The Fascinating World of Constellations: Connecting the Stars and Myths

Observing the sky would not be a simple matter without the adoption of a system to easily distinguish one area from another. In fact, by connecting the brightest stars with imaginary lines, it is possible to identify certain sectors of the sky, just as the ancient astronomers did, interpreting these alignments as representations of myths, characters, heroes and legends from their daily lives.

However, the term constellations does not only mean figures, but also certain areas that they represent, which include stars and objects of other nature such as: galaxies, nebulae, star clusters, etc.

Origin of constellations

There are currently 88 official constellations, and more than half of them have been handed down to us by the Greek astronomer Ptolemy, who, collecting previous records and studies, listed 48 of them in his Almagest. The names of many of them, therefore, are mostly of Babylonian or Greek origin, and have been preserved to the present day, as have the names of some of the stars.

This is all as far as the boreal sky is concerned, since the sky of the other hemisphere, or at least the southernmost part of it, is hidden from observers throughout the Mediterranean region because of the latitude. The Australian constellations are therefore of much more recent origin, dating back to the last four or five centuries, when the navigators who circumnavigated the globe used the southern sky to orient themselves. In this way, they provided valuable clues to the astronomers of the time, who completed our knowledge of the subject by drawing up new celestial maps.

Among these were Plancius, Bartsch, Hevelius, De Lacalle, and especially Bayer, who, as author of a famous atlas, introduced a system for designating the stars using the letters of the Greek alphabet in descending order of magnitude. Completing the work of these eminent scholars, the names and boundaries of each constellation were finally established in 1922 and 1930 by the I.A.U., the world's leading astronomical body.

However, constellations are perspectival entities, formed by bodies that appear to be part of a single system, but in reality share only the same celestial sector in three-dimensional space, sometimes at distances of millions of light-years from each other.

Constellation Visibility

Although the stars may be considered fixed, each constellation changes its position (relative to our own and not to the celestial background) due to the apparent motion of the sky. This is because the Earth, in addition to its own rotation, makes a revolutionary motion around the Sun, giving us a slightly different view of the celestial vault each night. Each of these constellations will then appear at the same time, shifted more and more to the west as the months pass, until it disappears below the western horizon and then reappears, after some time, from the eastern horizon.

Exceptions are the constellations near the poles, which rotate around the poles due to the sphericity of the Earth, describing complete circles between the zenith and the horizon. For this reason, they are called circumpolar and are visible only from the hemisphere to which the celestial pole they orbit belongs, remaining occulted in the other. They are divided into boreal and austral constellations.

Constellations of the Zodiac

The 12 constellations that make up the zodiac, or the strip of sky concentric with the ecliptic over which all the main bodies of the solar system seem to move, deserve a separate discussion. Among them, one in particular, the Sun, crosses a sector equal to 30° (sign of the zodiac) each month, making one complete revolution in a calendar year.

Since ancient times, it has been customary to identify each of these subdivisions with its namesake and corresponding constellation. Beginning with the first are: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces. However, the zodiac also includes the constellation of Ophiuchus, through which the Sun passes in December, but which is not one of the 12 traditional signs of the zodiac.

Precession of the Equinoxes

Due to the phenomenon of the "precession of the equinoxes", a slow and continuous movement of our planet's axis of rotation, which describes a circle in 26000 years, each celestial reference varies its position by about 1.5° every century. Every 2000 years there is a shift of 30°, so that each constellation occupies the place of the sign immediately following it. In fact, the equinoxes, the intersections of the ecliptic with the celestial equator, also known as the "Point of Aries" and the "Point of Libra," are now in Pisces and Virgo, respectively, and no longer in the signs from which they were originally named.