The Zodiac: Celestial Heritage of Ancient Civilizations

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The zodiacal belt and the ecliptic

The zodiac (zodiac comes from the Greek zōidiakos, meaning circle of small animals) corresponds to a celestial band of about 8.5° on either side of the plane of the ecliptic (plane of the apparent revolution of the Sun around the Earth). This region of the sky contains the constellations crossed by the Sun during its apparent annual motion. In other words, the zodiac is not a purely symbolic invention, but a geometric consequence of the Earth's rotation and the tilt of its axis.

The twelve zodiacal constellations, from Aries to Pisces, are defined by their position on this celestial band. Ancient Mesopotamian, Egyptian, and then Greek astronomers divided this circle into twelve equal sectors of 30°, corresponding to a complete cycle of 360°, i.e., a solar year.

The Zodiac: Celestial Heritage of Ancient Civilizations

Mesopotamian astronomers observed and catalogued the constellations located on the path of the Sun, Moon, and planets for several centuries before our era, as evidenced by the MUL.APIN catalogue (compiled around the 7th century BCE from older traditions). But it was only at the end of the 5th century BCE that Babylonian astronomers formally divided the ecliptic into twelve equal sectors of 30°, a mathematical division probably linked to their calendar of twelve lunar months.

It was then the great Greek astronomer Claudius Ptolemy (c. 100-170) who fixed the positions of over 1000 stars distributed across 48 constellations in his major work, the Almagest. Aware of the phenomenon of precession, he expressed their positions in ecliptic longitude and latitude rather than equatorial coordinates, laying the foundations of celestial cartography for the following fifteen centuries.

N.B.:
Ecliptic longitude and latitude are the coordinates of the system used by Ptolemy. Ecliptic longitude, analogous to terrestrial longitude, is measured in degrees along the ecliptic from the vernal point, while ecliptic latitude is measured in degrees north or south of the plane of the ecliptic. The vernal point is the point of intersection between the plane of the ecliptic and the plane of the celestial equator, corresponding to the position of the Sun when it passes from the southern celestial hemisphere to the northern hemisphere, at the time of the spring equinox. This system, sensitive to precession, differs from the modern equatorial system (right ascension and declination), now preferred because it follows the Earth's rotation rather than the plane of the ecliptic.

The twelve constellations crossed by the Sun

Unlike astrological signs, the constellations of the zodiac have unequal sizes and no longer correspond to the traditional dates of the horoscope. Due to the precession of the equinoxes (slow movement of the Earth's axis of rotation, about 26,000 years, changing the position of the equinoxes), the Sun no longer enters the constellations at the same periods as two millennia ago.

• Aries – constellation marked by the bright star Hamal and symbol of the beginning of spring.
• Taurus – hosts the open cluster of the Pleiades and the red star Aldebaran.
• Gemini – its two main stars, Castor and Pollux, represent the mythological twins.
• Cancer – a discreet constellation containing the open cluster Praesepe.
• Leo – easily recognizable by its lion shape and the bright star Regulus.
• Virgo – a large constellation containing the Virgo Cluster and the galaxy M87.
• Libra – a faint constellation symbolizing balance and justice.
• Scorpius – a remarkable constellation with the red star Antares, resembling a scorpion.
• Ophiuchus – crossed by the Sun but often omitted, it represents the Serpent Bearer.
• Sagittarius – rich in nebulae and star clusters, symbol of the archer.
• Capricornus – a small triangular constellation often associated with the sea-goat.
• Aquarius – a diffuse constellation, symbol of water and rain in mythology.
• Pisces – a zodiacal constellation representing two fish tied by a cord.

N.B.:
The International Astronomical Union (IAU) officially recognizes thirteen constellations crossed by the Sun, including Ophiuchus, between Scorpius and Sagittarius.

Comparison between astrological signs and real constellations

The following table illustrates the differences between the symbolic dates of astrological signs and the real positions of the Sun in the corresponding constellations, according to NASA data.

The zodiac through the ages

The constellations of the zodiac served as landmarks for navigation, agriculture, and timekeeping. The Egyptians, around 1000 BCE, oriented their temples according to the heliacal risings of the zodiacal stars. The Greeks, with Hipparchus of Nicaea (c. 190-120 BCE), discovered the precession of the equinoxes, explaining the slow drift of the constellations on the celestial sphere. Even today, these stellar figures remain a fundamental reference for astronomers and astrophysicists in the study of celestial dynamics.

The constellations of the zodiac constitute an ancient but still relevant cartography of our sky. They materialize the trajectory of the Sun, the Moon, and the planets against the starry background, offering a direct link between astronomical observation and human symbolism. Between celestial geometry and cultural heritage, the zodiac remains a witness to our way of reading the sky and understanding our place in the cosmos.

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