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 aphides. The word is Greek, and derived from aptein, to connect. The apsis, at the greatest distance from the sun, is called the aphelion, and at the least distance from the earth the perigee; while that at the least distance from the sun is termed the perihelion, and at the least distance from the earth the perigee.