Elongation is the measurement of two points in the sky as viewed from Earth. Read this Bright Hub article to find out more about it.
The Basics of Elongation
In astronomy, the elongation of a planet is the measurement of the angle between the planet and the Sun, and the Earth is used as the place of orientation to take this measurement. Inferior planets, which are closer to the Sun than the Earth, have different elongation periods than superior planets, which are further away from the Sun than the Earth. This makes Mercury and Venus inferior planets, and Mars, Jupiter, Saturn, Uranus, and Neptune are superior planets.
The possible values of elongations for planets depend on whether the planet is inferior or superior. An inferior planet can have a possible elongation ranging from zero degrees to its greatest possible elongation, though this value is less than ninety. For example, Mercury has a greatest elongation of twenty-eight degrees, and Venus has a greatest elongation of forty-eight degrees. A superior planet can have an elongation ranging from zero degrees to one hundred eighty degrees.
The sideral period of a planet is the time it takes to orbit the Sun and return to the same place relative to the stars. In regards to elongation, a planet's synodic period is the amount of time it takes for a planet to return to the same measurement of elongation. For example, Venus has a sidereal period of two hundred twenty-five days. However, the synodic period of the planet measures at five hundred eighty-four days. The values of planets synodic periods can only be approximated. This is because they vary slightly due to elliptical orbits and the fact that planets' orbits are all in a slightly different plane. The variation in magnitude, or how large a planet appears in the sky, also changes over a planets elongation period as the planets move away and towards each other through their orbits.
Elongation of Superior Planets
Different elongations have different specific names. For instance, a zero degree elongation is called a conjunction. An inferior conjunction, which only an inferior planet can have, takes place when the inferior planet is in a direct line between Earth and the Sun. In a superior conjunction, the sun is in a direct line between the Earth and the planet which is being measured.
Quadrature and opposition can only take place for superior planets. A ninety degree elongation is called quadrature. Superior planets have both an eastern and western quadrature. A one hundred eighty degree elongation is called opposition. At opposition, a planet can be found in the night sky at midnight from the observer's viewpoint at the meridian, which is the imaginary circle which is perpendicular to the horizon and celestial equator.
Superior planets are most easily viewed when they are at opposition. This is because they are both closest to the Earth at opposition and because they stay above the horizon for the duration of the night. When planet watching, be sure to check out ahead of time when the best times to see various are.
Image Source: http://commons.wikimedia.org/wiki/File:Positional_astronomy.png