Elongation
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. An elongation of 180° means the planet is opposite the Sun in the sky (opposition) — visible all night, rising at sunset and setting at sunrise. Any elongation between these extremes places the planet some angular distance away from the Sun.
Elongation determines the practical geometry of observation. A planet at 30° elongation rises or sets about two hours before or after the Sun — visible in twilight but not in full darkness. A planet at 90° elongation (quadrature) is highest in the sky at either sunrise or sunset, well-positioned for observation in a dark sky.
The maximum elongation a planet reaches depends on whether it is an inferior planet (orbiting closer to the Sun than Earth) or a superior planet (orbiting farther).
Inferior planets — Mercury and Venus — have a maximum elongation determined by their orbital geometry. Mercury’s maximum elongation is approximately 28°; Venus reaches approximately 47°. These planets are always near the Sun in the sky. They appear as morning stars (east of the Sun, rising before it) or evening stars (west of the Sun, setting after it) but never appear in the midnight sky. The concept of “greatest elongation” — maximum angular separation from the Sun — is central to planning inferior planet observations.
Superior planets — Mars, Jupiter, Saturn, and beyond — can reach any elongation from 0° to 180°. They are best observed at opposition (180° elongation), when they are closest to Earth, brightest, and up all night.
Etymology
Elongation comes from Latin elongatio — a lengthening, a drawing out — from elongare (to lengthen, to move away from). In astronomical usage, the planet is “drawn away” from the Sun in angular terms. The word entered astronomical vocabulary through medieval Latin translations of Arabic and Greek astronomical texts and was in regular use by European astronomers by the Renaissance.
A Concrete Example
Venus never appears more than about 47° from the Sun, so it is always a morning or evening phenomenon. When Venus is at or near greatest elongation, it is at its most favorably positioned — the maximum angular distance from the Sun in a dark enough sky to observe well. Ancient and medieval astronomers knew Venus as both the “morning star” (when east of the Sun at eastern elongation) and the “evening star” (when west of the Sun at western elongation), sometimes treating them as different objects before the identity was established.
Common Misconception
Elongation is sometimes confused with distance — the assumption being that a large elongation means the planet is far from Earth. This is not generally true. Mars at opposition (180° elongation) is closest to Earth. But superior planets at other elongations may be at various distances. And Venus is actually closest to Earth at inferior conjunction (0° elongation, passing between Earth and the Sun) — when it is at its smallest observed angle from the Sun and therefore hardest to observe. Maximum elongation and minimum distance are not the same thing.