The black drop effect is an optical phenomenon visible during a transit of Venus and, to a lesser extent, a transit of Mercury.
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 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.[1][2] 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 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.[3] Larger telescopes and better optics may have been a factor.