The astrological chart is a schematic representation of the sky at any given moment of time, projected upon the
ecliptic--or the apparent path of the Sun as seen from the Earth--which forms the circle in which the chart is enclosed. The longitudinal positions of the planets are plotted onto this circle, because the planets (except Pluto) and many stars, lie very close to the Sun's path in celestial latitude.
How this map of the sky is seen from the Earth is determined by where the
horizonis at the time for which the chart is cast. The horizon forms the boundary between what can be seen, or the visible sky, and sky which exists on the opposite side of the earth, which exists at the same time and space, but cannot be seen.
The line of the horizon cuts across the circle of the chart horizontally, and forms the most important angle of the chart: the
Ascendant, or the exact place where the Sun's path crosses the horizon in the East. It is at this point that all planets and many stars appears to rise up out of what cannot be seen and become apparent to the observer. This is because the Earth's daily rotation reveals sky objects from East to West, and makes them appear to be moving from the eastern horizon across the sky to the western horizon, where they disappear again to the observer, dipping down again to the unseen sky. The western horizon, where the Sun's path meets the horizon in the West, is called the Descendant.
The other very important "angle" of the chart is the
Midheaven(also called the M.C. for the Latin "Medium coeli", or "middle of the sky.") The Midheaven represents the highest point in the sky reached by the Sun, or its "culmination", as it crosses from one horizon to the other--the noon point in a chart which is plotted for dawn. At the Earth's equator, it is the point on the ecliptic which is directly overhead from the observer; as the observer moves north or south from the Equator, the midheaven appears to withdraw, so that from points north of the equator, the noon point of the Sun appears lies in the southern sky, and south of the equator, it appears in the northern sky.
The point opposite the Midheaven, which is in the unseen sky, and would be the midnight point in a chart cast for dawn, is the "anticulmination" of the Sun, or the
Imum Coeli, which is Latin for the "bottom of the sky." This is the last of the four "angles."
The angles are crucial to the understanding of the meaning of the sky map to the individual or event for which it was cast. There are no more individual points in chart. Much has been made by astrologers (deriving from the
Theosophicaltradition that is closely linked to much of modern astrological practice) of the quality of "coming into being" that they represent, as they represent going from the unseen to the seen. Since Theosophical astrology was tied to the idea of manifesting from the spiritual to the bodily form, the angles have come to symbolize this connection. However, even if this theory is discounted, as Bernadette Bradyref|Brady has noted, to all ancient peoples, the horizon was the place where the gods came into contact with the earth and became available to human supplication. Without this connection, the spiritual realm and the world had nothing to do with one another, and for that reason, astrology, which seeks to communicate between the two spheres, must use this place of connection to derive significance for the world from the sky.
For delineation of each of the angles, see:
#Bernadette Brady. Brady's Book of Fixed Stars. Weiser Books (York Beach, Maine, 1999) ISBN 1-57863-105-X.
Wikimedia Foundation. 2010.