
{{cite magazine
 |url=http://www.economist.com/science/displayStory.cfm?story_id=2705523
 |title= Transits of Venus – Kiss of the goddess
 |magazine= The Economist
 |date=27 May 2004
 |accessdate=2015-05-16
}}</ref>

Observing Mercury simultaneously during its transit in May 1832 with different instruments, [[Friedrich Wilhelm Bessel]] and [[Friedrich Wilhelm Argelander]] noticed a black drop effect (though the term had not been coined yet) with that instrument of less resolution.<ref>F. W. Bessel: ''Durchgang des Mercurs durch die Sonne.'' In: ''Astronomische Nachrichten.'' Band&nbsp;X (1832), No.&nbsp;228, Sp.&nbsp;185–196 ([http://articles.adsabs.harvard.edu/full/1832AN.....10..185B digital]).</ref> With precise measurements, a black drop effect was observed from outside the Earth's atmosphere during the 1999 and 2003 [[transit of Mercury|transits of Mercury]], although [[Mercury (planet)|Mercury]] has no significant atmosphere.<ref>
{{cite arXiv
 |last1= Schneider |first1=G.
 |last2=Pasachoff  |first2=J.M.
 |last3=Golub      |first3=L.
 |eprint=astro-ph/0310379
 |title=Space Studies of the Black-Drop Effect at a Mercury Transit
 |year=2003
}} PrePrint or article in Icarus 168, 249–256. Retrieved 2015-05-16.</ref>

{{nat|In the 8 June 2004}} 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.<ref name="Sky &amp; Telescope">{{cite web |url=http://www.skyandtelescope.com/observing/highlights/The-Disappearing-Black-Drop-153838145.html |title=The Disappearing Black Drop |last=Shiga |first=David |publisher=F+W Meida, Inc. |magazine=Sky & Telescope |date=24 May 2012 |accessdate=2015-05-16 |archive-url=https://web.archive.org/web/20120916205336/http://www.skyandtelescope.com/observing/highlights/The-Disappearing-Black-Drop-153838145.html |archive-date=16 September 2012 |url-status=dead }}</ref> Larger telescopes, better optics, and [[limb darkening]] may have been factors.

==1832 Mercury transit==
The [[Shuckburgh telescope]] of the [[Royal Observatory, Greenwich]] in London was used for the 1832 [[transit of Mercury]].<ref name=":3" /> It was equipped with a micrometer by Dollond and was used for a report of the events as seen through the small refractor.<ref name=":3">{{Cite book|url=https://archive.org/details/monthlynoticesr72socigoog|page=[https://archive.org/details/monthlynoticesr72socigoog/page/n132 103]|quote=Shuckburgh telescope eastern equatorial.|title=Monthly Notices of the Royal Astronomical Society|date=1831|publisher=Priestley and Weale|language=en}}</ref> By observing the transit in combination with timing it and taking measures, a diameter for the planet was taken.<ref name=":3" /> They also reported the peculiar effects that they compared to pressing a coin into the Sun.<ref name=":3" /> The observer remarked:

{{quote2|I afterwards observed, that immediately around the planet there was a dusky tinge, making it appear as if, in a small degree sunk below the sun's surface;"|''Royal Astronomical Society, Vol II, No. 13''<ref name=":3" /> }}

==Gallery==
<gallery>
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 [[James Cook]] and [[Charles Green (astronomer)|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). 
</gallery>

==See also==
*[[Shadow blister effect]]

==References==
{{Reflist}}

== External links ==
{{Commons category|Black drop effect}}
* [http://www.transitofvenus.nl/blackdrop.html The black drop effect]
* [https://web.archive.org/web/20101218112451/http://www.transitofvenus.org/history/black-drop The "Black Drop" effect]—explanation at the [http://www.transitofvenus.org/ TransitOfVenus.org] website
* [https://web.archive.org/web/20120916205336/http://www.skyandtelescope.com/observing/highlights/The-Disappearing-Black-Drop-153838145.html ''Sky and Telescope'': The Disappearing Black Drop]
* [https://www.youtube.com/watch?v=wylnvUor4Z0 Video Simulating Black Drop effect using your hand]

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[[Category:Astronomical transits]]
[[Category:Optical phenomena]]