The Constellations
Historically, constellations were groupings of stars that
were thought to outline the shape of something, usually with mythological
significance. There are 88 recognized constellations, with their names tracing
as far back as Mesopotamia, 5000 years ago.
The Historical Constellations
In some cases one can discern easily the purported shape; for example, the
constellation Leo shown on the right might actually look like a lion with the
dots connected as they are. In other cases the supposed shape is very much in
the eye of the beholder, as the example of Canis Minor (The Little Dog) shown on
the left indicates. This certainly could be a little dog, or a cow, or
a submarine, or . . .
Star Groupings and Asterisms
Some of the more familiar
"constellations" are technically not constellations at all. For
example, the grouping of stars known as the Big Dipper is probably familiar to
most, but it is not actually a constellation. The Big Dipper is part of a larger
grouping of stars called the Big Bear (Ursa Major) that is a constellation.
A well-known grouping of stars like the Big Dipper that is
not officially recognized as a constellation is called an asterism.
Constellations Are Not Physical Groupings
The apparent groupings of stars into constellations that
we see on the celestial sphere are not physical groupings. In most cases the
stars in constellations and asterisms are each very different distances from us,
and only appear to be grouped because they lie in approximately the same
direction. This is illustrated in the following figure for the stars of the Big
Dipper, where their physical distance from the Earth is drawn to scale (numbers
beside each star give the distance from Earth in light years).
The relative
distances to stars in the Big Dipper |
The Constellations of the Zodiac
The zodiac is an imaginary band 18 degrees wide and centered
on the ecliptic. The constellations that fall in the zodiac are called the 12
constellations of the zodiac. They were at one time thought to have great
mystical and astrological significance. Astrology is
bunk, but the
constellations of the zodiac are still of importance because the planets, as
well as the Sun and Moon, are all near or on the ecliptic at any given time;
thus, they are always found within one of the zodiac constellations.
Constellations in Modern Astronomy
In modern astronomy, the significance of constellations
is no longer mythological, but practical: constellations define imaginary
regions of the sky, just as the individual states each define an imaginary
region of the United States. Thus, to say that a planet is in the constellation
Leo is to partially locate the planet on the celestial sphere, just as saying
that Knoxville is in Tennessee is to partially locate the city on the surface of
the Earth. As for states, modern constellations have irregular boundaries that
have been agreed upon for various reasons, perhaps not always completely
logical.
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