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As we look up into the night sky,
the stars appear to be placed on a large, inverted bowl
above us. This inverted bowl is called the celestial
sphere. Since all objects on the celestial sphere appear
to be at the same, arbitrarily large distance from the
observer, it is usually not necessary to know the object's
true distance. It is only necessary to know the object's
angular position as projected onto the celestial sphere.
Since the celestial sphere appears
as a two dimensional, curved surface, two angular measurements
are required to specify one object's position relative
to another. Although any arbitrary coordinate system
could be used, these measurements are usually made in
two specific systems by amateur astronomers. Each is
characterized by a specific plane of reference which
determines a great circle when projected onto the celestial
sphere, and by a specific reference point on that great
circle. Coordinates are then specified by angular measure
around the great circle from the reference point and
by angular distance from the reference plane along another
great circle perpendicular to that plane.
Topocentric Coordinates
Also called alt-azimuth or horizon
coordinates, this system uses the plane of the local
horizon as the plane of reference. The reference point
within the plane is the geographic north point. (It
might just as easily have been the geographic south
point except that early civilizations and, hence, directional
conventions, first developed in the northern hemisphere.
The northern bias continues to this day.)
The azimuth, designated
theta, is measured along the horizon, eastward
from the north point. From the definition, azimuth ranges
from 0 degrees to 360 degrees. The common directions
of due north, east, south and west correspond respectively
to azimuths of 0, 90, 180, and 270 degrees.
Altitude, designated
a, is measured perpendicular to the horizon. Altitude
values range from -90 degrees to +90 degrees. If the
altitude of an object is negative, it is below the horizon.
If the altitude is greater than 90 degrees, it should
be measured from a point on the horizon 180 degrees
away in azimuth. This would bring the altitude back
to less than 90 degrees. The point directly overhead,
having +90 degrees altitude, is called the zenith.
Equatorial Coordinates
As the name suggests, the plane of
the Earth's equator is chosen as the plane of reference.
The projection of this plane intersects the celestial
sphere in a great circle called the celestial equator.
The reference point on the celestial equator is defined
with the aid of another plane, the plane of the Earth's
orbit, called the ecliptic.
The Sun appears to follow the ecliptic
as it moves day-to-day against the stellar background.
The path was named ecliptic because lunar and solar
eclipses occur along this path.
Since the equator is inclined about
23.5 degrees to the ecliptic, the two projected great
circles intersect at two points. That intersection where
the Sun appears to cross the celestial equator from
south to north is chosen as the reference point and
is known as the vernal equinox.
Right ascension,
designated alpha, is measured along the celestial
equator eastward from the vernal equinox. For convenience,
right ascension is normally measured in hours and ranges
from 0h to 24h. Note that, in the northern hemisphere,
right ascension increases clockwise when you are facing
north.
Declination, designated delta,
is measured perpendicular to the celestial equator.
Declination ranges from -90 degrees to +90 degrees.
Negative declinations indicate objects south of the
celestial equator. As with altitude, a declination greater
than 90 degrees should be measured from a right ascension
12h (=180 degrees) away so that the correct value is
always within the +/-90 degree range.
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