Zodiac

If you are looking for the East Asian zodiac, see Chinese zodiac. For other uses of the word, see Zodiac (disambiguation).

The zodiac is a belt-shaped region of the sky. It reaches about 8° north and south of the ecliptic. The ecliptic is the apparent path that the Sun seems to follow across the sky over a year. This apparent motion is caused by Earth's orbit around the Sun. Earth is tilted 23.4° compared to this path. The Moon and the brightest planets also appear within this belt as they move along their orbits.

The zodiac is divided into 12 equal parts called "signs." Each sign covers 30° of the sky. The signs roughly match the constellations named Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces.

People have used the signs for a long time to keep track of the time of year, because the Sun is in each sign during certain days. In Western astrology, each sign is linked with different qualities. Long ago, astronomers also used these qualities. The zodiac system and its way of measuring a circle in 360 degrees (°) came from Babylonian astronomy in the 1st millennium BC. It probably started during the Achaemenid Empire. By the 2nd century BC, it had been passed on to Greek astronomy, and it also helped shape the Hindu zodiac. The sexagesimal number system helped make this way of counting possible. Because of the precession of the equinoxes, the time of year when the Sun is in a certain constellation has changed since Babylonian times. The point of the March equinox has moved from Aries into Pisces.

The zodiac forms a celestial coordinate system, or more specifically an ecliptic coordinate system. In this system, the ecliptic is the starting point for measuring latitude. The Sun's position at the vernal equinox is the starting point for measuring longitude. Modern astronomers still use the ecliptic coordinate system to track objects in the Solar System.

Name

The English word "zodiac" comes from the Latin zōdiacus, the Latinized form of the Ancient Greek word zōdiakòs. That Greek word means "of or relating to little animals." Many of the twelve signs show animals and mythological hybrids.

In English, "zodiac" can also be used for the similar twelve-year cycle of the East Asian Chinese zodiac (see also Earthly Branches). Both systems use animals to stand for personality traits and life events. This is why the word is used when talking about both.

Usage

The zodiac was used in the Roman era. Its ideas came from Hellenistic astronomy, which had taken them from Babylonian astronomy of the Chaldean period, around the middle of the 1st millennium BC. That system had grown out of an older list of stars along the ecliptic. Ptolemy explained how the zodiac was built in his long 2nd century AD book, the Almagest.

The zodiac is still the basis of the ecliptic coordinate system used in astronomy, along with the equatorial system. Today, though, the word "zodiac" and the names of the twelve signs are mostly linked with horoscopic astrology. The word can also mean a band of the celestial sphere where the planets travel, about 8 degrees above and below the ecliptic. Each planet has its own zodiac band that includes its path. For example, the "zodiac of the Moon" is a band 5 degrees above and below the ecliptic. The "zodiac of the comets" can refer to the band that covers most short-period comets.

History

Further information: Former constellation

As early as the 14th century BC, a list of the 36 Egyptian decans was carved among the hieroglyphs on the tomb of Seti I. The decans also appeared in the temple of Ramesses II and on many Egyptian astrology monuments. The famous zodiacs of Dendera, dated 50 BC, show their symbols, which were identified by Karl Richard Lepsius. The idea of dividing the sky's path into zodiac signs comes from Babylonian astronomy during the first half of the 1st millennium BC. It used stars from earlier Babylonian star catalogues, such as the MUL.APIN catalogue, which was made around 1000 BC. Some constellations are even older, going back to Old Babylonian Empire sources. These include Gemini "The Twins," which came from Sumerian culture, and Cancer "The Crab."

Around the end of the fifth century BC, Babylonian astronomers divided the ecliptic into 12 equal "signs," each covering 30° of celestial longitude. This was the first known celestial coordinate system. It was probably introduced between 409 and 398 BC, during Persian rule. Modern astrologers start Aries at the Sun's position at the vernal equinox in the Northern Hemisphere (March equinox). The Babylonians instead fixed the signs to stars. They started Cancer at the "Rear Twin Star" (β Geminorum) and Aquarius at the "Rear Star of the Goat-Fish" (δ Capricorni). Because of the precession of the equinoxes, the time of year when the Sun is in each constellation has changed. The March equinox has moved from Aries into Pisces.

