The edge of the visible sky, and the origin of the Saturn return.
Saturn returns to its natal position at roughly 29, 58 and 88 years of age — the best known cycle in astrology, and one that requires no birth time to compute, only a date.
Until 1781 it was the last planet. Every scheme older than that ends here, which is why Saturn carries the traditional weight of limit and boundary: it was, literally, the boundary.
It ruled Aquarius as well as Capricorn before Uranus, pairing two signs that share a modality but not an element.
What it is
The outermost planet visible to the unaided eye, and so the boundary of the classical cosmos.