Celestial
Coordinates
By: JC Willette
The universe and our earth are in constant motion.
This presents huge problems finding objects in the sky and tracking them. The
earth is spinning on its axis once every 24 hours. The axis of the earth is also
moving in a circular motion but maintains a constant 23.4° tilt. This in effect
makes the earth like a wobbling gyroscope. Compounding things, the earth is also
moving around the Sun once a year and the Sun is moving around our galaxy every
220 million years. One more problem, the galaxy is moving through space. In
essence everything in the universe is moving constantly in different direction
and at different speeds. So how can we find an object in the sky reliably every
night after night at any time?
There are several catalogues of stars and of
deep-sky objects that specify the right ascensions and declinations of these
objects at some particular time, or epoch. Because of the slow movement of just
about everything in the universe, we can accomplish this cataloging of the
universe.
Declination
Declination is the celestial equivalent of latitude on Earth. We measure
this like latitude in degrees (°), minutes of arc (') and seconds of arc (")
from 0° at the celestial equator to 90° North and 90° South at the celestial
poles. As we look out level to the earth this will be 0° declination and
when we look directly overhead it is 90° declination. Remember that 90°
is over the North Pole.
Altitude is measured in degrees upward from
the horizon to the object. The point directly overhead at 90° altitude is
called the zenith. The nadir is "down", or opposite the zenith. We
sometimes use zenith distance instead of altitude, which is 90 deg minus
the altitude. As we look out level to the earth this will be 0°
altitude and when we look directly overhead it is 90° altitude. This
measurement has nothing to do with where the North Pole is located.
Right Ascension
Right ascension is the celestial equivalent of longitude on Earth. It is
measured in hours (h), minutes (m) and seconds (s), but is sometimes measured in
degrees. The zero point of right ascension is on the Greenwich meridian of
longitude.
Alt-Azimuth
Alt-Azimuth coordinates consist of a pair of measurements, altitude and
azimuth. Both coordinates are in relation to the observer and refer to the
position of an object in the sky.
Simply put, altitude simply tells you how
high above the horizon the object is located. For example, if Jupiter is at an altitude of 18º, that means it is just over one hand-span above the
horizon. (hold your hand out at arms length and spread you fingers apart as far
as possible, this is 15°). If it were at 30º, it would be 2 hand-spans above the
horizon and so on. At 90º is straight up above your head.
Azimuth is an angle of measure along the
horizon to a vertical line drawn from the object to the horizon. It is measured
in a clockwise direction from due North (0°) through due East (90°), due South
(180°), due West (270°), and back to due North (0° or 360°). These four
principle directions are called the cardinal points.
Coordinates of the same object will differ for
every observer on earth because each observer their own separate alt-azimuth
system. In addition, because of the Earth rotation, the altitude and
azimuth of any given object are constantly changing with time as seen from a
given location. This system can only identify celestial objects at any give time
and place and therefore not useful for indicating their permanent
direction in space.
