The ecliptic is the path the Sun appears to trace across the sky over the course of a year. It is not the Sun moving. It is Earth orbiting, and our perspective shifting. The ecliptic is the projection of Earth's orbital plane onto the celestial sphere.

The ecliptic matters because the planets and the Moon stay close to it. They all orbit in roughly the same plane, so they appear to travel along the same path as the Sun. That is why the zodiac constellations lie along the ecliptic. The word zodiac comes from a Greek phrase meaning circle of little animals, and the signs mark where the Sun is at different times of year.

The ecliptic is tilted relative to the celestial equator by about 23.5 degrees. That tilt is the reason we have seasons. When the Sun is north of the celestial equator, the northern hemisphere gets more direct sunlight and longer days. When it is south, the southern hemisphere does.

The ecliptic also intersects the celestial equator at two points: the vernal equinox and the autumnal equinox. Those points are the reference for right ascension and the starting point for the astronomical year.