All the planets and most of the moons in the Solar System revolve around the Sun in a single plane. Seen from the Earth, the Sun and planets appear to travel within a few degrees of an imaginary line named the Ecliptic. The Earthís axis and equator are tilted relative to the Ecliptic, so for half of the year the Sun appears to be north of the equator (so the northern hemisphere enjoys longer days) and for the other half the opposite is true. Twice a year, the Sunís path along the Ecliptic crosses the equator, and days and nights are equal in length at all latitudes - these are the vernal (spring) and autumnal (fall) equinoxes. They fall on about the 21st of March and September.

The Moonís orbit also broadly follows the Ecliptic, so at the time of the equinoxes the new and full moons will be essentially over the equator, the quarter moons will be at their furthest from it. Consequently, at sunrise and sunset the angle between the horizon and the Moonís path is either at itís greatest or smallest.

The equinoxes mark the beginnings of spring and autumn. The corresponding start dates for summer and winter are the solstices (from the Latin sol, sun, sistere, make stand). These are the days when the Sun is stationary at its greatest separation from the equator before gradually converging with it again. On these days the noon Sun is at its lowest or highest point in the sky for the year.

Inconstant Moon