The Dragon Above encompasses the heavens and all they contain. Flashes of color, streaks of light, and strange blossoms in the dark of night excite the imagination of the world’s inhabitants. Crossing the dome of the world is the Ring of Siberys, a wide swath of golden dust stretching from one horizon to the next. During the day, the Ring is faint; at night, it is a brilliant belt of scintillating specks weaving through the heavens.
Twelve moons hang like jewels in the night sky, some huge, others small and distant, and a few mere pinpricks of light, scarcely able to be seen with the naked eye. Each moon moves through its phases, waxing and waning in its own time. Farther out lie the stars, silent guardians of the greatest mysteries. Dragons and mortals alike gather these distant lights into constellations, making of them gods, omens, and portents of events to come.
Source: Eberron Campaign Guide
In the myth of the Progenitors, Siberys, Eberron, and Khyber worked together to create the thirteen planes. Following this effort, they rested in the emptiness that lies at the center of the planes—and there, they quarreled. Khyber killed Siberys and tore him apart, then Eberron enfolded Khyber and became a living prison, forming the world itself.
This could be literal truth. Perhaps the ring in the sky was once the body of a great cosmic wyrm. But even if it’s not literally true, it reflects truths about the world. From the surface, Eberron looks much like Earth; there is soil below your feet and moons and stars above. But the fact is that Eberron is not like Earth, and much of what can be seen isn’t what it appears to be. As far as anyone has determined, Eberron is the sole planet in its Material Plane, and the fulcrum where the thirteen planes come together. Go below the surface of Eberron and you find not a molten core, but the demiplanes of Khyber. So what are the celestial bodies of Eberron?
The Ring of Siberys. This brilliant equatorial band of light dominates the sky. Siberys dragonshards fall from the Ring of Siberys, and its golden color suggests that the entire ring may be comprised of these magical crystals. The Siberyan Theory, as taught at Arcanix and the Arcane Congress, postulates that all arcane magic manipulates energy that radiates from the Ring—and magic itself is the Blood of Siberys.
The Sun. In the Progenitor myth, the three rested in the Material Plane after creating the planes. They created the sun, Arrah, much as mortals might kindle a campfire. This fire remained even after their battle, and continues to provide light, heat, and comfort to the world, functioning much like the sun we’re used to in our own world. In the Sovereign Host, Dol Arrah is the Sovereign of Sun and Sacrifice; her name is, essentially, Warrior Sun.
The Stars. There are stars in the sky of Eberron, but they aren’t the anchors of distant solar systems. There are limits to the Material Plane, and the stars mark those limits; think of them as glittering points in a crystal sphere. The common constellations are figures of ancient dragons—Io, Tiamat, Chronepsis—though most people couldn’t actually say where these names come from. It’s generally assumed that they were handed down by one of the ancient kingdoms of Sarlona, or established by the ancestors of the Aereni—but in fact, the names are from a tradition spread by the sages of Argonnessen.
The Moons. Twelve orbiting moons are visible from Eberron— the former moon, Crya, hasn’t been seen since ancient times, when tales say the empire of the giants destroyed it. Each moon goes through standard lunar phases, but during the month that shares its name, the moon enters an ascendant phase; during this time, the moon is brighter than usual. Each moon is associated with certain personality traits, and it’s believed that people are influenced by the moon that’s ascendant at the time of their birth. But there’s a further complication, because the moons are also tied to the planes, as indicated on the Planar Destinations and Moons table. Each moon enters an additional ascendant phase whenever its associated plane is coterminous, and becomes unusually dim when the plane is remote. So while unusual, it’s possible for there to be two or three ascendant moons at a time if multiple coterminous periods converge.
The connection between the planes and moons is reinforced by the fact that while on a plane, its associated moon is the only one that can be seen in the sky (though not all planar layers have a visible sky, like the dismal caverns of Dolurrh). While on a moon’s associated plane, the phase of the moon doesn’t necessarily match its current phase on Eberron. It may be fixed in a single phase—such as in Lamannia, where the moon is always full—or its phase might even change from layer to layer. While on a plane, if the moon is visible, it can be identified with a successful DC 10 Wisdom (Nature) check.
To date, no humanoids have visited one of the moons. Because of this, their true nature remains a mystery. It’s possible that they are orbiting, habitable planetoids, home to exotic creatures and valuable resources. But some sages believe that they aren’t actually physical bodies at all, but rather, planar gateways—if an airship were to somehow fly high enough and attempt to land on a moon, perhaps it would instead pass into the sky of the associated plane. Determining the truth would be an epic adventure!
Source: Exploring Eberron