The black drop effect is an optical phenomenon which is visible during planet transits.
by Anne Mette Sannes and Knut Jørgen Røed Ødegaard
Blackdrop effect during the Venus-transit June 8th 2004.
As Mercury is about to enter the solar disk (2nd contact) and also when it’s about to leave the disk (3rd contact), it looks like a drop making the planet stick to the solar limb which makes it impossible to estimate the exact moment for the second and third contact. This phenomenon resulted in unsuccessful attempts to make exact measurements of the astronomical unit (AU) (the distance Earth– Sun) during Venus-transits in the nineteenth century.
For a long time, scientists believed that this black drop effect was due to Venus’ dense atmosphere, and the drop phenomenon was also considered as evidence that Venus in fact had an atmosphere. Later, scientists came to know that the phenomenon was due to an optical illusion caused by turbulence in the Earth’s atmosphere or that the observation equipment was not good enough. The black drop effect was in fact also observed during the Mercury-transits both in 1999 and 2003 although the observations were done outside the Earth’s atmosphere and despite the fact that Mercury does not have a visible atmosphere. Then it was proven once and for all that the black drop effect is an optical phenomenon.
During the transit of Venus on June 8th 2004, several observers reported that they did not see any black drop effect, or that it at least was much less prominent than reported from transits in the previous centuries. Larger telescopes with improved optics and the Suns limb darkening might have been important factors.
Main page about the Mercury transit