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, in which the planet's disc appears to remain connected to the edge of the sun's disc by a dark, teardrop-shaped smear just after it fully enters and just before it fully leaves the solar disc. The effect makes it difficult to time the exact moments a transiting planet crosses onto or off of the sun. It was once taken as evidence of an atmosphere around Venus, but is now understood to result from the darkening of the sun's own disc near its apparent edge combined with the optical imperfections of the viewing instrument, an explanation supported by the fact that the effect also appears during transits of Mercury, which has no significant atmosphere. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/