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. 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 to speak of.
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. Larger telescopes and better optics may have been a factor.
Skywatchers await the fleeting shadow of Venus: on 8 June, Venus will cross directly in front of the sun for the first time in 122 years.(Planetary Science)
May 14, 2004; Venus usually shines like a brilliant beacon in the morning or evening sky. But on 8 June, our sister planet will assume a darker...