Astronomy and astrology cannot be separated in the Middle Ages and the early modern period as they are today. For the educated medieval man or woman both the movement of the heavenly bodies and their effect on man were entirely natural and subject to scientific study and prediction. It was possible for learned men to argue about how the influence of the planets was transmitted, about whether astrologers were performing their profession competently, or only telling their employers what they wanted to hear, and even whether it was lawful to use information about the movement of the stars and planets to foretell the future or to excuse a person's behavior. But all of these arguments took as their basis a shared belief in the influence of the planets on earth in general, and on man in particular.
For the contemporary readers of the Planet Books, the earth was at the center of the universe; the planets (the moon, Mercury, Venus, the sun, Mars, Jupiter, and Saturn) the stars and the heavenly regions rotated around earth (and man). The entire system was set up and put in motion by God, and it was by His will that the planets influenced life on earth in a regular way.
This influence was physical: it affected the physical world and the bodies of human beings. The sun demonstrably affects plant life on earth - vegetation is renewed when the sun begins to rise higher in the sky, and the warmth of the sun is an obvious factor in the generation of life. Equally, the moon can be shown to affect the tides. Naturally, the other planets were also thought to affect the weather, although perhaps more subtly than the larger bodies.
The influence of the planets was understood to be "elemental" and it was expressed in humans by the temperaments on which humoral medicine was posited. Persons whose physical qualities were cold and dry, for example (like the element earth), were said to be of a melancholic temperament, and they could expect to be subject to particular physical weaknesses (as leprosy and hemorrhoids), and to evince particular psychological characteristics (in this case, sadness, despair, suspicion, etc.). The psychological factors were a direct outgrowth of the physical makeup of the individual. Saturn is the planet which is cold and dry, and it is, reasonably enough, the planet which governed persons whose essential nature was melancholic. To describe the relationship causally, a person born when Saturn's influence was strong naturally was thought to take on the character of the planet in his humoral make-up, and to express those humors both through his physical appearance and his character.
In addition, the planets were thought to have an ongoing influence on the health of humans. Prudent men made use of astronomically-based tables which recommended the correct days or zodiacal position of the moon for bleeding or purgation. They also consulted charts that told which planets and signs influenced different parts of the body. Many of the manuscript Planet Books are bound with such tables, together with astrologically-based weather information.
There are two approaches by which a reasonably well-educated medieval person could have ascertained which planets an individual was influenced by. The first was astronomical: one found the exact time of the person's birth, worked out by means of an ephemeris and mathematical interpolation the position of each of the planets and a couple of major constellations, figured out which "mansion" each planet resided in, where the zodiacal signs fell, and what aspects the planets had to one another. One then calculated which planets were in strong positions, which in weak, etc. The planet which turned out to be in the strongest position could be considered to the "lord of the horoscope", and it was this planet which should have had the greatest influence over the person. (Note that I have here compressed into three sentences a procedure which took a skilled practitioner several hours. Astrology is a remarkably complex art.)
The alternative approach was related to the "rectification" of a horoscope. If an astrologer cast a natal horoscope and the results turned out to obviously contradict known facts (or just to be too terrible to explain to a paying customer), then he rectified the horoscope, tweaked the time or the date of birth (on the grounds that the information given must have been in error), until things worked out better. In the same way, if one was confronted by a person who was noticeably quarrelsome, one reasonably guessed that he was born under Mars. This is the popular, simplified, version of a complex art, and it is this sort of astrology which is directly expressed in the poems and illustrations of the Planet Books.
Both of these books are good introductions to medieval astrology. I had
the best luck using them together:
The commentary in this text includes a very cogent and comprehensive
introduction to humoral medical theory: