A reference to the skies or heavens.

The imaginary great circle that lies above the earth's equator and meets the celestial sphere: The plane of the celestial equatoris perpendicular to the line linking the north and south celestial poles

The angular distance between a celestial body and the ecliptic, measured along a line at right angles to the ecliptic, which passes through both celestial poles: The symbol of the celestial latitude is the Greek letter β or beta.

Longitude measured (in degrees) along the ecliptic to the east from the vernal equinox: The Symbol for the celestial longitude is the Greek letter λ or lambda.

The angular distance between a celestial body and the first point of Aries, measured along the ecliptic.

The study of the motion of celestial bodies under the influence of gravitation from one or more other bodies: Celestial mechanics is the branch of astronomy that deals with the calculation of the orbits of celestial bodies, their gravitational attractions, such as those that produce the earth's tides.

It also refers to the orbits of artificial satellites and space probes and is based on the laws of motion and gravity laid down by Isaac Newton (English mathematician, 1642-1727, and physicist; remembered for developing the calculus and for his law of gravitation and his three laws of motion).

The points in the sky where the earth's axis, extended into space, intersects with the celestial sphere: The celestial poles are the points at which the earth's rotation axis pierces the celestial sphere.

All of the stars appear to orbit around this point, whose approximate position in the north is marked by Polaris, the "polestar".

There are three main types of celestial radio emissions:

1. Thermal emissions which occur as a result of the acceleration of electrically-charged particles in a hot gas.
2. Synchrotron or non-thermal, emissions are produced by the acceleration of charged particles, but the acceleration is caused by a magnetic field.
3. Radio spectral-line emissions are concentrated in a narrow band around one specific frequency; just as an optical spectral line corresponds to a single frequency in the visible electromagnetic spectrum.

Radio line emissions usually originate in clouds of hydrogen gas, a relatively common constituent of the universe, which is found in our galaxy's spiral arms, among other places.

The imaginary sphere surrounding the earth, with the stars and other astronomical objects attached to it: The celestial sphere exists on the inside of which the celestial bodies seem to lie.

The positions of bodies, such as stars, planets, and galaxies are specified by their coordinates on the celestial sphere.