There are few texts about astrology that survive intact from ancient Greece and Rome. Although there are fragments from early Greek writers, the oldest complete text is Astronomica, written in the early 1st century CE by a Roman, Marcus Manilius. Astronomica is a poem in five books totalling 4,258 lines. It purports to be an instructional text on the theory and practice of astrology, and it would be a very valuable record of astrological practice in the early Empire – if it were not for its fundamental defects.
What’s wrong with Astronomica, you wonder? Well, for one thing, Manilius wrote a lot about the constellations of the zodiac, but very little about the planets. In astrology, the positions of the planets in the sky at the time of the birth of the native (as astrologers call the person whose horoscope is being cast) are supremely important. Without making reference to the positions of the planets within the zodiac the only data you have are which sign was rising at the moment of the native’s birth (which is called the ascendant) and, perhaps, which sign the sun was in. Clearly this doesn’t allow for many variations on the native’s horoscope.
(Of course, the “horoscopes” printed in newspapers today are based solely on the sign the sun was in at the time of birth. Any astrologer, ancient or modern, who was worth his/her salt would scoff at such unsophisticated nonsense!)