Black Drop Effect
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. Ob ...
[...More Info...]       [...Related Items...]     OR:     [Wikipedia]   [Google]   [Baidu]