Elongation is the angular distance between a planet (or other solar system body) and the Sun, as seen from Earth. It is the single most important number for determining whether a planet is observable on a given night and where in the sky to look for it. What It Is Elongation is measured in degrees, from 0° to 180°. An elongation of 0° means the planet is in the same direction as the Sun (conjunction) — invisible, lost in solar glare.