In the heart of the endless sea of cogs, flywheels, and spindles that make up the Clockwork Nirvana of Mechanus, a relatively small cog ticks in perfect harmony with its surroundings. This cog would be completely unremarkable except for the spinning circular mechanism that sits at its center. The mechanism, called the Great Orrery, is a perfect model of the multiverse from the heart of the Material Plane to the most distant Outer Plane.
No one ever sees the Great Orrery move, but every time one looks away for even an instant - even for the blink of an eye - the mechanism has advanced a turn or the arrangement of individual cubes and spheres has been changed. No one knows who designed or built the Great Orrery, let alone why. Indeed, no one even understands why it functions the way it does. The internal gears are not in synch with the observed external movements, sometimes moving only in tiny increments, but other times completely rearranging themselves several times within a few minutes. It is said, though, that by studying the Orrery, you gain a glimpse into how the multiverse functions - at least at that moment in time.
The Great Orrery is a planar touchstone site, granting enhanced planar abilities to those who understand its secrets.