Julius Firmicus Maternus (280-360 A.D.) was a Sicilian lawyer from the upper nobility who retired from his career in order to devote himself to science and literature. He wrote Matheseos Libri VIII around 330 A.D. and dedicated to his friend, the consul Lollianus Mavortius. The material used for this book is mainly from Greek astrology sources.
The first chapter represents a letter written by the author to his friend Mavortius, Governor of the entire East. Mavortius was an intelligent and learned person who had lots of discussions with Julius Firmicus on many subjects, one being astrology. A big part of this letter is treating the conflict between astrologers and their opponents. To all the questions rose by those who deny astrology Julius Firmicus answers by showing that people can’t control anything in life, that Fate (represented by planets in his system) governs everything. He was a fatalistic astrologer who believed people can’t change their inherent destiny.
Here you can find a wide range of astrological subjects. The author writes about the signs, the exaltations and falls (he considers a planet is more fortunate in exaltation than in domicile), the Egyptian decans and terms, matutine and vespertine planets, duodecatemoria, houses, aspects, antiscia etc.
Matutine planets are those which rise before the Sun (oriental), vespertine are those which rise after the Sun (occidental). The duodecatemoria is a technique based on the degree a planet have in a sign to compute another position which will reveal the whole essence. But, there is a problem because the algorithm is erroneous. You can find the good one here.
The aspects between planets are analyzed like whole sign aspects, without orb, giving much importance to the direction from which are formed: a right aspect is made from the back of the sign (dexter), a left aspect is in front of the sign (sinister).
The first pages from Liber Tertius present Thema Mundi, the chart for the birthday of the Universe. This doctrine, writes Firmicus, comes by Mercury itself who transmitted to other intermediaries. Thema Mundi is a chart with all the planets in domicile at 15 degrees. I believe at least in this moment is much more mythology that science.
After that, you’ll find the explanations for the planets and luminaries in houses. I think the interpretations are quite delightful and a big part of them are matching with reality. The analyses always takes in account if the chart is diurnal or nocturnal, thus if the planets are in sect their effect is considerable better. Also, in many cases, the author introduces new aspects (very often the Moon), analyzes more than position in house, for example:
"Mercury located exactly on the ascendant in signs in which he rejoices, in a daytime chart, makes philosophers, teachers of the art of letters, or geometers […]or men skilled in sacred writings. Often he makes orators and lawyers, especially if in this house he is in his own sign or in other voiced signs. If either the Sun, Saturn or Jupiter are in aspect with Mercury in this house, he will make great men crowned with wreaths for being famous in sacred matters. […]But if Mars is in opposition or in square aspect or together with Mercury on the ascendant, the native is attacked by a variety of continual evils."