How to cast a horoscope

Note: Beliefs regarding the validity of astrology and its classification as a pseudoscience are discussed in the article "astrology".

The following instructions apply to contemporary mainstream western astrology. The procedures used in vedic astrology, Chinese astrology and some schools of western astrology differ significantly.

1. The native refers to the time and place for the event being charted, and it is 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 defined by the the earth's equator.
4. The plane of the ecliptic is defined by the orbit of the earth and the sun. For all practical purposes the plane of the equator and the plane of the ecliptic maintain a constant inclination relative 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 indefinitely 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 they are relatively minor; they are clearly 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 plane of the ecliptic, and its width is sufficient to allow for the fact that the plane of of the orbits of the moon and all other planets is not parallel to the plane of the ecliptic. It has a width of approximately 18º.
8. The medium coeli, or mid-heaven, is that 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 co-incide the limiting position for these points is at 90º 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-most point of the chart. In the course of a day, because of the earth's rotation, the entire circle of the ecliptic will pass through the ascendant and it will be advanced by about 1º. This provides us with the term rising sign, which is the sign of the zodiac appearing 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 that is familiar to almost everyone. As a very rough rule of thumb, if an event takes place at 6:00 a.m. its ascendant and sun sign will be the same; other rising signs can then be estimated at two hour intervals from there.
11. The houses[?] are a series of twelve intervals on the plane of the ecliptic. Astrologers have devised at least nine different ways of calculating the positions of those houses. 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 an other. 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. Using an Equal House System the ecliptic is divided into twelve equal spaces of 30º each. The first house begins at the ascendant and the others are numbered consecutively counterclockwise from that point. The first six are all below the horizon, and the later six are all above. The positions of these houses remains fixed relative to the native. The signs move through the houses. The planets move through the signs.
12. Most astrologers use the tropical zodiac in which the astrological year begins with the vernal equinox when the sun moves into the earth's northern hemisphere. This point is set at 0º of the sign Aries. This is a matter of some dispute with those who would favor the sidereal zodiac that 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 a sidereal zodiac believe that the position of the signs should be fixed relative to the constellations.
13. A cusp is the dividing point 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 equal houses. On this is superimposed the signs. In an equal house system the cusp between any two houses will fall at the corresponding point for each of the signs. Thus for a native whose ascendant is at 12º of Leo, the beginning of the second house will be at 12º of Virgo, of the third at 12º Libra, and so on. In other systems of houses which take into consideration the effects of the angle of intersection between the planes of the horizon and the ecliptic, the calculations will not be that simple. For these calculations it is essential to know the latitude of the event. Tables are available for these calculations, and more recently computer programmes have been written to do this.

Longitude is also necessary for determining 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 cities in a time zone had the same time. In reality there is an hour of difference between points at the beginning and end of a 15º average time zone. For political reasons the time zones cannot be identical bands. It would not be appropriate for a time zone boundary to cut through the middle of a city. 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 is then able to place the sun, moon and planets at their rightful celestial longitudes. Some astrologers will also take note of minor planetary bodies, fixed stars and yet other calculated points.

To complete the horoscope the astrologer will consider the aspects or relative angles between pairs of planets. Certain aspects carry greater importance. Those generally recognized by the astrological community are conjunction (0º), opposition (180º), square (90º), trine (120º) and sextile (60º). Other aspects of less importance are recognized by some. Understandably these aspects are more significant when they are exact, but they are considered to function within an orb of influence. The size of that orb will vary according to the importance of the particular aspect. Thus conjunctions are believed to operate over a larger orb than sextiles.

Reference: Dona Marie Lorenz, Tools of Astrology: houses, Topanga, Eomega Grove Press, 1973