The Zodiac sign is not the constellation
The constellation of Ophiuchus, 1729. Plate 22 from Atlas Coelestis, by John Flamsteed (1646-1710), the first Astronomer Royal.

Italian version

It is important to note that zodiac signs are determined by dividing the ecliptic, the sun’s apparent path throughout the year as seen from the Earth, into twelve sections. On the contrary, constellations are patterns made by connecting visible stars and were invented during the Egyptian era for easy memorisation of the times for sowing, harvesting, and Nile flooding based on the heliacal rising [1] of the stars.

Thus, the Western zodiac is based on the seasons and divides the sky into twelve sections, in analogy to the solar radiation levels during different times of the year. Although the Earth’s orbital axis precession causes the reference constellations to shift by one degree every 72 years, this doesn’t impact the symbolic design of the tropical zodiac, which remains intact.

[1] The heliacal rising of a star occurs once a year when it becomes visible above the eastern horizon just before sunrise.

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