Western astrology

Western astrology is the system of astrology most popular in Western countries. It is historically based on Ptolemy's Tetrabiblos, which in turn was a continuation of Hellenistic and ultimately Babylonian traditions.
Western astrology is largely horoscopic, that is, it is a form of divination based on the construction of a horoscope for an exact moment, such as a person's birth as well as location, in which various cosmic bodies are said to have an influence. Astrology in western popular culture is often reduced to sun sign astrology, which considers only the individual's date of birth.
Astrology is a pseudoscience and has consistently failed experimental and theoretical verification.
Astrology was widely considered a respectable academic and scientific field before the Enlightenment, but modern research has found no consistent empirical basis to it.

Core principles

A central principle of astrology is integration within the cosmos. The individual, Earth, and its environment are viewed as a single organism, all parts of which are correlated with each other. Cycles of change that are observed in the heavens are therefore reflective of similar cycles of change observed on earth and within the individual. This relationship is expressed in the Hermetic maxim "as above, so below; as below, so above", which postulates symmetry between the individual as a microcosm and the celestial environment as a macrocosm.
As opposed to Sidereal astrology, Western astrology evaluates a person's birth based on the alignments of the stars and planets from the perspective on earth instead of in space.
At the heart of astrology is the metaphysical principle that mathematical relationships express qualities or 'tones' of energy which manifest in numbers, visual angles, shapes and sounds – all connected within a pattern of proportion. An early example is Ptolemy, who wrote influential texts on all these topics. Al-Kindi, in the 9th century, developed Ptolemy's ideas in De Aspectibus which explores many points of relevance to astrology and the use of planetary aspects.

The zodiac

The zodiac is the belt or band of constellations through which the Sun, Moon, and planets move on their journey across the sky. Astrologers noted these constellations and so attached a particular significance to them. Over time they developed the system of twelve signs of the zodiac, based on twelve of the constellations through which the sun passes throughout the year, those constellations that are "Enlightened by the mind". Most western astrologers use the tropical zodiac beginning with the sign of Aries at the Northern Hemisphere vernal equinox always on or around March 21 of each year. The Western Zodiac is drawn based on the Earth's relationship to fixed, designated positions in the sky, and the Earth's seasons. The Sidereal Zodiac is drawn based on the Earth's position in relation to the constellations, and follows their movements in the sky.
Due to a phenomenon called precession of the equinoxes, there is a slow shift in the correspondence between Earth's seasons and the constellations of the zodiac. Thus, the tropical zodiac corresponds with the position of the earth in relation to fixed positions in the sky, while the sidereal zodiac is drawn based on the position in relation to the constellations.

The twelve signs

In modern Western astrology the signs of the zodiac are believed to represent twelve basic personality types or characteristic modes of expression. The twelve signs are divided into four elements fire, earth, air and water. Fire and air signs are considered masculine, while water and earth signs are considered feminine. The twelve signs are also divided into three qualities, also called modalities, Cardinal, fixed and mutable.

Note: these are only approximations and the exact date on which the sign of the sun changes varies from year to year.

Zodiac sign for an individual depends on the placement of planets and the ascendant in that sign. If a person has nothing placed in a particular sign, that sign will play no active role in their personality. On the other hand, a person with, for example, both the sun and moon in Cancer, will strongly display the characteristics of that sign in their make up.

Sun-sign astrology

Newspapers often print astrology columns which purport to provide guidance on what might occur in a day in relation to the sign of the zodiac that included the sun when the person was born. Astrologers refer to this as the "sun sign", but it is often commonly called the "star sign". These predictions are vague or general; so much so that even practicing astrologers consider them of little to no value on their own. Experiments have shown that when people are shown a newspaper horoscope for their own sign along with a newspaper horoscope for a different sign, they judge them to be equally accurate on average. Other tests have been performed on complete, personalized horoscopes cast by professional astrologers, and have shown no correlation between the horoscope results and the person it was cast for.

