Callisto is a moon orbiting Jupiter, named after Callisto (a moon orbiting Jupiter), named by Michael Brown after the Xena: Warrior Princess character. With a mean orbit ten PI from its host planet, Callisto is the largest known spherical mirror known to exist. In fact, we don't know anything about Callisto except what it reflects back on us. Since its discovery, philosophers all over the world have been reflecting on the subject without much success.
The Eye of Callisto
The most prominent feature on the surface of Callisto is something whimsically called the Eye by astronomers. While fixed in place, it occasionally changes colour, and has been reported as both brown and blue, and even green by a minority of observers. Automatic cameras have so far failed to record it, but it is clearly present when human observers look.