Saturn

Saturn is the second-largest planet in the solar system and the outermost of the five classical planets known since antiquity. It is a gas giant with a composition and internal structure broadly comparable to Jupiter, but it has the lowest mean density of any planet. Its defining visual feature is the ring system, which lies in the plane of Saturn's equator.

Through a telescope Saturn is one of the most striking objects in the sky. The ring plane is tilted relative to Saturn's orbit around the Sun, so the rings open and close from our point of view during a Saturnian year. When the rings are widely open, by roughly 26°, they can be seen around the planet and Saturn's shadow is easy to follow across them.

Near a ring-plane crossing the rings can nearly vanish. They may be unlit when the ring plane passes through the Sun, seen from their dark side when Earth and Sun are on opposite sides of the rings, or viewed edge-on from Earth so that the thin ring sheet reflects very little light. These intervals are especially interesting for telescopic observers because Saturn's appearance can change noticeably within days.

Saturn's fast rotation also produces strong flattening: its polar diameter is about ten percent smaller than its equatorial diameter. When the rings are nearly edge-on, Saturn appears about one magnitude dimmer than when the rings are well presented. Ring-plane crossing seasons recur roughly every 15 years; notable recent examples include 1995/1996, 2009/2010, and 2025.