Elongation (astronomy)

For other uses, see Elongation (disambiguation).
An angle ε is drawn between two straight lines from Earth to the Sun, and from Earth to the planet. This is demonstrated for different positions along circular orbits, both for planets closer to the Sun (where the angle is always less than 90°), and for outer planets (for which the angle can range from 0° to 180°), while distinguishing east and west sides.
This diagram shows various possible elongations (ε), each of which is the angular distance between a planet and the Sun from Earth's perspective.

In astronomy, a planet's elongation is the angular separation between the Sun and the planet, with Earth as the reference point.[1]

The greatest elongation is the maximum angular separation. Astronomical tables and websites, such as Heavens-Above, forecast when and where the planets reach their next maximum elongations.

Sometimes elongation may instead refer to the angular distance of the Moon relative Earth or the natural satellite of another planet from its central planet, for instance the angular distance of Io from Jupiter.

A quadrature occurs when the position of a body (moon or planet) is such that its elongation is 90° or 270°; i.e. the body-earth-sun angle is 90°.

  1. ^ Chisholm, Hugh, ed. (1911). "Elongation" . Encyclopædia Britannica. Vol. 9 (11th ed.). Cambridge University Press. p. 298.