Which of Saturn's major moons is the third-largest?

Iapetus is the third-largest of Saturn’s major moons.

Only Titan and Rhea are larger. Iapetus measures roughly 1,469 kilometers across and is the outermost of Saturn’s large moons, orbiting much farther away than the other major satellites. Its distant, inclined orbit makes it unusual among Saturn’s regular moons.

Giovanni Domenico Cassini discovered Iapetus in 1671. He noticed that it appeared much brighter on one side of Saturn than the other, eventually realizing that the moon has a bright trailing hemisphere and a dark leading hemisphere. Iapetus is tidally locked, so the same side always faces Saturn.

Its two-tone surface is not its only odd feature. A gigantic equatorial ridge extends about 1,300 kilometers and rises as high as 20 kilometers in places, giving Iapetus a distinctive walnut-like shape. It is also mostly icy, with a mean density only slightly greater than water. A common mix-up is confusing Iapetus with Rhea: Rhea is Saturn’s second-largest moon, while Iapetus ranks third.