Ecliptic Sphere -- Also called the Zodiac Sphere, this is the sphere resulting from projecting the plane of the Earth's orbit and points (its poles) 90° north and south of that plane.
Ecliptic Plane -- The ecliptic is a plane that passes through the centers of the Earth and the Sun. It represents the path the Sun's center takes each year on the celestial sphere as seen from the Earth or the Earth's path as seen around the Sun.
North Ecliptic Pole -- The point on the ecliptic sphere that are 90 degrees from the plane of the ecliptic, to the North or 'above'.
NEP -- North Ecliptic Pole
South Ecliptic Pole -- The point on the ecliptic sphere that are 90 degrees from the plane of the ecliptic, to the South or 'below'.
SEP -- South Ecliptic Pole
Obliquity of the Ecliptic -- The 23 1/2° angle (23°27') that represents the inclination of the ecliptic to the celestial equator. This also marks the maximum angular distance that the Sun can reach north or south of the celestial equator at the times of the solstices.
Celestial Latitude (Ecliptic Latitude) -- The angular distance of any object measured north or south of the plane of the ecliptic to the poles, from 0° to 90°.
Celestial Longitude (Ecliptic Longitude) -- The angular distance of any object as measured from zero Aries to a plane through an object.
Zodiac -- From a Greek word meaning the 'circle of animals', is a belt about 18° wide (9° above and 9° below the plane of the ecliptic) within which the planets travel. This circle is divided into 12 equal 30° sections, the signs of the zodiac -- Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces. At one point (over 2000 years ago), the signs of the zodiac corresponded with the constellations of the same name. However, due to precession, the signs have drifted westward (backward) until today they are almost an entire sign (30°) off.
Solstices -- The longest and shortest days of the year, when the Sun reaches its greatest angular distance from the equator. The longest day is the summer solstice (around June 21) and the shortest day is the winter solstice (around December 22). These two are reversed in the southern hemisphere.
Solstice Points -- The instant when the Sun is at either the summer or winter solstice.
Equinoxes (Spring and Fall) -- This is the instant when Sun crosses the celestial equator at either its ascending node (Spring Equinox, about March 21) or its descending node (Fall Equinox, September 23). At the Spring Equinox, the Sun moves north of the ecliptic plane, while at the fall Equinox, it moves from north to south.
Spring Equinox (Vernal Equinox)
Fall Equinox (Autumnal Equinox)
Zero Aries -- The intersection of the celestial equator and the ecliptic. This point undergoes a very slow backward movement.
Colures -- There are two, the equinoctial colure and the solstitial colure. The equinoctial colure is the hour circle that passes through the vernal and autumnal equinoctial points (RA 0h and 12 h). The solstitial colure is the hour circle that passes through the summer and winter solstices (0° points of the tropical zodiac signs Cancer and Capricorn at RA 6h and 18h).
Copyright (c) 1997 Michael Erlewine