Some people believe the Babylonian zodiac is reflected in the Hebrew Bible. E. W. Bullinger read the creatures in the book of Ezekiel (1:10) as middle signs of the zodiac. He saw the Lion as Leo and the Bull as Taurus. Some writers linked the signs to the twelve tribes of Israel and to the Hebrew calendar, which has twelve lunar months in a lunar year. Ernest L. Martin and others argued that the arrangement of the tribes around the Tabernacle, described in the Book of Numbers, matched the zodiac order. In this idea, Judah, Reuben, Ephraim and Dan stood for middle signs. Thomas Mann explored these ideas in his novel Joseph and His Brothers. In his version of the Blessing of Jacob, he gave each tribe a trait linked to the zodiac.

Babylonian star catalogues reached Greek astronomy in the 4th century BC through Eudoxus of Cnidus. In the Hellenistic world, Chaldea became so closely linked with astrology that "Chaldean wisdom" meant divination through the planets and stars to Greeks and Romans. Hellenistic astrology came partly from Babylonian and Egyptian astrology. Horoscopic astrology first appeared in Ptolemaic Egypt (305 BC–30 BC). The earliest surviving Greek text that uses the twelve-sign division is the Anaphoricus of Hypsicles of Alexandria. Later, Ptolemy laid the foundations of the Western astrological tradition with his Tetrabiblos. He explained the zodiac as a tropical coordinate system lined up with the equinoxes. This came after Hipparchus discovered precession around 130 BC.

According to Montucla, the Hindu zodiac was adopted from the Greek zodiac through contact with the Greek kingdom of Bactria. It uses the sidereal coordinate system, which is based on fixed stars. So it does not stay lined up with the seasons in the same way as the ecliptic and equator. During the Abbasid era, Greek books were translated into Arabic, and Islamic astronomers corrected Ptolemy's work. Al-Sufi's Book of Fixed Stars included pictures of 48 constellations. Mistakes in translation gave some stars names such as Hamal.

Interest in Greco-Roman magic came back in the High Middle Ages. This happened through Kabbalism, Renaissance magic and the Sefer Raziel HaMalakh. Astrology became part of Islamic culture in the 8th century AD. Its most common type, Genethlialogy, studied a person's life using the positions of the planets at birth, much like a horoscope. The Mughal king Jahangir also issued coins showing the twelve signs of the zodiac.

Twelve signs

Main article: Astrological sign

The modern zodiac has twelve signs. The list below shows their names in Latin, Greek, Sanskrit, and Babylonian. English speakers usually use the Latin names, but "Capricorn" is used instead of "Capricornus." The Sanskrit and older names refer only to the constellations, not to the zodiac signs. A poem by Isaac Watts helps people remember the signs. Another memory trick is "A Tense Gray Cat Lay Very Low, Sneaking Slowly, Contemplating A Pounce."

The table compares the dates when the Sun enters each sign. It uses the Ptolemaic tropical zodiac and two sidereal systems. One sidereal system was proposed by Cyril Fagan. The other, proposed by Steven Schmidt, has fourteen signs. It adds Ophiuchus and Cetus, whose IAU boundaries just touch the ecliptic. The first sign, Aries, begins at the moment of the vernal equinox, and the other dates move with it. The Gregorian calendar and the tropical year do not match exactly, so the dates change a little from year to year. Since 2008, the vernal equinox has fallen on 20 March UT, and it will keep doing so until 2043.

Each sign covers exactly 30 degrees of the zodiac. On average, the Sun stays in each sign for about one twelfth of a sidereal year, or 30.43 standard days. Earth's orbit is slightly oval, so the time varies from sign to sign. The Sun stays in Capricorn for about 29.4 days, and in Cancer for about 31.4 days (see Equation of time). Signs also take different amounts of time to rise, because Earth's axis is tilted. They are called "long" or "short" ascension.

