In Western astrology, astrological signs are the twelve 30° sectors of the ecliptic, starting at the vernal equinox (one of the intersections of the ecliptic with the celestial equator), also known as the First Point of Aries. The order of the astrological signs is Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces. Each sector is named for a constellation it passes through.

The concept of the zodiac originated in Babylonian astrology, and was later influenced by Hellenistic culture. According to astrology, celestial phenomena relate to human activity on the principle of "as above, so below", so that the signs are held to represent characteristic modes of expression.^[1] Modern discoveries about the true nature of celestial objects have undermined the theoretical basis for assigning meaning to astrological signs, and empirical scientific investigation has shown that predictions and recommendations based on these systems are not accurate.^{[2] [3]} Astrology is generally regarded as pseudoscience.

The twelve sector division of the ecliptic constitutes astrology's primary frame of reference when considering the positions of celestial bodies, from a geocentric point of view, so that we may find, for instance, the Sun in 23° Aries (23° longitude), the Moon in 7° Scorpio (217° longitude), or Jupiter in 29° Pisces (359° longitude). Beyond the celestial bodies, other astrological points that are dependent on geographical location and time (namely, the Ascendant, the Midheaven, the Vertex and the houses' cusps) are also referenced within this ecliptic coordinate system.

Various approaches to measuring and dividing the sky are currently used by differing systems of astrology, although the tradition of the Zodiac's names and symbols remain consistent.

Western astrology measures from Equinox and Solstice points (points relating to equal, longest and shortest days of the tropical year), while Jyotiṣa or Vedic astrology measures along the equatorial plane (sidereal year). Precession results in Western astrology's zodiacal divisions not corresponding in the current era to the constellations that carry similar names,^[4] while Jyotiṣa measurements still correspond with the background constellations.^[5]

In Western and Indian astrology, the emphasis is on space, and the movement of the Sun, Moon and planets in the sky through each of the zodiac signs. In Chinese astrology, by contrast, the emphasis is on time, with the zodiac operating on cycles of years, months, and hours of the day.

While Western astrology is essentially a product of Greco-Roman culture, some of its more basic concepts originated in Babylon.

Isolated references to celestial "signs" in Sumerian sources are insufficient to speak of a Sumerian zodiac.^[6] Specifically, the division of the ecliptic in twelve equal sectors is a Babylonian conceptual construction.^[7]

By the 4th century BC, Babylonian astronomy and its system of celestial omens had an influence on the culture of ancient Greece, as did the astrology of ancient Egypt by late 2nd century BC. This resulted, unlike the Mesopotamian tradition, in a strong focus on the birth chart of the individual and in the creation of Horoscopic astrology, employing the use of the Ascendant (the rising degree of the ecliptic, at the time of birth), and of the twelve houses. Association of the astrological signs with Empedocles' four classical elements was another important development in the characterization of the twelve signs.

The body of astrological knowledge by the 2nd century AD is described in Ptolemy's Tetrabiblos

The following table enumerates the twelve divisions of celestial longitude, with the Latin names (still widely used) and the English translation (gloss).

The longitude intervals, being a mathematical division, are closed for the first endpoint (a) and open for the second (b) – for instance, 30° of longitude is the first point of Taurus, not part of Aries. Association of calendar dates with astrological signs only makes sense when referring to Sun sign astrology.

Empedocles, a fifth-century BC Greek philosopher, identified Fire, Earth, Air, and Water as elements. He explained the nature of the universe as an interaction of two opposing principles called love and strife manipulating the four elements, and stated that these four elements were all equal, of the same age, that each rules its own province, and each possesses its own individual character. Different mixtures of these elements produced the different natures of things. Empedocles said that those who were born with near equal proportions of the four elements are more intelligent and have the most exact perceptions.^[8]

Each sign is associated with one of the classical elements,^[9]Be]] positive or extrovert, masculine signs; while Water and Earth signs are considered negative or introvert, feminine signs. The four astrological elements are also considered as a direct equivalent to Hippocrates' personality types (sanguine = air; choleric = fire; melancholic = earth; phlegmatic = water). A modern approach looks at elements as "the energy substance of experience"^[11] and the next table tries to summarize their description through keywords.^[12][13]

Classification according to element has gained such importance, that some astrologers start their interpretation of a natal chart, by studying the balance of elements shown by the position of planets and angles^[15] (especially the Sun, the Moon and the Ascendant).

