Interesting posts all, JA, but what do they have to do with your OP? The Babylonians were siderealists, but they did not do what you would call horoscopic astrology. When they worked out nativities they sometimes recorded a time roughly, like "late at night," but nothing precise enough to work out an ascendant. They didn't use houses. (See Francesca Rochberg,
The Heavenly Writing on this.) Very late in their era, some "proper" horoscopes were recorded, by which time this was probably some reverse diffusion from the Hellenists. On other threads, I recall your insisting on the importance of the ascendant calculation in horoscopic astrology.
What is your source for astrology as a practice (vs. mythological star and god lore) diffusing from Babylon at such an early date? I can't think of any sources that place it earlier than the mid-4th century BCE.
The origins of Hellenistic astrology as distinct from the Babylonian foundations are unfortunately lost or have yet to be discovered, but by the time the textbook sources emerge in the first century CE, western tropical astrology was well established, as the sources I cited in my two previous posts indicate. The Hellenists still used fixed stars for different purposes, notably the rising dates of stars, asterisms, or constellations as an agricultural, maritime, and religious calendar, based on pre-astrological Greek and Egyptian custom. The rising "star" in a horoscope, be it planet or fixed star, seems to have been important to the course of a person's life in some sources.
Speaking of the Babylonians, their ecliptic was based on the path of the moon, "Sin" ("shin") not of the sun, which is understandable, given that constellations were detectable at night. It was the same as the sun's pathway for zodiacal purposes. They developed signs because 30-degree intervals in their sexagesimal (base 60) arithmetic system made it easier to predict eclipses.
Ancient societies used different starting dates for their calendar years. The Babylonian calendar did start in the spring
https://en.wikipedia.org/wiki/Babylonian_calendar, which is probably why their month of Aries became the first sign. The Egyptian New Year was in July, probably in relation to the rising of Sirius; the Jewish calendar started in autumn (with Rosh Hashanah prayers for autumn rains,) various ancient Greek calendars started the new year with the first new moon after the summer solstice.
https://en.wikipedia.org/wiki/Attic_calendar
From Aries it was then a question of determining the correct degree of the spring equinox, based on the length of equal days and nights.
