Zodiac

The Earth's orbit around the Sun causes the apparent motion of the latter along the ecliptic (red). Earth is axially tilted 23.4° relative to this plane; its equator, extended to the stars, is shown in light blue.

Astrophotos of the twelve zodiac constellations

The zodiac is a belt-shaped region of the sky that extends approximately 8° north and south (as measured in

The signs have been used to determine the time of the year by identifying each sign with the days of the year the Sun is in the respective sign. In Western astrology, and formerly astronomy, the time of each sign is associated with different attributes. Though the zodiacal system and its angular measurement in 360

The zodiac was in use by the

Chaldean period (mid-1st millennium BC), which, in turn, derived from an earlier system of lists of stars along the ecliptic.^[8] The construction of the zodiac is described in Ptolemy's comprehensive 2nd century AD work, the Almagest.^[9]

Although the zodiac remains the basis of the

short-period comets.^[13]

History

Further information:

Former constellation

Early history

As early as the 14th century BC a complete list of the 36 Egyptian decans was placed among the hieroglyphs adorning the tomb of Seti I; they figured again in the temple of Ramesses II, and characterize every Egyptian astrological monument. Both the famous zodiacs of Dendera display their symbols, identified by Karl Richard Lepsius.^[14]

The division of the ecliptic into the zodiacal signs originates in Babylonian astronomy during the first half of the 1st millennium BC. The zodiac draws on stars in earlier Babylonian star catalogues, such as the MUL.APIN catalogue, which was compiled around 1000 BC. Some constellations can be traced even further back, to Bronze Age (Old Babylonian Empire) sources, including Gemini "The Twins", from Sumerian: 𒀯𒈦𒋰𒁀𒃲𒃲, romanized: ^MULMAŠ.TAB.BA.GAL.GAL "The Great Twins"; Cancer "The Crab", from Sumerian: 𒀯𒀠𒇻, romanized: ^MULAL.LUL "The Crayfish", among others.^[15][16]

Around the end of the fifth century BC,

celestial longitude, thus creating the first known celestial coordinate system. According to calculations by modern astrophysics, the zodiac was introduced between 409 and 398 BC, during Persian rule,^[17] and probably within a very few years of 401 BC.^[18] Unlike modern astrologers, who place the beginning of the sign of Aries at the position of the Sun at the vernal equinox in the Northern Hemisphere (March equinox), Babylonian astronomers fixed the zodiac in relation to stars, placing the beginning of Cancer at the "Rear Twin Star" (β Geminorum) and the beginning of Aquarius at the "Rear Star of the Goat-Fish" (δ Capricorni).^[19]

Due to the

precession of the equinoxes, the time of year the Sun is in a given constellation has changed since Babylonian times, the point of March equinox has moved from Aries into Pisces.^[20]

Because the division was made into equal arcs, 30° each, they constituted an ideal system of reference for making predictions about a planet's longitude. However, Babylonian techniques of observational measurements were in a rudimentary stage of evolution.[21] They measured the position of a planet in reference to a set of "normal stars" close to the ecliptic (±9° of latitude) as observational reference points to help positioning a planet within this ecliptic coordinate system.^[22]

In

ephemerides, the positions of significant astronomical phenomena were computed in sexagesimal fractions of a degree (equivalent to minutes and seconds of arc).^[24] For daily ephemerides, the daily positions of a planet were not as important as the astrologically significant dates when the planet crossed from one zodiacal sign to the next.^[23]

Hebrew astronomy and astrology

Knowledge of the Babylonian zodiac is said to be reflected in the

lunar year. Martin and others have argued that the arrangement of the tribes around the Tabernacle (reported in the Book of Numbers) corresponded to the order of the zodiac, with Judah, Reuben, Ephraim, and Dan representing the middle signs of Leo, Aquarius, Taurus, and Scorpio, respectively. Such connections were taken up by Thomas Mann, who in his novel Joseph and His Brothers attributes characteristics of a sign of the zodiac to each tribe in his rendition of the Blessing of Jacob.^{[citation needed}

]

Hellenistic and Roman era

The Babylonian star catalogs entered

Greek astronomy in the 4th century BC, via Eudoxus of Cnidus.^[15]

Babylonia or

Egyptian astrology.^[28] Horoscopic astrology first appeared in Ptolemaic Egypt (305 BC–30 BC). The Dendera zodiac

, a relief dating to c. 50 BC, is the first known depiction of the classical zodiac of twelve signs.

