aphelion apogee astronomical longitude autumnal equinox celestial equator celestial longitude celestial meridian circle colures ecliptic equator equinoctial equinoctial circle equinoctial colure galactic longitude geocentric longitude geodetic longitude great circle heliocentric longitude longitude meridian orbit perigee perihelion period small circle solstice solstitial colure summer solstice trajectory vernal equinox winter solstice zodiac zone