Mankind has a well-documented fascination with the heavens. The Sun, the moon, the planets and the stars, as well as other celestial objects like comets and events like eclipses, have all been a field of great interest. They have been used in all manner of divinations and believed to be able to foretell the future, for weal or woe. In your campaign world, celestial bodies can hold in truth all the fabled knowledge of yore, but there’s no reason to feel obligated to leave it at that.
There are a wide variety of possibilities for any of the features of the sky in a fantasy world. Let’s look at a few…
The Sun is often taken for granted, but it plays a huge role in the environment of your world. What if the sun is weak, or distant, and at best provides a twilight level of illumination for the world—what changes would that make to farming, wildlife, and society? What if two or more suns burn in the sky, and night is unknown—and what if suddenly one of the suns vanishes? Your world could traverse an odd path around its sun, one that leads to abnormally long seasons lasting years or even decades, or perhaps your campaign world is not heliocentric at all, but rather features the sun and all the other heavenly bodies rotating around the world. Playing around with the basic expectations of night and day can lead to a very distinct feel to your world.
The moon—or moons as the case may be—can affect a lot as well. In Dragonlance, for example, the phases of the three moons of Krynn affect the strength of magical spells. In the Ultima series of computer games, the phases of the two moons of Britannia were behind the secret of the moongates. In our own world the full moon is commonly associated with triggering the lycanthropic transformations of werewolves, as well as a general trend towards erratic behavior. Quite apart from that, in a fantasy setting the moon could be inhabited—perhaps the orbit is low enough for magical airships to traverse the distance between the world and the moon, leading to colonization and fantastic cities being erected. Alternately, a force of extraplanar invaders is using the moon as a beachhead for their invasion of your campaign world. Perhaps one of the moons is in fact a constructed citadel, home to a god or ancient power, watching over the world as guardian or tyrant (or both). What if the ancient citadel has been long abandoned, but contains magic artifacts of untold power waiting for someone to unravel the secrets of the lunar keep?
Other planets are possible too, though this depends on the assumptions of your setting. The default Points of Light setting for the current edition of D&D assumes a cosmology largely incompatible with a more traditional or realistic view of space, but there’s no reason your campaign world must be a part of the greater network of planes linked by the Astral Sea or other core assumptions. You might have other inhabited worlds in sailing distance of a flying ship, or you may decide that the world your player characters reside upon is the sole life-supporting world in the system, with others being hunks of lifeless rock. Still, such lifeless husks can be used to great effect in divinations, or in setting up some astronomical alignment—a time when great events or powerful magic may be possible due to the rare conjunction of all the planets for a short time.
Celestial events like comets or eclipses can be used in a similar fashion, of course. With enough preparation they can add a sense of urgency—“you must rescue the damsel before the eclipseâ€, perhaps, or “The demon-god will achieve apotheosis on the night the comet lights the skyâ€. Even if you as the DM maintain control and use the timing of the event as a narrative device, rather than a hard time limit on the adventure, it does contribute to the atmosphere and makes events feel more immediate and epic. By and large these kinds of events are more effective when planned ahead and the player characters are forewarned of the coming celestial circumstances, but an unexpected eclipse can lend tension and confusion to a scene as well.
The stars themselves are somewhat more distant, but can still be used for great effect. In our world, constellations are named for great heroes or deities, and the same can be true in your setting, allowing you an avenue to expand the myths and legends of your world. If your player characters manage to kill an evil deity, perhaps that deity’s constellation fades from the heavens, to be replaced by constellations representing one or more of your heroes. Other possibilities include the eldritch beings with whom certain warlocks form their pacts, or even strange invaders from other galaxies—though it feels like a science fiction premise, with spelljammers and other fantasy trappings it could be a novel approach to a campaign.
All in all there are as many possibilities as there are stars in the night sky. Spending a little bit of time thinking about what you could do with the heavens in your campaign can lead to a wide variety of new adventure sites, story hooks, or even full campaign plotlines.
