How to Find the
Constellations
[Key Words]
[General] [Right
Ascension] [Declination]
[So Where Am I Looking]
General
The sky has been mapped out just like our planet.
Earth is mapped out by latitude and longitude. Latitude measures
the distance away from the equator. It is a series of lines
parallel to the equator. The equator is 0°. Above that, the
numbers continue to rise until you reach the north pole, which is
90° North (90°). If you start on the equator and work your way
down, you will reach the south pole at 90° South (-90°).
Longitude measures the distance away from the prime meridian.
Lines that radiate from the north and south pole are used for
measurement. They work east and west until meeting up at the
180° mark, halfway around the world.
The sky has a similar system that is used for
position. It all lies on the celestial
sphere. This sphere is not a physical object, just like the
latitude and longitude lines on earth. Stars appear to be
flattened onto this celestial sphere. The celestial
equator is a line that runs all around the celestial sphere
inline with earth's equator. The celestial poles are formed where the north and
south poles intersect the celestial sphere. Keep in mind that
these are all imaginary and do not really appear in the sky.
Right Ascension
What astronomers use in place of longitude is called right
ascension (RA). Right ascension is measured in hours,
minutes, and seconds. The 0° mark is located at the vernal
equinox. The vernal equinox is the point in which the ecliptic
crosses the celestial equator on the suns way north. We can make
a line of right ascension by connecting an imaginary line from
the north celestial pole, through the north star, right over our
head (called the zenith) and down to the south pole. Because we
are rotating, this line will move with us. Every 24 hours, it
will cross the same point that it started at.
On the celestial sphere, lines of right ascension
don't rotate with us, unlike the one that we just made. There are
24 hours of right ascension, each one broken into 60 minutes, and
each minute broken into 60 seconds. So, if you looked at your
zenith, and you knew that you were looking at 00h 00m 00s of
right ascension, one second later you would be looking at 00h 00m
01s of right ascension, another second and it would be 00h 00m
02s, etc.
Declination
Declination (dec) is much simpler than right
ascension. It works just like our system of latitude. It is
measured in degrees, minutes, and seconds. As you go above the
celestial equator, numbers start at 0° and work up to 90° at
the north celestial pole. As you go below the celestial equator,
numbers start as 0° and go down to -90° at the south celestial
pole.
So Where Am I
Looking?
Knowing coordinates of stars is very useful for
using star maps. If you know the coordinates of a star, you can
then look them up on a star map, and then find them in the sky.
For example, if you saw the coordinates 16h 45m 08.9s RA -16°
42' 58" dec you could look them up on a star chart and find
Sirius, the brightest star outside our solar system. Here is one
you should go and look at: 05h 36m 12.8s RA -1° 12' 07"
dec.
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The Constellations Next Topic: The
Zodiac
[Key Words]
[General] [Right
Ascension] [Declination]
[So Where Am I Looking]
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