APSIS, in astronomy, a term used indifferently for either of the two points of a planet's orbit, where it is at greatest or least distance from the sun or earth; and hence the line connecting those points is called the line of the apsides. The word is Greek, and derived from apto, to connect. The apsis at the greatest distance from the sun is called the aphelion, and at the greatest distance from the earth the apogee; while that at the least distance from the sun is termed the perihelion, and at the least distance from the earth the perigee.