The Celestial - Coordinate System - Celestron FIRSTSCOPE 80 EQ Instruction Manual

4. UNDERSTANDING THE SKY
A. 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 360° 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 0°.
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 .
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 RA.
coordinates, evenly spaced around the 360°
equator, one every 15°. 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.
Figure 2
locations on the celestial sphere, because this makes
Astronomers prefer the time designation for R.A.
coordinates even though the coordinates denote
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
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