In 1930, the International Astronomical Union formalized the constellations as regions of the sky, not just the stellar patterns within them. These modern constellations have their roots in which the Greek astronomer Ptolemy was introduced in the second century AD. Different cultures have seen patterns in the sky that are unique in their history. To complicate matters further, unlike astrological signs, constellations are not of equal size and shape. The stars that make up a constellation are, for the most part, not physically related. They are only based on patterns that our ancestors saw when they looked at the sky and tried to understand everything.

In Western astrology, astrological signs are the twelve 30-degree sectors that make up the Earth’s 360-degree orbit around the sun. The signs are listed from the first day of spring, known as the first point of Aries, the vernal equinox. Astrological signs are Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces.

Over the centuries, the zodiacal divisions of Western astrology have become out of alignment with the constellations named after him by the earth’s axial precession, while measurements of Hindu astrology correct this change. Astrology (i.e., a system of doom based on celestial apparitions) also developed in Chinese and Tibetan cultures, but these astrologies are not based on the zodiac, but cover the whole sky. From their data, ancient astronomers in the Fertile Crescent knew that the estimated length of a year was 365 days and that they could predict events such as annual flooding or rainy season. Different ancient civilizations created different calendars and timekeeping maps based on the sun, stars, and planets.

The seasonal or tropical year is actually a hair shorter than a full-time job. This means that each year the location of the sun relative to the stars on a given day, for example June 21, shifts a very small amount. Meanwhile, astrology benefits from the rank and vitality of the Greek gods. Linked to the planets and constellations, these very human deities make astrology dramatic and zodiac sign exciting. Originally evolved to help with matters of state, art finds its enduring role in launching the fortunes of ordinary men and women. The Babylonians realize that the zodiac, the sequence of constellations along which the sun and planets seem to move as they pass through the sky, can serve as a measure of celestial time if it is divided into recognizable and equal segments.

Particularly important in the development of Western horoscopic astrology was the astrologer and astronomer Ptolemy, whose work Tetrabiblos laid the foundation for the Western astrological tradition. Under the Greeks, and Ptolemy in particular, the planets, houses, and signs of the zodiac were rationalized, and their function was established in a way that has changed little to this day. Ptolemy lived in the 2nd century AD, three centuries after the discovery of the precession of the equinoxes by Hipparchus around 130 BC.