Term: ecliptic, equinox

The stars on the celestial sphere don't move relative to each other. The apparent motion of the stars in the sky during a night is due to the rotation of the whole celestial sphere. There are a handful of objects, called by the Greeks "wanderers" or planets, that do move relative to the stars on the celestial sphere. The most obvious of these wanderers is the Sun. The Sun moves only about 1° per day, but over a year it completes a great circle on the celestial sphere. The path of the Sun on the celestial sphere is called the ecliptic. The ecliptic is inclined about 23.5° compared to the celestial equator. Thus during half the year the Sun is north of the celestial equator, and the Sun follows a path in the sky like the red star on the previous page. It rises north of east, culminates high in the sky, and sets north of west. During the other half of the year the Sun follows a path like the green star on the previous page. It rises south of east, culminates below the celestial equator, and sets south of west. The moments, two each year, when the Sun moves between hemispheres are called equinoxes; one happens around March 21, the other around September 21. The spring equinox (when the Sun moves from the southern hemisphere to the northern hemisphere) is called the vernal equinox

This is what the ecliptic looks like on the full celestial sphere. Note that the Sun slowly moves on the celestial sphere in the opposite direction that the celestial sphere rotates. Thus in this picture, if we stopped the sphere's rotation while the Sun was on the surface facing you, the Sun would be slowly creeping to your right. In the first picture, the Sun would be creeping to the east if the celestial sphere stopped rotating. But since the creep of the Sun along the ecliptic (one rotation per year) is so much slower than the daily rotation of the whole celestial sphere (one rotation per day), the Sun is seen to move from east to west in our sky.