is the Black Drop Effect?
Venus is near the inner edge of the Sun's disk, its edge appears to
smear and make a hazy connection with the solar disk. This makes Venus
look like a black drop dripping off the edge of the Sun. This smearing
prevented astronomers from timing accurately the moment when Venus made
"contact" with the edge of the Sun, dooming any chance of
obtaining an accurate measure of the Earth-Sun distance.
effect is caused by a combination of factors, including Earth's atmosphere,
the diffraction of light waves within a telescope, and the limitations
of the human eye.
see this phenomenon, hold your thumb and forefinger up to a light and
slowly bring them together. Notice how the gap between them seems to
smear before they actually make contact.