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Casting charts

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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. The process of interpreting a chart is known as delineation.

A horoscope (or astrological chart)

Casting the chart

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.

Key components

  • The Medium Coeli or Midheaven 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 90º from the ascendant.
  • 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 1º. This provides us with the term rising sign, which is the sign of the zodiac on the native's ascendant.
  • 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.
  • 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. 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. Since the ascendant/descendant axis divides the horoscope along the horizon, the first six houses fall below the horizon, and the other six fall above it.
  • The signs of the zodiac are superimposed upon the layout of houses as described above. Most western 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 50.25 seconds of arc every year, or 1° 23' 45" per century. 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.
  • Having established the relative positions of the signs in the houses, the astrologer positions chart points such as 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 (0º), opposition (180º), square (90º), trine (120º) and sextile (60º). 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 10º or less.