The earliest extant Greek text using the Babylonian division of the zodiac into 12 signs of 30 equal degrees each is the Anaphoricus of

tropical coordinate system, by which the zodiac is aligned to the equinoxes and solstices, rather than the visible constellations that bear the same names as the zodiac signs.^[34]

Hindu zodiac

According to mathematician-historian

sidereal coordinate system, which makes reference to the fixed stars. The tropical zodiac (of Mesopotamian origin) is divided by the intersections of the ecliptic and equator, which shifts in relation to the backdrop of fixed stars at a rate of 1° every 72 years, creating the phenomenon known as precession of the equinoxes. The Hindu zodiac, being sidereal, does not maintain this seasonal alignment, but there are still similarities between the two systems. The Hindu zodiac signs and corresponding Greek signs sound very different, being in Sanskrit and Greek respectively, but their symbols are nearly identical.^[36] For example, dhanu means "bow" and corresponds to Sagittarius, the "archer", and kumbha means "water-pitcher" and corresponds to Aquarius, the "water-carrier".^[37]

Middle Ages

During the

Astrology emerged in the 8th century CE as a distinct discipline in Islam,^[39]: 64 with a mix of Indian, Hellenistic Iranian and other traditions blended with Greek and Islamic astronomical knowledge, for example Ptolemy's work and Al-Sufi's Book of Fixed Stars. A knowledge of the influence that the stars have on events on the earth was important in Islamic civilization. As a rule, it was believed that the signs of the zodiac and the planets control the destiny not only of people but also of nations, and that the zodiac has the ability to determine a person's physical characteristics as well as intelligence and personal traits.^[40]

The practice of astrology at this time could be divided into 4 broader categories: Genethlialogy, Catarchic Astrology, Interrogational Astrology and General Astrology.^[39]: 65 However the most common type of astrology was Genethlialogy, which examined all aspects of a person's life in relation to the planetary positions at their birth; more commonly known as our horoscope.^[39]: 65

Astrology services were offered widely across the empire, mainly in bazaars, where people could pay for a reading.^[41] Astrology was valued in the royal courts, for example, the Abbasid Caliph Al-Mansur used astrology to determine the best date for founding the new capital of Baghdad.^[39]: 66 Whilst horoscopes were generally widely accepted by society, many scholars condemned the use of astrology and divination, linking it to occult influences.^[42] Many theologians and scholars thought that it went against the tenets of Islam; as only God should be able to determine events rather than astrologers looking at the positions of the planets.^[41]

In order to calculate someone's horoscope, an astrologer would use 3 tools: an astrolabe, ephemeris and a takht. First, the astrologer would use an astrolabe to find the position of the sun, align the rule with the persons time of birth and then align the rete to establish the altitude of the sun on that date.^[43] Next, the astrologer would use an Ephemeris, a table denoting the mean position of the planets and stars within the sky at any given time.^[44] Finally, the astrologer would add the altitude of the sun taken from the astrolabe, with the mean position of the planets on the person's birthday, and add them together on the takht (also known as the dustboard).^[44] The dust board was merely a tablet covered in sand; on which the calculations could be made and erased easily.^[41] Once this had been calculated, the astrologer was then able to interpret the horoscope. Most of these interpretations were based on the zodiac in literature. For example, there were several manuals on how to interpret each zodiac sign, the treatise relating to each individual sign and what the characteristics of these zodiacs were.^[41]

Early modern

An example of the use of signs as astronomical coordinates may be found in the Nautical Almanac and Astronomical Ephemeris for the year 1767. The "Longitude of the Sun" columns show the sign (represented as a digit from 0 to and including 11), degrees from 0 to 29, minutes, and seconds.^[45]

Mughal king Jahangir issued an attractive series of coins in gold and silver depicting the twelve signs of the zodiac.^[46]

• 
  A volvella of the moon. A volvella is a moveable device for working out the position of the Sun and Moon in the zodiac, 15th century
• 
  17th-century fresco of

  Cathedral of Living Pillar, Georgia

What follows is a list of the signs of the modern zodiac (with the ecliptic longitudes of their first points), where 0° Aries is understood as the vernal equinox, with their Latin, Greek, Sanskrit, and Babylonian names. But note that the Sanskrit and the name equivalents (after c.500 BC) denote the constellations only, not the tropical zodiac signs. The "English translation" is not usually used by English speakers. The Latin names are standard English usage (except that "Capricorn" is used rather than "Capricornus").

