Ecliptic

The ecliptic plane is the plane of Earth's orbit around the Sun, tilted 23.4° from the celestial equator. The great circle it traces on the celestial sphere is where the Sun and planets appear to move against the background stars, and where the Sun and Moon converge during eclipses — hence the name *ecliptic*.

The ecliptic crosses the celestial equator at two points. One of them is the vernal equinox, where the Sun crosses into the northern hemisphere at the beginning of spring; it is the zero point of right ascension in the celestial coordinate system.

The twelve classical zodiac constellations lie along the ecliptic, but the ecliptic also passes through Ophiuchus, acknowledged as the "13th constellation" along the Sun's apparent path. The modern constellation boundaries vary in size, so the Sun spends different lengths of time in front of each one, whereas every astrological sign covers exactly 30° (12 signs in 360°). In addition, because of precession — a wobble of Earth's axis lasting about 26,000 years — the Sun no longer occupies the constellations traditionally assigned to the astrological signs at each time of year; the "signs" have shifted about one constellation to the west. Astrological signs and astronomical constellations are therefore not the same thing.