Planetary Periods in Western Astrology
In Western Astrology from the Greek period forward through the Arabic period and the Latin astrology of the Middle Ages we find frequent references to planetary periods. In another article on this webpage. I have given an extensive discussion on alfridaries or firdar a system of periods apparently derived from the Persians. In this discussion I concentrate solely on the system of small, mean, and great periods.
A table of these periods as given by various sources follows:
The small periods are derived from recurrence cycles, cycles at the end of which the planets conjoin the Sun in approximately the same degree of the zodiac. These are derived as follows:
The Sun's value of 19 years is actually the metonic cycle of 19.0 years at the end of which an eclipse occurs in the same degree as at the beginning of the period. Here 254 lunar (tropical or sidereal) months equals 19 years (tropical or sidereal).
The Moon's value is derived from a relationship between the Egyptian year of exactly 365 days and the lunation cycle. In 25 Egyptian years there are exactly 309 lunation cycles. In fact dividing the number of days in 25 Egyptian years, 9125, by 309 one gets 29.5307 days which squares superbly with the modern value of 29.5306 days in an average lunation cycle. Or to put it another way, 309 lunations = 24.9999 Egyptian years. This gives an error of 0.04 days in 25 years. From the practical point of view, using the Egyptian calendar, every 25 years the phases of the moon would occur on the same days of the year. Note that both the Sun and the Moon derive their cycles from their relationship to each other.
Mercury's period is also derived from the relationship of a synodic cycle to the Egyptian year. 63 Mercury-Sun synods = 20.0005 Egyptian years. Again, as with the Moon's period, every 20 years conjunctions with Mercury would occur on roughly the same date. This produces an error of 0.18 days in 20 years.
Venus's period of 8 years is its recurrence cycle with the Sun. Using Egyptian years one gets 5 Venus-Sun synods = 7.99889 Egyptian years. Every 8 years Venus conjunctions would occur on the same dates. This cycle is also very close to 99 lunar months which equal 8.009 Egyptian years. This is an error of 0.41 days in 8 years.
Mars' period of 15 years is its recurrence cycle with the Sun. Seven Mars-Sun synods = 14.9577 Egyptian years. This is not as precise as the previous examples but after all Mars is slower. This is an error of 15.44 days in 15 years.
Jupiter's period of 12 years is its recurrence cycle with the Sun. 11 Jupiter synods = 12.0212 Egyptian years, an error of 7.727 days in 12 years.
Saturn's period of 30 years is its recurrence cycle with the Sun. 29 Saturn synods = 30.0402, an error of 14.67 days in 30 years.
Obviously with Saturn, Jupiter, and Mars the recurrence is with the signs rather than the degree. But the Sun, Moon, Mercury and Venus are very close to the degree.
The great periods are not derived from actual astronomical cycles, but are the sums of the degrees allocated to each planet in both the system of terms according to the Egyptians and that of Ptolemy, at least for the planets. The Sun's and Moon's Maximum periods have a different origin. Among other things the period for the Sun is the period of life in the Hindu Vimshottari Dasas while the Moon's period is the same as the period of life in the Ashottari Dasas. However, the actual origin of these periods seems to be derived from the belief that the greatest possible semi-arc of the Sun was 120 within the bounds of the civilized world. Since such a semi-arc corresponds to a latitude of about 49 this is a reasonable value for the civilizations of that time. The Moon's great period is derived from this. It was reckoned that the Moon had to be at least 12 from the Sun to be visible at sunset. The Sun's semi-arc minus that 12 gives us 108, the great period of the Moon.
At some point in the future I will present additional material as to the use of these periods and their possible importance for modern Astrology.