The planets

In modern Western astrology the planets represent basic drives or impulses in the human psyche. These planets differ from the definition of a planet in astronomy in that the Sun, Moon, and recently, Pluto are all considered to be planets for the purposes of astrology. Each planet is also said to be the ruler of one or two zodiac signs. The three outer planets have each been assigned rulership of a zodiac sign by astrologers. Traditionally rulership of the signs was, according to Ptolemy, based on seasonal derivations and astronomical measurement, whereby the luminaries being the brightest planets were given rulership of the brightest months of the year and Saturn the coldest furthest classical planet was given to the coldest months of the year, with the other planets ruling the remaining signs as per astronomical measurement. It is noteworthy that the modern rulerships do not follow the same logic.

Classical planets

The astrological 'planets' are the seven heavenly bodies known to the ancients. The Sun and Moon, also known as 'the lights', are included as they were thought to act like the astronomical planets. Astrologers call inner planets Mercury, Venus and Mars, the 'personal planets', as they represent the most immediate drives. The 'lights' symbolise respectively the existential and sensitive fundamentals of the individuality.
The following table summarizes the rulership by the seven classically known planets of each of the twelve astrological signs, together with their effects on world events, people and the earth itself as understood in the Middle Ages.

Symbol	Planet	Rules the sign	Effect on world events	Person's nature	Effect in life	Effect in earth
	Sun	Leo		Wisdom, generosity, 'sunny' disposition	Good fortune	Gold
	Moon	Cancer		Wandering	Travel, lunacy	Silver
	Mercury	Gemini and Virgo	Action	Changeability, eagerness, quickness, 'mercurial' temperament	Rapid change	Mercury
	Venus	Libra and Taurus	Fortunate events	Beauty, amorousness	Good luck	Copper
	Mars	Aries, classically also Scorpio	War	Strength, endurance, 'martial' temperament	Conflicts, misfortune	Iron
	Jupiter	Sagittarius, classically also Pisces	Good times, prosperity	Cheerful, magnanimous, 'jovial' temperament	Good fortune	Tin
	Saturn	Capricorn, classically also Aquarius	Disastrous events	Wisdom, stability, persistence, 'saturnine' temperament	Accidents, disease, treachery, bad luck	Lead

Modern modifications to the Ptolemaic system

Additional planets

These are the planets discovered in modern times, which have since been assigned meanings by Western astrologers.

Symbol	Planet	Representing	Rules the sign	Claimed effects on world events	Claimed effects on people
	Uranus	Platinum as a planetary metal; monogram 'H' for planet's discoverer, William Herschel	Aquarius	Innovation, technology	Sudden or disruptive change
	Neptune	Trident, weapon of sea-god Neptune	Pisces	Change of contemporary musical taste	Confusion, sensitivity
	Pluto	Pluto's bident; PL monogram for astronomer Percival Lowell who predicted a planet beyond Neptune	Scorpio	Demolition of old, bad political systems	Transformation, fate, death

Sidereal and tropical astrology

There are two camps of thought among western astrologers about the "starting point", 0 degrees Aries, in the zodiac. Sidereal astrology uses a fixed starting point in the background of stars, while tropical astrology, used by the majority of Western astrologers, chooses as a starting point the position of the Sun against the background of stars at the Northern hemisphere vernal equinox each year. The consequence of the Tropical approach is that when we say the Sun or a planet is in a certain zodiac sign, observation of it in the sky will show that it does not lie within that constellation at all.
As the Earth spins on its axis, it "wobbles" like a top, causing the vernal equinox to move gradually backwards against the star background, at a rate of about 30 degrees every 2,160 years. Thus the two zodiacs would be aligned only once every 26,000 years. They were aligned about 2,000 years ago when the zodiac was originally established.
This phenomenon gives us the conceptual basis for the Age of Aquarius, whose "dawning" coincides with the movement of the vernal equinox across the cusp from Pisces to Aquarius in the star background.