Astronomical Magnitude [m] 
Indicates the visual brightness of an object.
Positive = dim, Negative = bright
The brightest star (Sirius) reaches 0m, whereas 6m is the limit to the unaided eye.
Venus, the brightest planet, reaches 4m.
The moon at first quarter is 8m, about the same as the brightest Iridium flares

Azimuth (degrees) 
Azimuth defines a point on the horizon in degrees counting from geographic North clockwise to the East.
(i.e. angular distance around the horizon, where north is 0 degrees, east is 90 degrees.)

Altitude (degrees) 
The altitude of a celestial object is its angular distance above the horizon.
The altitude of an object on the horizon is 0°, and that of an object at the zenith, 90°. 
Right Ascension 
Right Ascension (abbreviated R.A.) is similar to longitude on the Earth, and is measured in hours of time
along the celestial equator (divided into 24 equal portions).0 hours is a point on the celestial equator called the vernal or March equinox.

Declination 
Declination is measured in degrees, and refers to how far above the imaginary "celestial equator" an object is.
It is measured as 0 degrees at the equator, +90 degrees at the North Pole, and 90 degrees at the South Pole.
(Polaris, the North Star, is at (close to) declination +90)

Great Circle 
A great circle bisects the celestial sphere into two equal hemispheres

Celestial Sphere 
6th century B.C. Pythagoreans thought the stars as lying on the inside surface of a giant
celestial sphere which rotates around us once a day.

Coordinate Systems 
Any spherical coordinate system begins with a measurement made along a great circle,
with measurements above or below the great circle.
Three great circles are used as the basis of three different celestial coordinate systems:
Horizon (altitude and azimuth). Tied to the observer and so do NOT rotate with the Celestial Sphere.
Celestial Equator (right ascension and declination).
Tied to the Celestial Sphere and rotate with it in its diurnal rotation.
Ecliptic (celestial latitude and celestial longitude)
