Understanding where to locate celestial objects, and how those objects move
across the sky is fundamental to enjoying the hobby of astronomy. Most amateur
astronomers adopt the simple practice of "star-hopping" to locate
celestial objects by using star charts or astronomical software which identify
bright stars and star patterns (constellations) that serve as "road
maps" and "landmarks" in the sky. These visual reference
points guide amateur astronomers in their search for astronomical objects.
And, while star-hopping is the preferred technique, a discussion of using
setting circles for locating objects is desirable since your telescope is
provided with this feature. However, be advised, compared to star-hopping,
object location by use of setting circles requires a greater investment
in time and patience to achieve a more precise alignment of the telescope's
polar axis to the celestial pole. For this reason, in part, star-hopping
is popular because it is the faster, easier way to become initiated in the
|IMPORTANT NOTICE! Never use a telescope or spotting scope to look at the Sun! Observing the Sun, even for the shortest fraction of a second, will cause irreversible damage to your eye as well as physical damage to the telescope or spotting scope itself. |
The Celestial Sphere
Understanding how astronomical objects move: Due to the Earth's rotation,
celestial bodies appear to move from East to West in a curved path through
the skies. The path they follow is known as their line of Right Ascension
(R.A.). The angle of this path they follow is known as their line of Declination
(Dec.). Right Ascension and Declination is analogous to the Earth-based
coordinate system of latitude and longitude.
Understanding celestial coordinates: Celestial objects are mapped
according to the R.A. and Dec. coordinate system on the "celestial
sphere," the imaginary sphere on which all stars appear to be placed.
The Poles of the celestial coordinate system are defined as those 2 points
where the Earth's rotational axis, if extended to infinity, North and South,
intersect the celestial sphere. Thus, the North Celestial Pole is that point
in the sky where an extension of the Earth's axis through the North Pole
intersects the celestial sphere. In fact, this point in the sky is located
near the North Star, or Polaris.
On the surface of the Earth, "lines of longitude" are drawn between
the North and South Poles. Similarly, "lines of latitude" are
drawn in an East-West direction, parallel to the Earth's equator. The celestial
equator is simply a projection of the Earth's equator onto the celestial
sphere. Just as on the surface of the Earth, imaginary lines have been drawn
on the celestial sphere to form a coordinate grid. Celestial object positions
on the Earth's surface are specified by their latitude and longitude.
The celestial equivalent to Earth latitude is called "Declination,"
or simply "Dec," and is measured in degrees, minutes or seconds
north ("+") or south ("-") of the celestial equator.
Thus any point on the celestial equator (which passes, for example, through
the constellations Orion, Virgo and Aquarius) is specified as having 0°0'0"
Declination. The Declination of the star Polaris, located very near the
North Celestial Pole, is +89.2°.
The celestial equivalent to Earth longitude is called "Right Ascension,"
or "R.A." and is measured in hours, minutes and seconds from an
arbitrarily defined "zero" line of R.A. passing through the constellation
Pegasus. Right Ascension coordinates range from 0hr0min0sec up to (but not
including) 24hr0min0sec. Thus 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 prime (0h0m0s) Right Ascension grid line carry increasing
With all celestial objects therefore capable of being specified in position
by their celestial coordinates of Right Ascension and Declination, the task
of finding objects (in particular, faint objects) in the telescope can be
simplified. The setting circles, R.A. and Dec. of the telescope may be dialed,
in effect, to read the object's coordinates, positioning the object in the
vicinity of the telescope's telescopic field of view. However, these setting
circles may be used to advantage only if the telescope is first properly
aligned with the North Celestial Pole.
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 the Earth's axial rotation). During any 24 hour period,
stars make one complete revolution about the pole, making concentric circles
with the pole at the center. By lining up the telescope's polar axis with
the North Celestial Pole (or for observers located in Earth's Southern Hemisphere
with the South Celestial Pole), astronomical objects may be followed, or
tracked, simply by moving the telescope about one axis, the polar axis.
If the telescope is reasonably well aligned with the pole, therefore, very
little use of the telescope's Declination flexible cable control is necessary-virtually
all of the required telescope tracking will be in Right Ascension. (If the
telescope were perfectly aligned with the pole, no Declination
tracking of stellar objects would be required). For the purposes of casual
visual telescopic observations, lining up the telescope's polar axis to
within a degree or two of the pole is more than sufficient: with this level
of pointing accuracy, the telescope can track accurately by slowly turning
the telescope's R.A. flexible cable control and keep objects in the telescopic
field of view for perhaps 20 to 30 minutes.