The Harmonics  is probably earlier than the Tetrabiblos. Although Ptolemy is well known for his astronomy and astrology, he was also one of the ancient sources for theories of harmony.
The work studies this universal field of knowledge for which music and geometry are considered subsets. Much of the material in Harmonics is exclusively concerned with music, including setting up exact number ratios for the different modes.
The Harmonics Book III applies his previous exposition of intervals, scales, and ratios to ethics and psychology, and to astronomy and astrology. Unfortunately, some of Book III has been lost to us. In Chapter 9 of Book III, Ptolemy presents the astrological aspects and their ratios to one another. Curiously, in this presentation Ptolemy does not use degree numbers for the aspects. Instead, he considers the twelve zoidia as discrete units and the whole number relationships between them that yield moriai and epimoriai. This discussion might have fit better into the Tetrabiblos Book I, within his treatment of the qualities and relationships between whole zoidia.
Ptolemy’s Digression: Astrology’s Aspects and Musical Intervals
Statement of the Problem
This essay addresses a problem in the development and continuity of astrology: how do astrologers, past and present, account for the astrological aspects? Aspects are the means by which a planet or position (Ascendant, Lot of Fortune) has contact from another planet or other planets. Once an astrologer has designated planet or a position to answer a question posed to the astrological chart, aspects to the designated position provide information to answer a question and determine an outcome.
What do have Bill Gates and Kepler, Mahatma Gandhi and Pablo Picasso, Maria Shriver and Descartes, Nelson Mandela and Jim Jones have in common? No, I’m not joking! They all have the same aspect in their birth chart: Jupiter conjunct Pluto. In non-astrological language this says that seen from the earth Jupiter and Pluto are seen in the very same direction in heaven. It is as if they can shake hands. Of course there is, astronomically spoken, still a big distance between the two, but when two planets are seen very close to each other in heaven; astrologers call this ‘conjunct’. A conjunction means power and the astrological meaning of these planets will be strongly interwoven.
Pluto is a (dwarf) planet and has many faces: dark, intense, and confronts us with shadows and problems. Pluto wants power, whether in secret or in the open. But the same Pluto also has the power to bring to the surface what was hidden, has real psychological knowledge and an incredible willpower and persistence.
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