Caelum

What Caelum does to the sky above you Caelum replaces Minecraft's default skybox with a simulation built on real astronomical data. The mod swaps the vanilla star field for actual star positions and colors, lets the Moon move independently of the Sun so lunar phases occur naturally, and calculates the positions of the Sun, Earth, Moon, and planets using Keplerian orbital elements. The result is a sky that changes over the course of a Minecraft year: different stars rise and set with the seasons, and the whole celestial sphere tilts based on your latitude - your Z coordinate. Near the poles you get polar days and polar nights where the Sun never sets or never rises. Solar and lunar eclipses happen on the same dates they would in real life, within the tolerance of the simulation. Planet transits are also possible. You can adjust the size of the Sun and Moon in the config; vanilla makes them several times larger than reality, which produces more eclipses than would occur. ## Configuration you can use The mod exposes a long list of toggles. You can choose between vanilla stars, custom stars, or no stars at all. Custom stars can be added or replaced entirely through resource packs. Star color and star size are configurable independently of the vanilla fallback. You can change the length of the day-night cycle, the length of the lunar month, and the length of the Minecraft year (which controls which stars are visible when). Latitude effects can apply to the whole sky, stars only, or be disabled. You also set which Z coordinates count as the north and south poles. ## Loaders, versions, and where it runs Caelum runs on Forge and NeoForge. The supported Minecraft versions are 1.20.1 and 1.21.1 - no other versions are listed. The mod is client-side required. Server-side support is optional but recommended: if the mod is present on the server, all clients must also have it. With the mod only on the client, polar daylight still looks like daylight but the game logic treats it as night - mobs spawn, daylight sensors stay dark. ## Compatibility notes you should know Shader support is partial at best. The Sun and Moon positions and their shadows are correct for many shaders, but custom stars, planets, and other sky additions almost certainly won't render because shaders handle sky drawing themselves. There are no guarantees with other mods that touch sky rendering, OptiFine included. The simulation uses circular orbits for the Moon and Earth and assumes no axial tilt for Earth, so it's an approximation, not a perfect model.