10
U
U
N
N
D
D
E
E
R
R
S
S
T
T
A
A
N
N
D
D
I
I
N
N
G
G
T
T
H
H
E
E
S
S
K
K
Y
Y
The Celestial-Coordinate System
The celestial-coordinate system is an imaginary projection of the earth's geographical coordinate system onto the
celestial sphere, which gives the appearance of rotating overhead at night. This celestial grid is complete with
equator, latitudes, longitudes and poles. The celestial equator is a full 360º circle dividing the celestial sphere into
the northern celestial hemisphere and the southern celestial hemisphere. Like the earth's equator, it is the prime
parallel of latitude and is designated 0º. The celestial parallels of latitude are called “coordinates of declination
(DEC).” As with the earth's latitudes, they’re named for their angular distances from the equator. These distances
are measured in degrees, minutes and seconds of arc. There are 60 minutes of arc in each degree, and 60 seconds of
arc in each arc minute. Declinations north of the celestial equator are "+" and declinations south are "-". The north
pole is +90 and the south pole is -90 .
The celestial meridians of longitude are called “coordinates of right ascension” (RA). Like the Earth's longitude
meridians, they extend from pole to pole. There are 24 major RA coordinates, evenly spaced around the 360º
equator, one every 15º. Like the earth's longitudes, RA coordinates are a measure of time, as well as angular
distance. We speak of each of the earth's major longitude meridians as being separated by one hour of time because
the earth rotates once every 24 hours (one hour = 15°). The same principle applies to celestial longitudes since the
celestial sphere appears to rotate once every 24 hours. Right ascension hours are also divided into minutes of arc
and seconds of arc, with each hour having 60 minutes of arc and each arc minute being divided into 60 arc seconds.
Astronomers prefer the time designation for RA coordinates even though the coordinates denote locations on the
celestial sphere. Using time makes it easier to tell how long it will be before a particular star will cross a particular
north-south line in the sky. RA coordinates are marked off in units of time eastward from an arbitrary point on the
celestial equator, in the constellation Pisces. The prime RA coordinate which passes through this point is designated
“O hours O minutes O seconds.” We call this reference point the vernal equinox where it crosses the celestial
equator. All other coordinates are names for the number of hours, minutes and seconds that they lag behind the
prime coordinate after it passes overhead, moving westward.