These twelve signs have been arranged into a nursery rhyme as a mnemonic device:^[51]

The ram, the bull, the heavenly twins,
And next the crab, the lion shines,
    The virgin and the scales,
The scorpion, archer, and the goat,
The man who holds the watering-pot,
    And fish with glittering scales.

— Isaac Watts

The following table compares the

The beginning of Aries is defined as the moment of vernal equinox, and all other dates shift accordingly.^[52] The precise Gregorian times and dates vary slightly from year to year as the Gregorian calendar shifts relative to the tropical year. These variations remain within less than two days' difference in the recent past and the near-future, vernal equinox in UT always falling either on 20 or 21 March in the period of 1797 to 2043, falling on 19 March in 1796 the last time and in 2044 the next. The vernal equinox has fallen on 20 March UT since 2008, and will continue to do so until 2043.^[53]

Symbol	Constellation	Tropical zodiac dates[54]	Lahiri ayanamsa )	Based on IAU boundaries[58]
	Aries	March 21 – April 19	April 14 – May 14	April 18 – May 13
	Taurus	April 20 – May 20	May 15 – June 15	May 13 – June 21
	Gemini	May 21 – June 20	June 16 – July 16	June 21 – July 20
	Cancer	June 21 – July 22	July 17 – August 16	July 20 – August 10
	Leo	July 23 – August 22	August 17 – September 16	August 10 – September 16
	Virgo	August 23 – September 22	September 17 – October 17	September 16 – October 30
	Libra	September 23 – October 22	October 18 – November 16	October 30 – November 23
	Scorpio	October 23 – November 21	November 17 – December 15	November 23 – November 29
	Ophiuchus[a]	—	—	November 29 – December 17
	Sagittarius	November 22– December 21	December 16 – January 14	December 17 – January 20
	Capricorn	December 22 – January 19	January 15 – February 12	January 20 – February 16
	Aquarius	January 20 – February 18	February 13 – March 14	February 16 – March 11
	Pisces	February 19– March 20	March 15 – April 13	March 11 – April 18

As each sign takes up exactly 30 degrees of the zodiac, the average duration of the solar stay in each sign is one twelfth of a sidereal year, or 30.43 standard days. Due to Earth's slight orbital eccentricity, the duration of each sign varies appreciably, between about 29.4 days for Capricorn and about 31.4 days for Cancer (see Equation of time). In addition, because the Earth's axis is at an angle, some signs take longer to rise than others, and the farther away from the equator the observer is situated, the greater the difference. Thus, signs are spoken of as "long" or "short" ascension.^[59]

Constellations

These two maps of the constellations, made two centuries apart, both show the zodiac constellations along a curved line representing the ecliptic.

In

Sidereal astrology remedies this by assigning the zodiac sign approximately to the corresponding constellation. This alignment needs re calibrating every so often to keep the alignment in place.

The ecliptic intersects with 13 constellations of Ptolemy's Almagest,^[61] as well as of the more precisely delineated IAU designated constellations. In addition to the twelve constellations after which the twelve zodiac signs are named, the ecliptic intersects Ophiuchus,^[62] the bottom part of which interjects between Scorpio and Sagittarius. Occasionally this difference between the astronomical constellations and the astrological signs is mistakenly reported in the popular press as a "change" to the list of traditional signs by some astronomical body like the IAU, NASA, or the Royal Astronomical Society. This happened in a 1995 report of the BBC Nine O'Clock News and various reports in 2011 and 2016.^[63][64][65]

Some "parazodiacal" constellations are touched by the paths of the planets, leading to counts of up to 25 "constellations of the zodiac".^[66] The ancient Babylonian MUL.APIN catalog lists Orion, Perseus, Auriga, and Andromeda. Modern astronomers have noted that planets pass through Crater, Sextans, Cetus, Pegasus, Corvus, Hydra, and Scutum, with Venus very rarely passing through Aquila, Canis Minor, Auriga, and Serpens.^[66]

