The word retrograde applies, in astrology, to the apparent backward motion through the zodiac of a planet. An old encyclopedia of astrology describes this retrograde motion as "the effect of a slow-moving train as viewed from another train traveling parallel to it but at a more rapid rate, wherein the slower train appears to be moving backwards. However, in the case of the celestial bodies it is not a matter of their actual speed of travel, but of the rate at which they change their angular relationship."*