CELESTIAL DEFINITIONS 2

Declination Declination
(abbreviated Dec) is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of the celestial equator, along the hour circle passing through the point in question.

Right Ascention
Right ascension (abbreviated RA) is the angular distance measured eastward along the celestial equator from the vernal equinox to the hour circle of the point in question. When combined with declination, these astronomical coordinates specify the direction of a point on the celestial sphere in the equatorial coordinate system.

Celestial Equator
The celestial equator is a great circle on the imaginary celestial sphere, in the same plane as the Earth's equator. In other words, it is a projection of the terrestrial equator out into space.

Azimuth
An azimuth is an angular measurement in a spherical coordinate system. The vector from an observer (origin) to a point of interest is projected perpendicularly onto a reference plane; the angle between the projected vector and a reference vector on the reference plane is called the azimuth.

Zenith
The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e. the direction in which gravity pulls, is toward the nadir.

Nadir
The nadir is the direction pointing directly below a particular location; that is, it is one of two vertical directions at a specified location, orthogonal to a horizontal flat surface there.

Aries -
first Point ( ) The point of intersection of the ecliptic and the celestial equator (equinoctial) at which the sun is moving from a south to north declination. Also known as the spring equinox or the vernal equinox.

GHA
(Greenwich Hour Angle) GHA indicates the position past the plane of the Greenwich meridian measured in degrees. Equivalent to longitude on earth. It can be followed by East or West.

LHA
(Local Hour Angle) the angle between the meridian of the celestial object and the meridian of the observer.

SHA
(Sideral Hour Angle) The sidereal hour angle of a body on the celestial sphere is its angular distance west of the vernal equinox generally measured in degrees. The SHA of a star changes slowly, and the SHA of a planet doesn't change very quickly, so SHA is a convenient way to list their positions in an almanac.