Gravitation was an Age written by Atrus. It has never been depicted in media, but it did appear in a journal entry that was published on Cyan's website in the late 1990s.
Atrus spent many months calculating the necessary orbits and angles for this complex Age's cosmic display of light and shadow. Many moons interact with each other, causing them to skirt off at right angles, hang at the horizon, or rise and fall swiftly. Six suns, all of varying color and intensity, dance in the sky. Meteor showers light up the night. The beauty is, as Atrus admits, fleeting, for such a complex Age would not remain stable for very long at all, and the orbits of the moons and stars would quickly decay, causing them to smash into each other with incredible, unobservable brilliance.