Some other constellations are mythologically associated with the zodiacal ones: Piscis Austrinus, The Southern Fish, is attached to Aquarius. In classical maps, it swallows the stream poured out of Aquarius' pitcher, but perhaps it formerly just swam in it. Aquila, The Eagle, was possibly associated with the zodiac by virtue of its main star, Altair. Hydra in the Early Bronze Age marked the celestial equator and was associated with Leo, which is shown standing on the serpent on the Dendera zodiac.^[15]

Precession of the equinoxes

The zodiac system was developed in

Western astrology takes the tropical approach, whereas Hindu astrology takes the sidereal one. This results in the originally unified zodiacal coordinate system drifting apart gradually, with a clockwise (westward) precession of 1.4 degrees per century.

For the tropical zodiac used in Western astronomy and astrology, this means that the tropical sign of Aries currently lies somewhere within the constellation Pisces ("

The sidereal coordinate system takes into account the

The discovery of precession is attributed to Hipparchus around 130 BC. Ptolemy quotes from Hipparchus' now-lost work entitled "On the Displacement of the Solstitial and Equinoctial Points" in the seventh book of his 2nd century astronomical text, Almagest, where he describes the phenomenon of precession and estimates its value.^[32] Ptolemy clarified that the convention of Greek mathematical astronomy was to commence the zodiac from the point of the vernal equinox and to always refer to this point as "the first degree" of Aries.^[68] This is known as the "tropical zodiac" (from the Greek word trópos, turn)^[69] because its starting point revolves through the circle of background constellations over time.

The principle of the vernal point acting as the first degree of the zodiac for Greek astronomers is described in the 1st century BC astronomical text of Geminus of Rhodes. Geminus explains that Greek astronomers of his era associate the first degrees of the zodiac signs with the two solstices and the two equinoxes, in contrast to the older Chaldean (Babylonian) system, which placed these points within the zodiac signs.^[68] This illustrates that Ptolemy merely clarified the convention of Greek astronomers and did not originate the principle of the tropical zodiac, as is sometimes assumed.

Ptolemy demonstrates that the principle of the tropical zodiac was well known to his predecessors within his astrological text, the Tetrabiblos, where he explains why it would be an error to associate the regularly spaced signs of the seasonally aligned zodiac with the irregular boundaries of the visible constellations:

The beginnings of the signs, and likewise those of the terms, are to be taken from the equinoctial and tropical points. This rule is not only clearly stated by writers on the subject, but is especially evident by the demonstration constantly afforded, that their natures, influences and familiarities have no other origin than from the tropics and equinoxes, as has been already plainly shown. And, if other beginnings were allowed, it would either be necessary to exclude the natures of the signs from the theory of prognostication, or impossible to avoid error in then retaining and making use of them; as the regularity of their spaces and distances, upon which their influence depends, would then be invaded and broken in upon.^[34]

In modern astronomy

Astronomically, the zodiac defines a belt of space extending 8°

The Sun's placement upon the vernal equinox, which occurs annually around 21 March, defines the starting point for measurement, the first degree of which is historically known as the "

The convention of measuring celestial longitude within individual signs was still being used in the mid-19th century,[77] but modern astronomy now numbers degrees of celestial longitude continuously from 0° to 360°, rather than 0° to 30° within each sign.^[78] This coordinate system is primary used by astronomers for observations of solar system objects.^[79]

The use of the zodiac as a means to determine astronomical measurement remained the main method for defining celestial positions by Western astronomers until the Renaissance, at which time preference moved to the equatorial coordinate system, which measures astronomical positions by

In Unicode, the symbols of zodiac signs are encoded in block "Miscellaneous Symbols". They can be forced to look like text by appending U+FE0E, or like emojis by appending U+FE0F:[82]

See also

References

External links

Wikimedia Commons has media related to:
Zodiac (category)

• "A Treatise on Zodiacal Signs and Constellations: Unique Jewels on the Benefits of Keeping Time" is a manuscript that dates back to 1831 with a focus on Arabic, Coptic and Syriac calendars.
• Zodiac Constellations at Constellation Guide