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(glossary entry)


The elongation of the Moon is the apparent angular distance of the center of the Moon from the center of the Sun as seen by a particular observer. - Jim Mosher

Additional Information

  • The elongation angle is listed as the "Sun-Observer-Target" or "SOT" angle in the lunar output from JPL Horizons.
  • It is an approximate determinant of the theoretical percent illumination of the Moon, which is calculated as if the Moon were a perfect sphere (see: phase). - Jim Mosher
  • The exact value of the elongation is dependent on the location of the observer. - Jim Mosher
  • If the observer is on Earth, the elongation angle is computed neglecting the effects of atmospheric refraction (since that is the angle which determines the lighting pattern, even though the observed separation may be distorted by differences in refraction). - Jim Mosher
  • The Moon's elongation approaches zero near the time of New Moon and approaches 180° near the time of Full Moon, but it rarely reaches either of these limits. - Jim Mosher
  • It is thought by some that the Moon is intrinsically invisible when the elongation is less than some critical value (see: Danjon limit). - Jim Mosher
  • See also: Saber's Beads, which is a curious optical phenomenon related to the very young (and extremely thin) moon's crescent (only a couple of hours after New Moon). Saber's Beads are also observable at the very OLD moon's crescent, a couple of hours before New Moon. - DannyCaes Feb 12, 2008

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