Terms: ecliptic, equinox

The stars on the celestial sphere don't move (much) relative to each other. Over our lifetimes the constellations maintain their shape. The apparent motion of the stars in the sky during a night (which we describe here as the rotation of the celestial sphere) is due to the rotation of the Earth. There are a handful of objects, called by the Greeks "wanderers" or planets, that do move relative to the stars. That is to say that a planet is not a permanent part of any constellation. Instead it will move through a constellation visiting different parts of the celestial sphere. The most obvious of these wanderers is the Sun. While each day the Sun (and the celestial sphere) circles our sky moving from east to west, the Sun is also moving relative to the stars. The Sun's motion through the constellations is much slower and in a direction opposite to the rotation of the celestial sphere: The Sun moves about 1° east per day on the celestial sphere. Over a year the Sun 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's daily path in the sky is like the red star on the below figure. It rises north of east, culminates high in the sky, sets north of west, and spends more than 12 hours above the horizon. This is summer in Minnesota. During the other half of the year the Sun's daily path is like the green star. It rises south of east, culminates below the celestial equator, sets south of west, and spends less than 12 hours above the horizon. This is winter in Minnesota.

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 sky map. Note that celestial equator is labeled, from the vernal equinox going east, 1h, 2h, 3h... this is the Right Ascension to be described on the next page. The eastward ecliptic is labeled 15°, 30°, 45° ... this is "celestial longitude" often denoted by the Greek letter lambda: . The Sun slowly moves east on the ecliptic, increasing its celestial longitude by about 1° per day. Notice that the Sun's motion on celestial sphere in the opposite direction that the celestial sphere rotates. Thus on the sky map, if we stopped the sphere's rotation while the Sun was on the surface facing you, the Sun would be slowly creeping to your left: eastward. 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.