|Moons of Mirrodin|
The metallic world of Mirrodin has five moons that also serve as the worlds suns. Each is a different color, corresponding to the colors of mana - white, blue, black, red, and green. Mirrodin's suns are not stars. They are enormous orbs of mana, not balls of space plasma. As celestial objects go, they obey rules of magic rather than the physics we understand.
For example, the mana suns don't exert gravitation. They don't cause tides in the Quicksilver Sea, and if they alter each other's orbits at all, it's not because of gravity. There may be some kind of attractive or relational force that binds the suns to Mirrodin, but that's a matter for the thaumaturgical theorists.
Mirrodin's suns do have orbits, however, or at least they have motion behavior that you could compare to orbiting. They move through Mirrodin's sky like our sun does through ours, or like the suns might relative to a planet in a multiple star system. However, their orbits are irregular, so it’s possible for any number of the suns, from zero to all five, to be overhead at any given place or time on the world.
Their motions through the sky are the subject of great interest to Mirrans. The goblins, for example, attribute great mystical import to the red sun, which they call the Sky Tyrant, and celebrate with wild festivals when the Sky Tyrant is directly above Kuldotha, the Great Furnace. The new green sun (the fifth mana orb that is behind the Fifth Dawn set) has brought unprecedented amounts of light streaming into the dense, copper-treed Tangle forest. The elves refer to it as the True Sun, as it's the only one that passes directly over certain parts of the Tangle.
The suns probably do not have strictly elliptical "orbits." From any given vantage point, they appear to rise, and eventually they set. But their movements across the sky and their positions relative to one another differ day by day. There may be a model that makes sense of it, but if there is, it probably involves a significant amount of mystical weirdness, and isn't buildable with some wires and rubber balls.
We haven't ever talked about whether the suns can collide, but they haven't yet, so—probably not? I'm not sure anyone knows what two huge mana-suns would even do, even if they did come close to each other. Harmlessly phase through each other like holograms? Mingle and disrupt, and start shining hybrid mana? Maybe a certain silver golem knows.
Is it ever night on Mirrodin? In some spots, and for short periods, I believe the answer is yes. There are certainly areas of shade, such as the deepest valleys of the Oxidda Chain and the densest groves of the Tangle. But there are probably some rare spots and rare times at which, from that location, all five suns appear to be hidden, and true night falls. The vast majority of the time, though, one sun or another is visible in the sky, and often more than one. By and large, shade is night on Mirrodin