Each of the four elements manifests in three modalities: Cardinal, Fixed and Mutable.^[16] As each modality comprehends four signs, these are also known as Quadruplicities.^[9]*Be]]*Be]]*Be]]*Be]]*Be]]*Be]]Be]]

The combination of element and modality provides a basic sign characterization.

For instance, Capricorn is a cardinal earth sign, meaning that it is associated with action (cardinal modality) in the material world (earth element).

That can translate into ambition or practical application to the concrete, everyday necessities of life.^[20] The next table displays the twelve combinations of elements and modalities.

Rulership is the connection between planet and correlated sign and house.^[21] In traditional Western astrology, each sign is ruled by one and only one of the seven visible planets (note that in astrology, the Sun and Moon are termed The Lights, while the other bodies are called planets, which literally means wanderers, i.e. wandering stars as opposed to the fixed stars). The traditional rulerships are as follows: Aries (Mars), Taurus (Venus), Gemini (Mercury), Cancer (Moon), Leo (Sun), Virgo (Mercury), Libra (Venus), Scorpio (Pluto), Sagittarius (Jupiter), Capricorn (Saturn), Aquarius (Uranus), and Pisces (Neptune).^[22][23]

Psychologically-oriented astrologers often believe that Saturn is the ruler or co-ruler of Aquarius instead of Uranus; Neptune is the ruler or co-ruler of Pisces instead of Jupiter, and that Pluto is the ruler or co-ruler of Scorpio instead of Mars. Some astrologers^[24] believe that the planetoid Chiron may be the ruler of Virgo, while other group of modern astrologers claim that Ceres is the ruler of Taurus instead. Other astrologers, still, use the former planets Pallas, Vesta, Juno and Hygiea in their delineations and rulerships, for example Vesta to Taurus and Pallas to Virgo.

Debate continues between those who consider the newly discovered planets as rulers or co-rulers of certain signs and those that do not.

Some astrologers do not even use the astrological signs at all (mostly Cosmobiologists and Uranian Astrologers/Hamburg School).

Therefore, they do not take into account planetary rulerships and the essential dignities when interpreting an astrological chart.

Note that, if one starts from Leo and Cancer, the traditional planetary rulers are arrayed outward in the same order from the sun as they occur in the natural solar system. The Lights ruling Leo and Cancer, Mercury ruling Virgo and Gemini, Venus ruling Libra and Taurus, Mars ruling Scorpio and Aries, Jupiter ruling Sagittarius and Pisces, Saturn ruling Capricorn and Aquarius. The result is a symmetry of traditional rulerships across the 0° Leo/Aquarius axis. Note that modern rulerships, which attribute Pluto as ruler of Scorpio, break this symmetry.

The following table shows both, traditional^[22] and modern,^[25] rulerships.

Signs can also be listed by opposite, resulting in six couples who are:

To note that Fire and Air (light elements) signs are opposited between them, like happens for Earth and Water ones (heavy elements).^[26] Spring signs are opposite to autumn ones, and winter signs are related to summer ones.^{[27][28][29][30]}

A traditional belief of astrology, known as essential dignity, is the idea that the Sun, Moon and planets are more powerful and effective in some signs than others, because the basic nature of both is held to be in harmony. By contrast, they are held to find some signs to be weak or difficult to operate in because their natures are thought to be in conflict. The most important of these categories are Dignity, Detriment, Exaltation and Fall.

In traditional astrology, other levels of Dignity are recognised in addition to Rulership.

These are known as Exaltation (see below), Triplicity, Terms or bounds, and Face or Decan, which together are known as describing a planet's Essential dignity, the quality or ability to give of one's true nature. Contemporary traditional astrologers like John Frawley or J Lee Lehman explain further on the concept of Essential Dignity.^[31]

The following table summarizes the positions described above:

In addition to essential dignity, the traditional astrologer considers Accidental dignity of planets. This is placement by house in the chart under examination. Accidental dignity is the planet's "ability to act." So we might have, for example, Moon in Cancer, dignified by rulership, is placed in the 12th house it would have little scope to express its good nature.^[33] The 12th is a cadent house as are the 3rd, 6th and 9th and planets in these houses are considered weak or afflicted. On the other hand, Moon in the 1st, 4th, 7th or 10th would be more able to act as these are Angular houses. Planets in Succedent houses of the chart (2nd, 5th, 8th, 11th) are generally considered to be of medium ability to act. Besides Accidental Dignity, there are a range of Accidental Debilities, such as retrogradation, Under the Sun's Beams, Combust, and so forth.

