My French stinks, Langdon thought, but my zodiac iconography is pretty good. Taurus was always the bull. Astrology was a symbolic constant all over the world.
The Da Vinci Code
Astrology is well defined in dictionaries:
The study of the positions and motions of celestial bodies in the belief that they have an influence on the course of natural earthly occurrences and human affairs.
I don't understand why "Astrology was a symbolic constant all over the world". How should we understand it in this context?