Celestron 21049 Instruction Manual - Page 8

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Celestron 21049 Instruction Manual
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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
the celestial equator. The lines are broken
down into degrees, minutes of arc, and seconds of
down into degrees, minutes of arc, and seconds of
arc. Declination readings south of the equator carry
arc. Declination readings south of the equator carry
a minus sign (-) in front of the coordinate and those
a minus sign (-) in front of the coordinate and those
north of the celestial equator are either blank (i.e., no
north of the celestial equator are either blank (i.e., no
designation) or preceded by a plus sign (+).
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
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
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
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
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
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.
lag behind this coordinate after it passes overhead moving toward the west.
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The daily motion of the Sun across the sky
The daily motion of the Sun across the sky
is familiar to even the most casual observer.
is familiar to even the most casual observer.
This daily trek is not the Sun moving as
This daily trek is not the Sun moving as
early astronomers thought, but the result of
early astronomers thought, but the result of
the Earth's rotation. The Earth's rotation
the Earth's rotation. The Earth's rotation
also causes the stars to do the same,
also causes the stars to do the same,
scribing out a large circle as the Earth
scribing out a large circle as the Earth
completes one rotation.
completes one rotation.
The size of the
The size of the
circular path a star follows depends on
circular path a star follows depends on
where it is in the sky.
where it is in the sky.
Stars near the
Stars near the
celestial equator form the largest circles
celestial equator form the largest circles
rising in the east and setting in the west.
rising in the east and setting in the west.
Moving toward the north celestial pole, the
Moving toward the north celestial pole, the
point around which the stars in the northern
point around which the stars in the northern
hemisphere appear to rotate, these circles
hemisphere appear to rotate, these circles
become smaller. Stars in the mid-celestial
become smaller. Stars in the mid-celestial
latitudes rise in the northeast and set in the
latitudes rise in the northeast and set in the
northwest. Stars at high celestial latitudes
northwest. Stars at high celestial latitudes
are always above the horizon, and are said
are always above the horizon, and are said
to be circumpolar because they never rise
to be circumpolar because they never rise
and never set. You will never see the stars
and never set. You will never see the stars
complete one circle because the sunlight
complete one circle because the sunlight
during the day washes out the starlight.
during the day washes out the starlight.
However, part of this circular motion of
However, part of this circular motion of
stars in this region of the sky can be seen by
stars in this region of the sky can be seen by
setting up a camera on a tripod and opening
setting up a camera on a tripod and opening
the shutter for a couple hours.
the shutter for a couple hours.
processed film will reveal semicircles that
processed film will reveal semicircles that
revolve around the pole. (This description
revolve around the pole. (This description
of stellar motions also applies to the
of stellar motions also applies to the
southern hemisphere except all stars south
southern hemisphere except all stars south
of the celestial equator move around the
of the celestial equator move around the
south celestial pole.)
south celestial pole.)
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The celestial sphere seen from the outside showing R.A. and DEC
The
The
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).
Figure 7
Figure 8
8
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