Each sign can be divided into three 10° sectors known as decans or decanates, though these have fallen into disuse.

The first decanate is said to be most emphatically of its own nature and is ruled by the sign ruler.^[34] The next decanate is sub-ruled by the planet ruling the next sign in the same triplicity.

The last decanate is sub-ruled by the next in order in the same triplicity.^[35]

While the element and modality of a sign are together sufficient to define it, they can be grouped to indicate their symbolism.

The first four signs, Aries, Taurus, Gemini and Cancer, form the group of personal signs.

The next four signs, Leo, Virgo, Libra and Scorpio form the group of interpersonal signs.

The last four signs of the zodiac, Sagittarius, Capricorn, Aquarius and Pisces, form the group of transpersonal signs.^[36]

Dane Rudhyar presented the tropical zodiac primary factors,^[37] used in the curriculum of the RASA School of Astrology.

The tropical zodiac is the zodiac of seasonal factors as opposed to the sidereal zodiac (constellation factors).

The primary seasonal factors are based on the changing ratio of sunlight and darkness across the year.

The first factor is whether the chosen time falls in the half of the year when daylight is increasing, or the half of the year when darkness is increasing.

The second factor is whether the chosen time falls in the half of the year when there is more daylight than darkness, or the half when there is more darkness than daylight.

The third factor is which of the four seasons the chosen time falls in, defined by the first two factors.

Thus^[38][39]

In Indian astrology, there are five elements: fire, earth, air, water, and ether.

The master of fire is Mars, while Mercury is of earth, Saturn of air, Venus of water, and Jupiter of ether.

Jyotish astrology recognises twelve zodiac signs (Rāśi),^[40] that correspond to those in Western astrology. The relation of the signs to the elements is the same in the two systems.

A nakshatra (Devanagari: नक्षत्र, Sanskrit nakshatra, from naksha- 'map', and tra- 'guard') or lunar mansion is one of the 27 divisions of the sky, identified by the prominent star(s) in them, as used in Hindu astronomy and astrology (Jyotisha).^[41]

Chinese astrological signs operate on cycles of years, lunar months, and two-hour periods of the day (also known as shichen). A particular feature of the Chinese zodiac is its operation in a 60-year cycle in combination with the Five Phases of Chinese astrology (Wood, Fire, Metal, Water and Earth).^[42] Nevertheless, some researches say that there is an obvious relationship between the Chinese 12-year cycle and zodiac constellations: each year of the cycle corresponds to a certain disposal of Jupiter.

For example, in the year of Snake Jupiter is in the Sign of Gemini, in the year of Horse Jupiter is in the Sign of Cancer and so on. So the Chinese 12-year calendar is a solar-lunar-jovian calendar.

The following table shows the twelve signs and their attributes.

In Chinese astrology, the zodiac of twelve animal sign represents twelve different types of personality. The zodiac traditionally begins with the sign of the Rat, and there are many stories about the Origins of the Chinese Zodiac which explain why this is so. When the twelve zodiac signs are part of the 60-year calendar in combination with the four elements, they are traditionally called the twelve Earthly Branches. The Chinese zodiac follows the lunisolar Chinese calendar^[43]]]]]]]]]]]]]*]]*nd thus the "changeover" days in a month (when one sign changes to another sign) vary each year. The following are the twelve zodiac signs in order.

The five elements operate together with the twelve animal signs in a 60-year calendar. The five elements appear in the calendar in both their yin and yang forms and are known as the ten Celestial stems. When trying to calculate the relevant year of the cycle in relation to the Gregorian calendar, an easy rule to follow is that years that end in an even number are Yang (representing masculine, active and light), those that end with an odd number are Yin (representing feminine, passive and darkness).^[45]