Astronomy Answers: From the Astronomical Dictionary

Astronomy Answers
From the Astronomical Dictionary

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The description of the word you requested from the astronomical dictionary is given below.

the elongation

The elongation of a celestial body is a measure for the distance in the sky between that celestial body and the Sun. The following different definitions are used:

  1. the angular distance in the sky between the center of that body and the center of the Sun, as seen from the center of the Earth
  2. the difference in geocentric ecliptic longitude between the center of the celestial body and the center of the Sun

For a celestial body on the ecliptic, both definitions yield the same results.

The elongation cannot exceed 180 degrees by definition. Superior planets can have any elongation up to the maximum 180 degrees, but inferior planets have a maximum elongation that is less than 180°. The maximum elongation of Mercury is about 28°, and that of Venus about 48°.