A birth chart is a map of the heavens as the newborn child would see it from the place of his or her birth, except that it also includes the sky below the horizon. There are ten moving planets in the chart with which astrology is chiefly concerned. To determine the location of the Earth, astrologers use classical astronomical theory to perform planetary calculations. 

The calculations involve fixing the planet Earth and determining the other planets in the solar system relative to it (as if all the planets rotate around the Earth). In the symbolic terms of astrology, the center of the chart represents Earth. There are 12 equal signs which form an encircling band along the ecliptic, each sign occupying a fixed 30 degree field. Against these fields move the planets, passing at various rates out of one sign and into the next in a perpetual circuit.

As the planets move, the angles they form with Earth can be measured for any given moment. In astrology, the moment of birth is taken as the decisive time in a person's life, and the birth chart is calculated with the angular degrees that show how the planets appeared in the heavens at that precise moment. In this way, a person’s birth chart reveals a pattern unique for his or her time and place of birth. 

Once a birth chart is calculated, an astrologer interprets the person's life according to the positions of the planets in different Signs, Houses and Aspects based on compiled statistical interpretation data collected over the past 6,000 years.