Black drop

In astronomy, a black drop is an optical illusion visible every time two bodies are apparently so close as to be about to touch: it seems that the profiles of the two bodies extend towards each other. This optical illusion is due to a slight astigmatism of optical instruments, or of the eye of the observer. The phenomenon can for example be observed in the apparent touching of the Moon and Sun during a solar eclipse: the irregularities on the edge of the Moon (the limb) seem to extend towards the edge of the Sun.