The north and south poles of the heavens are the points on the celestial sphere directly above the north and south poles of the earth.
The celestial equator is the great circle in which the plane of the earth's equator intersects the celestial sphere. It passes through the east and west points of the horizon and through the zenith—or point directly overhead—at the earth's equator.
The ecliptic is the great circle in which the plane of the earth's orbit intersects the celestial sphere. The celestial equator and the ecliptic are inclined to each other at an angle of 23½°, which is called the obliquity of the ecliptic. The two points in which the celestial equator and the ecliptic intersect are called respectively the vernal equinox and the autumnal equinox.
The vernal equinox is an important point of reference on the celestial sphere.
As the position of a point on the earth's surface is determined by its longitude and latitude so the position of an object on the celestial sphere—star, sun, planet—is determined by its Right Ascension and Declination.
The Declination of a celestial object is its distance north or south of the celestial equator, measured in degrees, minutes and seconds of arc, on a great circle of the celestial sphere passing through the object and north and south poles of the heavens. These great circles are called hour circles and they correspond to the meridians or circles of longitude on the earth's surface. The declination of an object in the heavens corresponds to the latitude of a point on the earth's surface. The Right Ascension of a point on the celestial sphere corresponds to the longitude of a point on the earth's surface. It is measured—as longitude is measured—in degrees, minutes and seconds of arc or in hours, minutes and seconds of time—eastward along the celestial equator from the hour circle passing through the vernal equinox to the foot of the hour circle passing through the object. The hour circle passing through the vernal equinox is the zero meridian for the celestial sphere just as the meridian of Greenwich is the zero meridian on the earth's surface.
The mean distance of the earth from the sun is 92,900,000 miles and is called the astronomical unit.
The sun with its satellites advances through the universe at the rate of 4 astronomical units in a year or approximately one million miles a day.