Black drop effect
Just after second contact, and again just before third contact during the transit, a small black "teardrop" appears to connect Venus' disk to the 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' 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 smearing of the image of Venus by turbulence in the Earth's atmosphere or imperfections in the viewing apparatus.  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.
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. Larger telescopes, better optics, and limb darkening may have been a factor.
The black drop effect as depicted by Torbern Bergman in 1761.
- "Explanation of the Black-Drop Effect at Transits of Mercury and the Forthcoming Transit of Venus". AAS. December 2003. Retrieved 2015-02-26.
- "Transits of Venus – Kiss of the goddess". The Economist. 27 May 2004. Retrieved 25 September 2006.
- Schneider, G.; Pasachoff, J.M.; Golub, L. (2003). "Space Studies of the Black-Drop Effect at a Mercury Transit". arXiv:astro-ph/0310379.
- "The Disappearing Black Drop". SkyandTelescope.com. 24 May 2012. Retrieved 2012-05-25.
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