The black drop effect is an optical phenomenon visible during a transit of Venus.

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 accurately time the exact moment of second or third contact. 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 known to be an optical effect due to the Earth's turbulent atmosphere. Indeed, with precise measurements a black drop effect was observed during the 1999 and 2003 transits of Mercury, although Mercury has no atmosphere.

In the June 8, 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 [1] (http://skyandtelescope.com/news/article_1277_1.asp). Bigger telescopes and better optics may have been a factor.

External links

• Explanation of the Black-Drop Effect at Transits of Mercury and the Forthcoming Transit of Venus (http://www.aas.org/publications/baas/v35n5/aas203/26.htm)—abstract at AAS website
• The "Black Drop" effect (http://www.transitofvenus.org/blackdrop.htm)—explanation at the TransitOfVenus.org (http://www.transitofvenus.org/) website
• Sky and Telescope: Where Was the Black Drop? (http://skyandtelescope.com/news/article_1277_1.asp)