AY Honors/Stars/Terms/en

a. Celestial sphere

The celestial sphere is an imaginary rotating sphere of "gigantic radius", with the Earth at its center. All objects in the sky can be thought of as lying upon the sphere.

b. Celestial pole

The two celestial poles are the imaginary points where the Earth's spin axis intersects the celestial sphere. The north celestial pole currently has nearly the same coordinates as the bright star Polaris (which is Latin for "Pole Star").

c. Celestial equator

The celestial equator is a great circle on the imaginary celestial sphere, which is actually the plane of the terrestrial equator extended out into the universe (i.e., it could be constructed by extrapolating the Earth's equator until it touches the celestial sphere).

d. Horizon

The horizon is the line that separates earth from sky.

e. Right ascension

Right Ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. It is equivalent to terrestrial longitude.

f. Declination

Declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Dec is comparable to latitude, projected unto the celestial sphere, and is measured in degrees north and south of the celestial equator. Therefore, points north of the celestial equator have positive declination, while those to the south have negative declination.

g. Transit

A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, as seen by an observer at some particular vantage point.

h. Conjunction

Conjunction means that, as seen from some place (usually the Earth), two celestial bodies appear near one another in the sky.

i. Ecliptic

The Ecliptic is the apparent path of the Sun traced out along the sky in the course of the year. More accurately, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun.