The zodiac belta road drawn on the sky

Look at the sky at the same time every night for a year and the stars behind the Sun appear to change. In March the Sun sits in front of one group of stars. By June it has moved to another. By the following March it has returned. This is not the Sun moving. It is the Earth travelling around the Sun while we watch from the moving platform, so the background shifts.

The path the Sun seems to trace against the stars over one year is called the ecliptic. It is a great circle across the sky. The Earth and the other planets travel around the Sun in nearly the same flat plane, so the Moon and the planets are always found close to this line. None of the visible planets wanders more than a few degrees from it.

The zodiac is that road and a margin on either side: a band about sixteen to eighteen degrees wide, with the ecliptic running down its centre. Every ancient culture that watched the sky closely noticed that the wandering lights stayed inside it. The word itself comes from the Greek for a circle of animals, because the earliest constellations along it were mostly creatures.

The idea of dividing the road into twelve equal parts came from Babylonian astronomers, and the scheme took its settled form around the fifth century BCE. Several reasons for twelve are usually offered, and they fit together.

• The Moon. There are slightly more than twelve full cycles of the Moon in one year (about 12.37). Twelve is the whole number that fits.
• Arithmetic. Twelve divides evenly by two, three, four and six, which made it a convenient base for the calculations the astronomers did by hand.
• The year. The Babylonian calendar used a year of about 360 days for calculation. Twelve months of thirty days each, and twelve signs of thirty degrees, let the Sun’s position and the calendar match.

An important point follows. The twelve parts are equal measurements, thirty degrees each. They are a coordinate system, like lines of longitude. They were named after the constellations that lay near them, but a sign is a measured span of the road, not a drawing on the sky.

A constellation is a pattern of stars with a boundary agreed by astronomers. In 1930 the International Astronomical Union fixed the boundaries of all 88 constellations. These are not equal. Virgo covers about 44 degrees of the ecliptic. Scorpius covers about 7. Cancer covers about 20.

The Sun’s path also passes through a thirteenth constellation, Ophiuchus, for about two and a half weeks each December. Astrologers do not count it, because the zodiac of signs is a system of twelve equal measures and not a map of what is behind the Sun.

Both facts are true at once. The sign Virgo is a thirty-degree stretch. The constellation Virgo is a larger irregular patch of sky. The names overlap, but the two do not coincide. Whether this matters depends on what you are using the zodiac for, which is the subject of the next chapter.

India has watched the sky for a very long time. The oldest layer of Indian sky-lore uses the nakshatras, twenty-seven or twenty-eight star groups along the Moon’s path, mentioned in Vedic texts such as the Taittirīya Saṃhitā and the Atharvaveda. The twelve Rāśis appear in Sanskrit texts a little later.

Most historians of astronomy date the arrival of the twelve-sign zodiac in India to the early centuries of the Common Era and link it to exchange with Greek astrology in the Hellenistic world. The Yavanajātaka of Sphujidhvaja, a verse text of the second century CE, is the usual landmark. Some traditional scholars hold that the signs were known earlier within the Vedic tradition. That disagreement is unsettled and the site does not take a side.

What Jyotisha did with the signs is its own. It joined them to the nakshatras, built a detailed system of timing on the Moon’s position, and chose to measure the zodiac against the fixed stars, which is the subject of the next chapter.

The twelve Rāśis, with the degrees each one covers on the sidereal zodiac:

1. ♈︎Meṣa Aries0°–30° · Fire · Movable
2. ♉︎Vṛṣabha Taurus30°–60° · Earth · Fixed
3. ♊︎Mithuna Gemini60°–90° · Air · Dual
4. ♋︎Karka Cancer90°–120° · Water · Movable
5. ♌︎Siṃha Leo120°–150° · Fire · Fixed
6. ♍︎Kanyā Virgo150°–180° · Earth · Dual
7. ♎︎Tulā Libra180°–210° · Air · Movable
8. ♏︎Vṛścika Scorpio210°–240° · Water · Fixed
9. ♐︎Dhanus Sagittarius240°–270° · Fire · Dual
10. ♑︎Makara Capricorn270°–300° · Earth · Movable
11. ♒︎Kumbha Aquarius300°–330° · Air · Fixed
12. ♓︎Mīna Pisces330°–360° · Water · Dual