In astrology, a horoscope is a chart or diagram representing the positions of the planets, other celestial bodies, and sensitive angles at the time of any moment in time or any event, such as a person's birth. The term horoscope is derived from Greek words meaning, "a look at the hours." Other commonly used names for the horoscope in English include natal chart, natus, birth chart, astrological chart, celestial map, sky-map, nativity, cosmogram, Vitasphere, soulprint, radix, or simply chart, among others.

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A horoscope (or astrological chart) - Y2K Chart -- This particular chart is calculated for January 1, 2000 at 12:01:00 A.M. Eastern Standard Time in New York City, New York, USA. (Longitude: 074W00'23" - Latitude: 40N42'51")


Opinions about the validity of astrology, or its classification as a pseudoscience are considered in the head article on astrology.

Using an ephemeris and a table of houses an astrologer calculates the geocentric positions of the Sun, Moon, and planets for a specific time and place in order to erect a formatted horoscope. This diagram, called a chart is a stylized map of the heavens. The Sun or the Earth is placed in the centre (depending on whether the ephemeris was heliocentric or geocentric) with the remaining elements around the outside: the planets, the lunar nodes, the ascendant and midheaven, and the houses. Then the angles between the planets are determined. These angles are the astrological aspects. Different systems of tri-secting arcs produce houses of different size.

In common usage, the word horoscope also refers to the astrologer's interpretation of the astrological chart.

In particular, many newspapers and magazines carry horoscope columns, describing planetary positions and influences for the various astrological signs. Most astrologers regard those as nearly worthless, since a horoscope is actually highly personalized, and cannot be generalized to thousands of readers.

How to cast a horoscope

In order to understand and visualize the spherical geometry of the construction of a horoscope, we need to begin with some basic terms.

The techniques described here belong to western astrology.

  1. The native refers to the time and place of the event being charted, and considered to be at the centre of the celestial sphere. This term is a general one that includes not only birth times as they are commonly understood, but any event for which a horoscope may be drawn.
  2. The celestial sphere is a sphere of arbitrary radius upon which the items appearing on the horoscope are projected without regard to their distance from the native.
  3. The plane of the equator is the plane of the earth's equator projected into space.
  4. The plane of the ecliptic is defined by the orbits of the earth and the sun. For practical purposes the plane of the equator and the plane of the ecliptic maintain a constant inclination to each other of approximately 23.5°.
  5. The plane of the horizon is centred on the native, and is tangential to the earth at that point. In a sphere whose radius is infinitely large this plane may be treated as nearly equivalent to the parallel plane with its centre at the earth's center. This greatly simplifies understanding the geometry of the horoscope. Some writers on astrology have considered the effects of parallax, but most would agree that (apart from that of the Moon) they are relatively minor, and are beyond the scope of this article.
  6. The axis of the plane of the horizon has end points above, the zenith, and below, the nadir.
  7. The zodiac refers to a band on the celestial sphere containing the signs. It is centered on the ecliptic, and its width is sufficient to allow for the fact that the orbits of the moon and all other planets are not parallel to the plane of the ecliptic. It is approximately 18 wide.
  8. The medium coeli or mid-heaven is the point on the ecliptic that is furthest above the plane of the horizon; its opposite point is known as the imum coeli. For events occurring where the planes of the eccliptic and the horizon coincide the limiting position for these points is at 90 from the ascendant.
  9. The ascendant is the eastern point where the ecliptic and horizon intersect. Its opposite point in the west is the descendant. In draughting a horoscope the ascendant is traditionally placed as the left-hand side point of the chart. During the course of a day, because of the earth's rotation, the entire circle of the ecliptic will pass through the ascendant and will be advanced by about 1. This provides us with the term rising sign, which is the sign of the zodiac on the native's ascendant.
  10. The sun sign is the sign of the zodiac in which the sun is located for the native. This is the single astrological fact most familiar to people. If an event occurs at sunrise the ascendant and sun sign will be the same; other rising signs can then be estimated at approximately two hour intervals from there.
  11. The houses are a series of twelve divisions of the plane of the ecliptic. Astrologers have devised at least nine different ways of calculating these house divisions. Just as this article does not seek to discuss the validity of astrology, it is also beyond its scope to attempt to resolve questions which may be disputed among astrologers. The use of a particular system of house division is often more a result of what calculations are available than of any conscious consideration of one system's merits over that of another. Similarly, explanations in this article based on the Equal House System are not meant to give any theoretical preference to that system; it is simply the system whose geometry is easiest to understand. In the case of the Equal House System the ecliptic is divided into twelve equal houses of 30 each. The first house begins at the ascendant and the others are numbered counterclockwise from that point. The first six are therefore below the horizon, and the other six are above. The positions of these houses remains fixed relative to the native. The signs and planets all move through the twelve houses during the course of a day, and the planets move through the signs over the course of months or years.
  12. Most astrologers use the tropical zodiac in which the astrological year begins with the vernal equinox, when the sun crosses the celestial equator and enters the sign of Aries. This is a matter of some dispute with those who favor the sidereal zodiac which takes into account the precession of the equinoxes. Because of a "wobble" in the earth's axis of rotation over a period of about 26,000 years the point at which the vernal equinox advances in the sky by about 50 seconds of arc every year. Advocates of the sidereal zodiac believe that the position of the signs should be fixed relative to the constellations. Adherants to the tropical view are content to use a view of the heavens as seen 2000 years ago as still valid for work today.
  13. A cusp is the boundary between two signs or houses. For some the cusp includes a small portion of the two signs or houses under consideration.

