The horoscope 12 sectors around the circle of the ecliptic, starting from the eastern horizon with the ascendant or rising sign. These 12 sectors are called the houses and numerous systems for calculating these divisions exist. Tables of houses have been published since the 19th Century to make this otherwise demanding task easier.
Houses
The chart thus begins with a framework of 12 houses. Upon this the signs of the zodiac are superimposed. In the equal house system the cusp between any two houses will fall at the same degree for each of the 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 commonly calculated by computer. Most astrology computer programs allow the user to choose from a variety of house systems.
Placements of the planets
Having established the relative positions of the signs in the houses, the astrologer positions the sun, moon, and planets at their proper 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 vertex, equatorial ascendant, etc. Many astrologers also use what are commonly referred to as Arabic parts (or Greek Lots), the most common of which is the Part of Fortune (Pars Fortunae).Aspects
To complete the horoscope the astrologer will consider the aspects or relative angles between pairs of planets. More exact aspects are considered more important. The difference between the exact aspect and the actual aspect is called the orb. Those generally recognized by the astrological community are Conjunction (0°), Opposition (180°), Square (90°), Trine (120°), Sextile (60°), Semi-Square (45°), Sesqisquare (135°), and Quincunx (150°). 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 8° or less for aspects involving the Sun, Moon, and Jupiter and smaller orbs for the other points. Some astrologers, such as practitioners of Cosmobiology, and Uranian astrology, use minor aspects (15°, 22.5°, 67.5°, 72°, 75°, 105°, 112.5°, 157.5°, 165°) with much narrower orbs.Ascendant
Longitude is necessary in order to determine the position of the Ascendant because horoscopes use local time.Having constructed the horoscope, the astrologer can begin the task of interpreting the chart. This interpretation depends upon which branch of horoscopic astrology is being used.
Adapted from the Wikipedia article Horoscope, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki
