The positions of the stars in the celestial sphere have remained almost unchanged since prehistoric times. Each of the early civilizations grouped the stars into various constellations. Historically, some constellations were taken to represent gods or mythical beings (or sometimes their dwellings). Other groups were thought to resemble animals or objects on earth—although in most cases, seeing the resemblance demands a vivid imagination. Stories and myths were woven around the gods and animals, however, and this helped people to remember the constellations’ names and positions. For example, Orion, the Hunter, is seen to be keeping at bay Taurus, the Bull.
The various civilizations gave different names to their own constellations, but many of the modern names are of Greek or Roman origin. Early Arab astronomers were responsible for naming many of the individual stars. This dates from the time when Arab culture and astrology flourished in the valleys of the rivers Tigris and Euphrates, during the first millennium A.D. Navigators and explorers at that time began to venture farther into the Southern Hemisphere and gradually became aware of new constellations in the southern skies.
Of special interest to early astronomers and astrologers were the constellations of the zodiac. These form a broad band across the sky along the line of the ecliptic (the path through the heavens followed by the sun, the moon, and the planets of the solar system). The 360 degrees of the ecliptic were divided into 12 sections, each corresponding to a zodiacal sign; this division into 12 was probably made because there are 12 complete lunar cycles (actually about 12.4) in one year. As a result, the sun “occupies” each sign for approximately one calendar month.
The constellations of the zodiac were named in the order in which the sun appeared to occupy them; the first sign being Aries, the Ram, chosen some 20,000 years ago. The start of the zodiac—called the First Point of Aries— was selected to coincide with the position in the sky where the plane of the ecliptic crossed the celestial equator. When the sun is at this point, its declination is at a minimum (0°), and the lengths of day and night are equal throughout the world; this is also called the vernal equinox and marks the commencement of spring in the Northern Hemisphere on March 20 or 21. After the vernal equinox, the sun occupies the successive signs of the zodiac until, at the next vernal equinox, it is once again at the beginning of its cycle.
The exact point on the celestial equator at which it is crossed by the ecliptic recedes by about 50 seconds of arc each year. This means it has receded by about 28° of arc since the early astronomers devised the zodiacal system; as a result it now lies in the sign of Pisces (the Fishes), instead of in Aries. Despite this recession, however, the vernal equinox is still sometimes called the first point of Aries, and it has retained the old symbol y, which is also the sign of the constellation of Aries.
Using the stars as signposts
The linking of the stars into groups and geometric patterns, as constellations, was of great help in locating individual stars. The simplified patterns still serve as signposts for identifying stars or neighboring constellations. With the exception of the moon, the most prominent feature in the night sky is the Milky Way, or Lactea Via, the poetic name for the great luminous belt of stars that encircles the whole of the celestial sphere. In its Greek mythological interpretation, Hercules was at the breast of Juno while she slept, and some of her milk fell and spread across the vault of the heavens. The Arabs and Persians had their own myth and regarded the Milky Way as a great river alongside which grazed gazelles, camels, and stallions. They described various star clusters as tents, date palms, oases, and even a treasure chest full of glittering jewels.
The constellation of the Big Bear (Ursa Major) is the best known to those who live in the Northern Hemisphere. The hind part of it consists of a group of seven bright stars that roughly form the shape of a plow (or of a saucepan or dipper). Its configuration makes it an excellent signpost for most of the major stars and constellations of the northern sky.
For the stars and constellations near the celestial equator, seen to best advantage in the winter months, Orion is the clearest signpost. It is perhaps the most spectacular constellation of all. Orion’s belt points to Aldebaran, the eye of the Bull (Taurus), which is being hunted by Orion, and leads on from there to the Pleiades. In the other direction, the belt points to Sirius (in Canis Major), the dog of Orion, the Hunter.
There are similar sets of constellations in the Southern Hemisphere, the most notable of which is the Southern Cross (Crux). This configuration helps to locate the south celestial pole, although there is no visible star near that point in the sky. It also locates Canopus, Rigel, and the False Cross. Ptolemy drew up a list of 48 constellations, of which 21 appear in the northern half of the celestial sphere and 15 occur south of the celestial equator. The remaining 12 form the zodiac, those special constellations that are arranged at equal intervals along the path of the sun in the plane of the ecliptic. Today, there are 88 named constellations, many of which are scarcely visible to the naked eye. The total number of stars visible without a telescope (brighter than magnitude 6.0) is about 6,000.