Elongation (astronomy)
in astronomy, angular separation between the Sun and a planet, with the Earth as a reference point
In astronomy, a planet's elongation is the angular separation between the Sun and the planet, with Earth as the reference point.
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°.
Of inferior planets
The greatest elongation of a given inferior planet occurs when this planet's position, in its orbital path around the Sun, is at tangent to the observer on Earth. Since an inferior planet is well within the area of Earth's orbit around the Sun, observation of its elongation should not pose that much a challenge (compared to deep-sky objects, for example). When a planet is at its greatest elongation, it appears farthest from the Sun as viewed from Earth, so its apparition is also best at that point.
When an inferior planet is visible after sunset, it is near its greatest eastern elongation.
The public source identifies “Elongation (astronomy)” as in astronomy, angular separation between the Sun and a planet, with the Earth as a reference point. This brief keeps that definition visible, then builds a research path around Elongation, astronomy and angular.
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This entry incorporates text from “Elongation (astronomy)” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.