Glossary of Astronomical terms used at this site
Altitude The angular distance between an object in the sky and the horizon - mentally bring the object down to Earth - perpendicularly, as it were. An object at the Zenith would have an altitude of 90 degrees, an object at the horizon an altitude of 0 degrees. A point midway to these two - an altitude of 45 degrees, and so on.
Aperture The diameter of the main light collecting element of a telescope or other optical instrument. For an optical telescope this would be the diameter of the Objective mirror or lens.
Azimuth We talk of mentally bringing an object in the sky - perpendicularly to the horizon on Earth. Start from the direction North, move eastwards along the horizon till you reach the point where you bought a celestial object down to Earth. The angle made by this sweep, is the Azimuth. An object due North will have an Azimuth of 0 degrees. Sun rising at Equinox - exactly due East, will have an Azimuth of 90 degrees and so on.
Elongation The angular distance between the Sun and any other object in the sky - usually used for the Moon and the other planets. For the inferior planets Mercury and Venus, the elongation is restricted to a limited range. The maximum values, east and west, reached during each orbit, are called greatest elongation. The greatest elongation for Mercury lies between 18° and 28° and that for Venus varies between 45°-47°. Any elongation is possible for the planets further from the Sun than the Earth.
Horizon The great circle encompassing the boundary between the visible half of the celestial sphere and the invisible half - from the standpoint of the observer. This is a circle that is the locus of points 90 degrees away from the observer's Zenith (overhead point - see below).
Magnitude A measure of brightness of a star or any other celestial object.The magnitude scale started with a ranking given by Hipparcus to stars - magnitude 1 being the brightest of stars that he could discern and six being the faintest. Later, we got to know that the magnitude scale is actually a logarithmic scale of brightness, so that two stars differing from each other in magnitude by 1, differ in brightness by a factor of 2.512.
Zenith The overhead point. The astronomical zenith is formally defined as the extension upwards to infinity of a plumb line.
(Glossary, still being built up. Words are added, as encountered in the website)