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The Celestial - Coordinate System.

The celestial-coordinate system is an imaginary projection of the Earth's geographical coordinate

system onto the celestial sphere which seems to turn overhead at night. This celestial grid is complete

with equator, latitudes, longitudes and poles.

The Earth is in constant motion as it rotates on its axis. Actually the celestial-coordinate system is

being displaced very slowly with respect to the stars. This is called precession and is caused by

gravitational influences from the Sun, Moon and other celestial bodies.

The celestial equator is a full 360º circle bisecting 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.)", and like the Earth's

latitudes they are 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 .

sphere, because this makes it easier to tell how long it will be before a particular star will cross a

particular north-south line in the sky. So, R.A. coordinates are marked off in units of time eastward

from an arbitrary point on the celestial equator in the constellation Pisces. The prime R.A. 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

The celestial meridians of longitude are

called "coordinates of right ascension

(R.A.)", and like the Earth's longitude

meridians they extend from pole to pole.

There are 24 major R.A. coordinates, evenly

spaced around the 360º equator, one every

15º.

Like the Earth's longitudes, R.A.

coordinates are a measure of time as well as

angular distance. We speak 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

R.A.

coordinates

coordinates denote locations on the celestial

even

though

the