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8
The Celestial Coordinate System
To help find objects in the sky, astronomers use a celestial
coordinate system that is similar to our geographical
coordinate system here on Earth. The celestial coordinate
system has poles, lines of longitude and latitude, and an
equator. For the most part, these remain fixed against the
background stars.
The celestial equator runs 360 degrees around the Earth and
separates the northern celestial hemisphere from the
southern. Like the Earth's equator, it bears a reading of zero
degrees. On Earth this would be latitude. However, in the
sky this is referred to as declination, or DEC for short.
Lines of declination are named for their angular distance
above and below the celestial equator. The lines are broken
down into degrees, minutes of arc, and seconds of arc.
Declination readings south of the equator carry a minus sign
(-) in front of the coordinate and those north of the celestial
equator are either blank (i.e., no designation) or preceded by
a plus sign (+).
The celestial equivalent of longitude is called Right Ascension, or R.A. for short. Like the Earth's lines of longitude, they run from pole to
pole and are evenly spaced 15 degrees apart. Although the longitude lines are separated by an angular distance, they are also a measure of
time. Each line of longitude is one hour apart from the next. Since the Earth rotates once every 24 hours, there are 24 lines total. As a
result, the R.A. coordinates are marked off in units of time. It begins with an arbitrary point in the constellation of Pisces designated as 0
hours, 0 minutes, 0 seconds. All other points are designated by how far (i.e., how long) they lag behind this coordinate after it passes
overhead moving toward the west.
Motion of the Stars
The daily motion of the Sun across the sky
is familiar to even the most casual
observer. This daily trek is not the Sun
moving as early astronomers thought, but
the result of the Earth's rotation. The
Earth's rotation also causes the stars to do
the same, scribing out a large circle as the
Earth completes one rotation. The size of
the circular path a star follows depends on
where it is in the sky. Stars near the
celestial equator form the largest circles
rising in the east and setting in the west.
Moving toward the north celestial pole, the
point around which the stars in the
northern hemisphere appear to rotate, these
circles become smaller. Stars in the mid-
celestial latitudes rise in the northeast and
set in the northwest. Stars at high celestial
latitudes are always above the horizon, and
are said to be circumpolar because they
never rise and never set. You will never
see the stars complete one circle because
the sunlight during the day washes out the
starlight. However, part of this circular
motion of stars in this region of the sky can
be seen by setting up a camera on a tripod
and opening the shutter for a couple hours.
The processed film will reveal semicircles
that revolve around the pole. (This
Figure 6
The celestial sphere seen from the outside showing R.A. and DEC
.
Figure 7
All stars appear to rotate around the celestial poles. However, the appearance of this motion
varies depending on where you are looking in the sky. Near the north celestial pole the stars
scribe out recognizable circles centered on the pole (1). Stars near the celestial equator also
follow circular paths around the pole. But, the complete path is interrupted by the horizon.
These appear to rise in the east and set in the west (2). Looking toward the opposite pole, stars
curve or arc in the opposite direction scribing a circle around the opposite pole (3).