Sidereal Orbital Period of Jupiter

Jupiter advances almost exactly one zodiac constellation per Earth year, completing the circuit in 11.86 — a coincidence with the number twelve that embedded itself in timekeeping. Chinese astronomy named it the Year Star and used its station among the twelve branches to count the years of the cycle that still names them; Indian astronomy's Brihaspati cycle runs on the same clock.

Dynamically the period matters for its near-misses rather than its matches. Five Jupiter orbits come close to two of Saturn's — the 5:2 "great inequality" whose 900-year modulation of both orbits baffled astronomers until Laplace explained it in 1785, closing the era's biggest open question about the solar system's stability. The same period sets the 399-day synodic rhythm of oppositions, when Jupiter rises at sunset as the brightest star-like object in the night sky, just as it did for the Babylonian scribes whose tablets record it.