The chart thus begins with a framework of 12 houses. Upon this the signs of the zodiac are superimposed. In an equal house system the cusp between any two houses will fall at the same degree for each of the signs. Thus for a native whose ascendant is at 12 of Leo, the second house will begin at 12 of Virgo, the third at 12 Libra, and so on. In house systems that take into consideration the effects of the angle of intersection between the planes of the horizon and the ecliptic, the calculations are more complicated. For these calculations it is essential to know the latitude of the event. Tables are available for these calculations, but they are now normally calculated by computer. Most computer programs allow the user to choose from a variety of house systems. The most commonly used is the Placidus system.

Longitude is also necessary in order to determine the position of the ascendant. This is because charts use Local Time. Time zones were developed in the 19th century as a by-product of the development of railways. This permitted train schedules to be written based on the certainty that any two places in a time zone used the same time. In reality there is an hour's difference between points at the beginning and end of a 15 average time zone. For political reasons the time zones cannot all be the same size. It would not be practical for a time zone boundary to cut through the middle of a town or small country. Time zone boundaries were also the subject of political manipulation in the Pacific islands when they sought to be the first places on earth to see the new millennium. Adjustments are therefore made for the difference in one's actual longitude and the longitude of the nominal meridian associated with clock time.

Having established the relative positions of the signs in the houses, the horoscopist positions the sun, moon and planets at their rightful celestial longitudes. Some astrologers also take note of minor planetary bodies, fixed stars, asteroids (for example, Chiron) and other mathematically calculated points and angles such as the Ascendant (ASC), the MC, the DC, and the IC, the Vertex, Equatorial Ascendant, etc. Many astrologers also use what are commonly referred to as Arabic Parts (or Greek Lots), the most famous of which is the Part of Fortune (Pars Fortuna).

To complete the horoscope the astrologer will consider the aspects or relative angles between pairs of planets. Certain aspects are considered more important than others. Those generally recognized by the astrological community are conjunction (0), opposition (180), square (90), trine (120) and sextile (60). Other aspects are used by some astrologers. Understandably these aspects are more significant when they are exact, but they are considered to function within an orb of influence, the size of which varies according to the importance of each aspect. Thus conjunctions are believed to operate with a larger orb than sextiles. Most modern astrologers use an orb of 10 or less.

Reference: Dona Marie Lorenz, Tools of Astrology: houses, Topanga, Eomega Grove Press, 1973de:Horoskop et:Horoskoop nl:Horoscoop pl:Horoskop fi:Horoskooppi


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