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 constellationis 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.