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 Constellation of the Big Dipper as Seen From Kauai 

(Public Domain) 

It is important to make this distinction because later we shall consider groupings that are physical groupings, such as star clusters and binary star systems.

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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