The New Student's Reference Work/Ecliptic

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Eclip′tic, in astronomy, the term applied to the apparent path of the sun in the heavens in the course of a year.  It is the circle in the celestial sphere to which longitudes and latitudes are referred, and the circle in whose plane eclipses occur, since an eclipse cannot take place unless the moon be in or near the ecliptic.  The circle of the sphere is inclined to the equinoctial at an angle of 23° 27′ 5″, and cuts it in two opposite points named the equinoxes.  The angle of the inclination of the equator and ecliptic is a variable quantity, and is called the obliquity of the ecliptic.  The result of this angle is the change of the seasons.