Saturn Orbit Semi-Major Axis

Saturn marks the rim of the ancient sky: the most distant planet visible to the naked eye, and therefore, for every civilisation before Herschel, the end of the solar system. Its slow crawl through the constellations — the slowest motion of any classical planet — made it the timekeeper of astrology's longest cycles and gave English the word saturnine. At 9.58 au, sunlight is one per cent of Earth's, and a signal from Cassini took 68 to 84 minutes to come home.

The distance figures in one elegant piece of history: Christiaan Huygens, having worked out in 1659 that Saturn's "handles" were a detached ring, reasoned from the orbit's size and period about the system's proportions using exactly the Kepler relations this constant plugs into. Titan, which he had discovered four years earlier, gave him the planet's mass relative to the Sun by the same third law — celestial weighing, two centuries before anyone knew a mass in kilograms.