Astronomy and the Zodiac

Astronomy has a lot to do with stars and planets. It also has a lot to do with people, their personalities and behavior. And it can even predict the future. One of the most basic concepts in astrology – and astronomy – is the zodiac signs, the constellations that mark the path that the Sun appears to travel across the sky over the course of the year. When we ask someone their horoscope sign, what we’re really asking is which constellation it was hidden in at the time of their birth.

The twelve astrological signs are based on a belt that extends nine degrees on either side of the ecliptic plane that marks the Sun’s apparent annual path. Each of the astrological signs takes up a constellation within that belt. The system originated in ancient Babylonian astronomy during the first millennium BC and was adopted into Greek astronomy by the 2nd century AD. It became the basis for the later Hellenistic astrology that influenced Roman and Western astrology as well.

Although the constellations that make up the astrological signs were defined a long time ago, they don’t all perfectly line up with the path that the Sun travels. The reason is a wobble in Earth’s rotational axis, known as precession. The effect is very slow, taking 25,800 years to complete a full cycle.

This means that over two thousand years, the dates that the Sun travels into each of the constellations has shifted by a few days. Astronomers take this into account when they draw up long-term planetary predictions. Public astronomer Marek Kukula explains why the traditional zodiac signs don’t always align with the Sun’s journey.

Astrology’s traditional zodiac starts at the point of 0 degrees Aries on the ecliptic. This is also known as the first point of Aries. The band that astrology defines, however, does not start at this point, but rather at the vernal equinox, or the spring equinox. This convention was established by the astronomer Ptolemy in the 2nd century AD, and was incorporated into his influential catalog of constellations, the Almagest.

The 12 astrological signs correspond to the constellations that the Sun enters into and hides out of each year. The corresponding astronomical constellations are:

Aries: This fire sign represents the courage of those born between March 21 and April 20. Aries are competitive and confident, often taking a head-first approach to life.

Taurus: Those born between April 22 and May 22 are represented by the bull. Taureans are practical and patient, with a strong sense of responsibility.

Gemini: The twins of this air sign are imaginative and witty. They’re able to keep their cool, even in stressful situations.

Leo: Ruled by the Sun, those born between July 23 and August 21 are loyal, generous, and caring. They’re devoted to their friends and family.

Libra: This is a sign of balance and justice, symbolized by the scales of Themis. Virgos are analytical and observant, and their compassion is evident in how they treat others. Scorpio: Those born between October 24 and November 22 are driven by instincts. They are often intuitive, and they’re not afraid to speak their mind.