(I) The aberration of starlight is an optical illusion caused by the motion of the Earth. (II) Rain falling appears vertical when seen from the window of a stationary train; when seen from the window of a moving train, the rain appears to follow a sloping path. (Ill) In the same way light from a star "falling" down a telescope seems to follow a sloping path because the Earth is moving. (IV) Optical aberration is one of a number of defects that impair the image in an optical instrument. (V) Thus, an apparent displacement or aberration in the position of the star takes place.