latitude and longitude, celestial objects may
also be located using Right Ascension and
Declination. For example: You can locate
Los Angeles, California, by its latitude (+34
°
)
and longitude (118
°
). Similarly, you can
locate the Ring Nebula (also known as
"M57") by its Right Ascension (18hr) and its
Declination (+33
°
).
•
• R
RIIG
GH
HT
T A
AS
SC
CE
EN
NS
SIIO
ON
N ((R
R..A
A..)):: This Celestial
version of longitude is measured in units
of hours (hr), minutes (min), and seconds
(sec) on a 24 hour "clock" (similar to how
Earth's time zones ar determined by
longitude lines). The "zero" line was
chosen to pass through the constellation
Pegasus, a sort of cosmic Greenwich
meridian. R.A. coordinates range from 0hr
0min 0sec to 23hr 59min 59sec. There are
24 primary lines of R.A., located at 15-
degree intervals along the celestial
equator. Objects located further and
further East of the zero R.A. grid line (0hr
0min 0sec) carry higher R.A. coordinates.
•
• D
De
ec
clliin
na
attiio
on
n ((D
De
ec
c..)):: This celestial version
of latitude is measured in degrees, arc-
minutes, and arc-seconds (e.g., 15
°
27'
33"). Dec. locations North of the celestial
equator are indicated with a plus (+) sign
The celestial map also contains two poles
and an equator just like a map of the Earth
(see Fig. 6). The celestial poles are defined
as those two points where the Earth's North
and South poles, if extended to infinity,
would cross the celestial sphere. Thus, the
North Celestial Pole is that point in the sky
where the North Pole crosses the celestial
sphere. The North Star, Polaris, is located
very near the North Celestial Pole.
So just as an object's position on the Earth's
surface can be located by its
Fig. 6
(e.g., the Dec. of the North
celestial pole is +90
°
). Any point on
the celestial equator (such as thee
constellations of Orion, Virgo, and
Aquarius) is said to have a
Declination of zero, shown as 0
°
0' 0". All
celestial objects therefore may be located
with their celestial coordinates of Right
Ascension and Declination.
LINING UP WITH THE CELESTIAL POLE
Objects in the sky appear to revolve around
the celestial pole. (Actually, celestial objects
are essentially "fixed" and their apparent
motion is caused by Earth's rotation). During
any 24 hour period, stars make one
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