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About Constellations
Look up at the sky on a clear night and you will see vast patterns
of bright stars. Close your eyes and think about the shapes of these
patterns. Do they remind you of people, or animals, or mythological
beings such as a winged horse or a great warrior holding his sword and
shield? Ancient astronomers imagined they saw people, animals, and
mythological beings in the sky. They called them constellations.
What's a constellation?
A constellation is a group of stars that appears to form a pattern
or picture like Orion the Great Hunter, Leo the Lion, or Taurus the
Bull. Constellations are easily recognizable patterns that help people
orient themselves using the night sky. In some cases, constellations
may have had ceremonial or religious significance. In other cases, the
star groupings helped to mark the passage of time between planting
and harvesting.
There are 48 “ancient” constellations and they are the brightest
groupings of stars – those observed easily by the unaided eye. There are
actually 50 “ancient” constellations; astronomers divided one of the
constellations (Argo) into 3 parts. In 1930 the International
Astronomical Union officially listed 88 modern and ancient
constellations (with Argo divided into 3 parts) and drew a boundary
around each. There are now 88 modern constellations and boundaries.
The boundary edges meet,
dividing the imaginary sphere — the
celestial sphere — surrounding
Earth into 88 pieces. Astronomers
consider any star within a
constellation boundary to be part of
that constellation, even if it is not
part of the actual picture. A simple,
geometric star pattern lies at the
heart of each constellation. The
stars in these patterns may appear
to be close to each other, but they
are often very far apart.
Generally, there is little resemblance between the star pattern
and the fully illustrated object or figure that represents the
constellation. For example, consider the Northern Hemisphere’s winter
constellation Orion the Hunter. The star pattern on which it is based —
four bright stars at the corners of a trapezoid and three stars in a row
near the center — doesn’t look much like a person. The ancients used a
lot of imagination when they created the constellations.
Now astronomers name stars based on where their coordinates
are found on the celestial sphere. This is an imaginary sphere
surrounding Earth. Earth’s north and south poles can be extended in
space to this sphere, marking the north and south celestial poles, the
poles around which the sphere spins. Polaris marks the intersection of
the extended North Pole and the sphere. Earth’s equator, extended into
space, intersects the sphere at the celestial equator, dividing it into
northern and southern hemispheres.
All stars and objects in space, such as constellations, can be
mapped relative to the poles and equator of the celestial sphere. Their
position north or south of the celestial equator — essentially their
latitude — is called “declination.” Their position east or west essentially
is their longitude, or “right ascension”, measured in hours, minutes, and
seconds. On Earth, we measure our longitude east or west from
Greenwich, England; right ascension on the celestial sphere is
measured from the intersection of the ecliptic (plane of Earth’s orbit)
and the celestial equator.