Constellations

These two star maps, made two hundred years apart, both show the zodiac constellations along a curved line called the ecliptic.

In tropical astrology, the zodiacal signs are not the same as the constellations linked to them. Over time the constellations move away from the signs because of the precession of equinoxes. The constellations are also different sizes along the ecliptic. This means the Sun stays in each constellation for different amounts of time. For example, Virgo covers about 5 times as much ecliptic longitude as Scorpius. Each zodiac sign is exactly one twelfth of the full circle, but the time the Sun spends in each sign changes a little because of the shape of the Earth's orbit. Sidereal astrology matches each sign roughly to its constellation, and this match sometimes needs to be adjusted.

The ecliptic passes through 13 constellations. They are listed in Ptolemy's Almagest and in the more carefully drawn IAU designated constellations. Besides the twelve constellations that give the signs their names, it also passes through Ophiuchus, whose lower part lies between Scorpius and Sagittarius. Sometimes the news wrongly reports this as a change to the traditional signs, made by groups such as the IAU, NASA, or the Royal Astronomical Society. This happened in a 1995 report on the BBC Nine O'Clock News and in several reports in 2011 and 2016. Some other constellations are linked to the zodiac in myths, such as Piscis Austrinus, Aquila, and Hydra.

Precession of the equinoxes

Further information: Axial precession, Epoch (astronomy), Sidereal and tropical astrology, Astrological age, and Ayanamsa

The zodiac system was developed in Babylonia, about 2,500 years ago, during the "Age of Aries". It is thought that people at that time did not know about the precession of the equinoxes. Today, the zodiac can be used in two ways. It can be sidereal, with the signs fixed to the stellar background. Or it can be tropical, with the signs fixed to the point (vector of the Sun) at the March equinox. Western astrology uses the tropical approach, and Hindu astrology uses the sidereal one. Over time, these two systems slowly move apart, about 1.4 degrees each century. In the tropical zodiac, the sign of Aries now lies within the constellation Pisces, in the Age of Pisces.

The sidereal system takes the ayanamsa into account. Bhāskara II's 12th-century book Siddhanta Shiromani explains ways to measure the precession of the equinoxes. It is based on older work in the Suryasiddhanta. The Greek astronomer Hipparchus is credited with discovering precession around 130 BC. Later, Ptolemy described it in his Almagest. Geminus of Rhodes and Ptolemy, in his Tetrabiblos, show that Greek astronomers used the tropical zodiac. They started its first degree at the spring equinox. This is why it is called "tropical," from the Greek word for "turn."

In modern astronomy

In astronomy, the zodiac is a band of sky about 8° or 9° north and south of the ecliptic. The Moon and the main planets stay inside this band. It is part of the ecliptic coordinate system, a celestial coordinate system built around the ecliptic. In this system, celestial longitude is measured in degrees east of the vernal equinox. The zodiac is narrow because most planets travel on paths that are only slightly tilted compared to the orbital plane of the Earth. Stars in the zodiac can be hidden behind the Moon and other objects in the Solar System. This is called an occultation. These events can help scientists estimate the size of a minor planet or check whether a star has a close companion.

Each year, the Sun reaches the vernal equinox around 21 March. This point is where measuring starts. Its first degree is called the "first point of Aries". The first 30° is called the sign Aries, but it no longer lies near the constellation Aries because of precession. Today it is near the end of Pisces, and it has been there since the 2nd century AD. Each of the twelve signs covers 30°. Modern astronomers count longitude continuously from 0° to 360°. Western astronomers during the Renaissance began using equatorial coordinates, which use right ascension and declination instead of celestial latitude. These became their main system. The word "zodiac" is also used for the zodiacal cloud, a cloud of dust that moves among the planets. It is also used for the zodiacal light, which is caused by sunlight scattering off that dust.

Unicode characters

In Unicode, the zodiac signs have their own symbols. They are found in a group called "Miscellaneous Symbols". An extra code can be added after a symbol to make it look like plain text, or like a small emoji picture.