As is well known, the celestial sphere is imagined to be spanned
by an imaginary line called the celestial equator, which is in the
same plane as the earth's equator, and as it were, a vast concentric
circle. The points in the celestial sphere opposite the north and
south terrestrial poles are called the north and south celestial
poles, and the celestial equator is midway between these. Owing to the
special form of the earth the north celestial pole is visible to us
in the northern hemisphere, and marked very nearly by the pole-star,
its height above the horizon being equal to the latitude of the place
where the observer stands. Owing to the daily rotation of the earth
the whole celestial sphere seems to revolve daily on the axis of the
north and south celestial poles, carrying the sun, moon and stars
with it. To this the apparent daily course of the sun and moon is
due. Their course seems to us oblique, as we are north of the equator.
If the earth's axis were set vertically to the plane of its orbit round
the sun, then it would follow that the plane of the equator would pass
through the centre of the sun, and that the line drawn by the sun in
its apparent revolution against the background of the celestial sphere
would be in the same plane. That is, the sun would seem to move round
a circle in the heavens in the same plane as the earth's equator,
or round the celestial equator. But the earth's axis is inclined at
23 1/2° to the plane of its orbit, and therefore the apparent path
traced by the sun in the celestial sphere, which is the same path as
the earth would really follow to an observer on the surface of the sun,
is inclined at 23 1/2° to the celestial equator. This is the ecliptic,
and is really the line of the plane of the earth's orbit extended to
cut the celestial sphere.