A phenomenon seen during transits of Venus and Mercury across the Sun. The black drop is a dark area that appears to link the limb of the Sun to the following limb of the transiting planet for a short time after second contact, and its preceding limb for a short time before third contact. It results from a combination of atmospheric seeing effects, diffraction in the telescope, and the limb darkening of the Sun. The phenomenon means that accurate timing of transits is impossible.
Subjects: Astronomy and Astrophysics.