Horizon and Equator Coordinate Systems
Explanation of Terms
CELESTIAL SPHERE
 sphere:

a surface in threedimensional space all of whose points are equidistant from a
fixed point (the center).
 great circle:

a circle on the surface of a sphere which divides the sphere into two equal parts.
 small circle:

any circle on the surface of a sphere which is not a great circle.
 celestial sphere:

a model of the sky as a spherical shell. All celestial bodies (Sun, Moon,
stars,...) are considered located on the celestial sphere.
It has the following
properties:
 It is imaginary.
 It has an indefinite radius large enough that its center may be
anywhere at all. Its center is often taken as the center of
the Earth. Since the offset between the center of the Earth and
the Earth's surface is ``small,'' the celestial sphere's center could
also be taken at an observer's position.
 Although objects are various distances from Earth, each is
projected onto the sphere so that
each star has an apparent place on the sphere.
Relative posistions on the sphere are specified by angles.
HORIZON SYSTEM
 zenith:

the direction straight up, i.e., directly overhead.
 nadir:

the direction diametrically opposite to the zenith.
 horizon:

1. the great circle midway between zenith and nadir
2. the great circle formed by the
intersection of the celestial sphere with a plane perpendicular to the
line from an observer to the zenith.
 meridian:

the great circle passing through the observer's zenith, and north and
south points on the horizon. It is both a
vertical circle and an hour circle. The observer's meridian is the most
important of all circles of reference.
 vertical circle:

any great circle passing through both the observer's
zenith and nadir. Vertical circles receive their name
from the fact that they are perpendicular to the horizon.
 altitude:

the angle from the horizon along the vertical circle to the object.
 azimuth:

the angle from the north point of the horizon
clockwise to the foot of the vertical circle through the
object.
 transit:
 when a star crosses the observer's meridian; a.k.a.culminate
EQUATOR SYSTEM
 celestial pole:

one of the two points determined by extending the Earth's axis of
rotation to the celestial sphere. The north and south celestial poles
are the centers of rotation of the celestial sphere.
 celestial equator:

1. the great circle on the celestial sphere midway between the
north and south celestial poles.
2. the great circle on the celestial sphere determined by
extending the Earth's equator to the celestial sphere.
 diurnal circle:

the apparent path of a star in the sky during one day due to the the
rotation of the Earth. Diurnal circles are parallel to the celestial
equator.
 hour circle:

any great circle on the celestial sphere which passes through the north
and south celestial poles.
 vernal equinox:

the point where the Sun crosses the celestial equator from
south to north each year. Also known as the First Point of Aries.
 right ascension:

the angle measured in units of time $15\xb0\; =\; 1h$ eastward along the
celestial equator from the vernal equinox to the foot of the hour
circle through the object. Symbol:
or $\alpha $ or R.A.
 declination:

the angle between a object on the celestial sphere and the celestial equator, measured north
or south along the hour circle through the
object. Symbol:
or $\delta $ or dec.
 hour angle:

the angle westward along the celestial equator from
the observer's meridian to the hour circle through the object.
Symbol: t or H.A.
 ecliptic:

the apparent path of the Sun on the celestial sphere over a year.