Ecliptic

As the Earth follows its annual orbit around the Sun, the relative positions of those two bodies changes with respect the the background stars. From the point of view of an observer on Earth, therefore, the Sun appears to move slowly relative to its backdrop of stars, passing from one constellation to another as the year passes, and ultimately completing a circuit of the entire sky. The apparent path of the Sun through the sky is known as the Ecliptic, an imaginary line that traces a path across the Celestial Sphere.

Under most circumstances, the stars 'behind' the Sun aren't visible to an Earth-bound observer because of the brightness of the Sun itself (the exceptions are at times of eclipse, when the Sun's light is darkened sufficiently to make the stars visible). The corollary of the Sun's apparent movement along the Ecliptic is that the visible stars opposite the Sun's brilliant disc also appear to move. This is the reason that the night sky changes as the year passes, with different constellations visible in different seasons.

Astronomical Effects

The major planets in the Solar System all orbit the Sun in nearly the same plane, and this means that, to an observer on Earth, the planets also move through the sky following the Ecliptic closely. The relationship is not exact, so the planets will rarely fall exactly on the line of the Ecliptic, but they will never stray far from it. The Moon, too, orbits the Earth in very nearly the same plane. The fact that solar eclipses can only occur when the Moon actually crosses the line of the Sun's path (and hence the Earth, Moon and Sun are exactly aligned) is the source of the the name Ecliptic.

The rotation of the Earth on its axis does not align with the Ecliptic: the planet is 'tilted' by more than 23° relative to the path of its orbit. It is this tilt gives rise to the seasons, and it also means that the Celestial Equator - which is based on the Earth's own equator - follows a quite different path across the Celestial Sphere from the Ecliptic.

There are two points in the sky where the Ecliptic and the Celestial Equator cross one another. These mark the points where the Earth's axial tilt aligns with the plane of the Solar System, and the practical result, when the Sun appears at one of those points, is an equinox, where day and night on Earth are effectively of equal length. These two crossing points of the Ecliptic and Equator correspond to the two annual equinoxes: the Vernal Equinox in March, and the Autumnal Equinox in September.

The point of intersection between the Ecliptic and the Celestial Equator at the Vernal Equinox is termed the First Point of Aries (for historical reasons, though it currently falls within the boundaries of Pisces). The First Point of Aries marks the reference point for coordinates on the Celestial Sphere, defining the position of zero hours right ascension and zero degrees declination.

Ecliptic Coordinates

The circle that the Ecliptic traces across the Celestial Sphere can be used as the basis of its own coordinate system. Defining the Ecliptic as the 'equator' of a system like this means that the Sun, Moon and major planets always remain close to 0° latitude (Ecliptic coordinates use degrees of longitude and latitude, rather than the right ascension and declination used on the Celestial coordinate system).

Mapping the sky using Ecliptic coordinates locates objects relative to the plane of the Solar System, so increasing or decreasing latitudes place objects further outside that plane. At 90° north and 90° south are the two Ecliptic Poles, marking the points directly 'above' and 'below' the Solar System's primary plane. The Northern Ecliptic Pole is in the constellation Draco, very close to the Cat's Eye Nebula, while the Southern Ecliptic Pole is in Dorado, very nearly in the direction of the Large Magellanic Cloud.

Ancient Traditions

The course of the Sun through the stars was known in ancient times, though somewhat misunderstood. Based on the assumption that the Earth was fixed at the centre of the Solar System - and indeed the entire cosmos - it was thought that the Sun and the planets literally followed the path of the Ecliptic, with the Sun actually travelling though a circuit of constellations each year.

The cycle of constellations through which the Ecliptic passed was therefore seen as profoundly significant, and the original twelve star-groups gave rise to the Zodiac. Gradual changes in the Earth's attitude over the centuries mean that the Ecliptic has changed slightly since the original twelve constellations were defined, but even in ancient times it also passed through a thirteenth constellation, Ophiuchus, interposed between the classical Ecliptic constellations of Libra and Sagittarius.