- describe the advantages of the altitude-azimuth system of reference and the equatorial coordinate system of reference
- describe how to determine the location of a star in the sky using both the altitude-azimuth system and the equatorial coordinate system
Big & Little Dippers
To an untrained observer, the night sky is full of small points of
light. Some make recognizable patterns, which can be described so that
others can find them. But how do you describe exactly where a
particular star is found in the sky? There are two systems of reference
used by astronomers to locate any object in the sky.
based on the observer, and what he or she can see from his or her
location. Imagine standing in the middle of a field, facing due north,
and imagine a bright star just off to the right. The location of that
star can be described using two coordinates: in what horizontal (or
cardinal) direction it is found (azimuth), and how high up in the sky
it is (altitude). Both coordinates are measured in degrees.
is measured from the
horizon upward to the object, and goes no higher than 90? – anything at
a greater angle starts coming back down toward the horizon. Right on
the horizon, a star would have an altitude of 0?. An altitude of
exactly 90? brings you to the point directly above your head, and is
called the zenith
. If your star is about halfway between the horizon and the zenith, above your head, its altitude would be 45?.
is measured from the
point on the horizon due north and moving eastward to the point
directly under your star and goes to a maximum of 360?– an azimuth of
360? brings you back to due north.
Locating a star using altitude and azimuth
In order to find a star with an azimuth of 64 degrees and an altitude
of 31 degrees, you would begin by facing due north. Turn to the right
(eastward) 64 degrees, and then look about a third of the way from the
horizon to the zenith.
While this is a straightforward system of reference, it is not very
practical. While you might observe a star at an azimuth of 64 degrees
and an altitude of 31 degrees, an observer at any other location on the
Earth would have to use slightly different coordinates based on their
position. Further, since the stars appear to revolve around the Earth,
even from your location, your bright star will find itself at different
coordinates within the hour.
What is required is a coordinate system that does not depend on the
observer, but on a fixed reference point that does not change over
time. For points on the Earth, longitude and latitude coordinates serve
this purpose – a city located at 113 degrees west and 55 degrees north
will always be located at 113 degrees west and 55 degrees north,
whether you are traveling to it from Vancouver, British Columbia, or
Iqaluit, Nunavut. The equatorial coordinate
system does the same for points in the sky.
Declination (or, dec)
instead of latitude, and measures a star’s north-south position
relative to the celestial equator. Recall that the equator is a fixed
object, relative to the Earth. A positive declination indicates being
north of the equator, while a negative declination indicates being
south of the equator. Just like altitude, declination is measured in
degrees, up to +/- 90 degrees. Polaris, being directly above the north
celestial pole, would have a declination of +90 degrees.
Right Ascension (or, RA)
, is used
instead of longitude. It measures the east-west position of a star
relative to the point where the Sun crosses the equator on the vernal
equinox. This is a point that remains fixed relative to the Earth, and
is analogous to the point on the Earth where the Prime Meridian passes
through the equator – all longitudinal readings are based on their
distance east or west of that point. So, too, is the right ascension
measured for stars. RA is measured in hours, minutes and seconds of the
vernal equinox, moving eastward along the equator. 24 hours is
equivalent to 360?.
Locating a star using Right Ascension and Declination
While the equatorial coordinate system is not as simple as the
altitude-azimuth system, its elegance lies in the fact that is fixes
stars’ locations in the sky, regardless of the location of the
observer, the time of day or the annual motions of the Earth around the
Did you know?
it was first used as far back as the 2nd century by the Greek scientist
Hipparchos, the equatorial coordinate system was not widely used until
the invention of the telescope in the early 1600s. Telescopes are often
mounted on an “equatorial mount,” which allows the telescope to rotate
across the sky in the same way the stars appear to move. As more
astronomers began using telescopes with these mounts, the equatorial
coordinate system was adopted for general use. Today, it is the most
widely used coordinate system by both professional and amateur