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

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.

Here is a Web page devoted to constellations and their stars, and a Java applet allowing interactive display of stars and constellations. See also this Java applet (select from the list of constellations that appears for a scrollable map centered on that constellation). Finally, here is a constellation pronunciation guide.

Drawing the Constellations

Here are two pieces of software that allow you to construct maps of the sky at specified times (now, or in the past or future) that include constellations:
  1. Starry Night is a $28 shareware program for the Macintosh that simulates the appearance and motion of the sky from a user chosen vantage point and at a user chosen time.

  2. Free Star Maps may be downloaded on demand from the Mount Wilson Observatory in postscript format. A forms interface allows the map to be customized, and links on the Mount Wilson page explain how to download a free postscript viewer if your don't already have one.
Each of these programs can provide information about the celestial sphere beyond just drawing the constellations. Here is a map of northern hemisphere constellations characteristic of late Winter evenings in the Southern United States that was constructed using the Mount Wilson software described above.