What Is the Zodiac

The zodiac is one of the most commonly used — and at the same time one of the most confusing — words in astrology and astronomy. It is used in horoscopes, in scientific observations, and in lunar calendars. Let's figure out what it really is, what types of zodiac exist and how they differ.

What is the zodiac in simple words?

The zodiac is a conventional belt on the celestial sphere, stretched along the ecliptic — the circle along which the Sun makes its apparent journey during the year. The belt captures about 8° in each direction from the ecliptic, that is about 16° wide. Inside it move all the main "zodiacal" objects: the Sun, the Moon and the planets of the Solar System.

The belt is divided into 12 signs of 30°, which bear the names of twelve constellations: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces. When they say "the Sun is in Aries" or "the Moon is in Pisces", they mean the sign, not the constellation.

Twelve signs or thirteen constellations?

A common misconception is that the signs of the zodiac equal the constellations. That is not so. Modern astronomy distinguishes 88 constellations in the sky, and 13 of them lie on the ecliptic (the famous twelve plus Ophiuchus). The signs, in turn, are a mathematical construction: equal 30° sectors named after the constellations.

Because of the precession of the Earth's axis (a slow "wobble"), the equinox points shift by about 1° every 72 years. Over the past centuries, the vernal equinox point has moved forward almost a whole sign, so the real constellations have long ceased to match "their" signs.

What zodiac systems exist?

In different cultures and epochs the zodiac was measured from different points. Three main systems have taken shape, which is what lunar.day uses:

The tropical zodiac "moves" with the seasons: its beginning always coincides with the vernal equinox around March 20. The sidereal one — with the fixed stars, so its signs are shifted relative to the tropical ones by roughly an ayanamsha (by 24°). The astronomical approach rejects equal sectors: here a sign is the real region of a constellation within the boundaries of the International Astronomical Union (IAU).

Why is the zodiac important in a lunar calendar?

The position of the Moon in the zodiac is the second most important parameter of the lunar calendar after the lunar day and the phase. It is believed that each sign sets the character of the day:

On lunar.day you can choose the tropical, sidereal or astronomical system and see which sign the Moon is in on any day and for any city.

Where did the zodiac come from?

The first zodiacal schemes appeared in Babylon about 2.5 thousand years ago. Greek astronomy combined them into a system of 12 equal signs. In India the scheme received a sidereal binding to the nakshatras — the lunar constellations. In China its own zodiac took shape — from 12 animals rather than constellations. We devote separate articles to the individual cultures.

How to use the zodiac on lunar.day

The "Tropical / Sidereal / Astronomical" switch is present on the pages of the lunar calendar and the Moon phase. In the sidereal mode you additionally need to choose an ayanamsha — the algorithm for counting the shift. By default the Fagan-Bradley measurement system is used, the most common one in Western Vedic calculations.

Summing up

The zodiac is a conventional belt of 12 signs, along which the Sun, the Moon and the planets move. It comes in three forms — tropical, sidereal and astronomical — and in each system the counting starts from its own point. The tropical one suits seasonal questions, the sidereal one — the stellar traditions of the East, the astronomical one — an exact match with the real constellations. Switch between the systems on lunar.day in one click and watch the position of the Moon for your city.