Caelum

Positions of the major solar system bodies are calculated from their orbital elements for the given date, so eclipses and transits fall on the same dates they do in reality, within a tolerance. Each player's day length can also depend on latitude, which produces polar days and nights near the poles. The sky and celestial objects are rendered on the client.

The mod can show the Vanilla sky objects, a custom set of points of light or none at all, and resource packs can add or replace the custom set. Moons and planets can be added or edited. Color and size of the custom points apply only to them, and the speed of time can be changed so days and nights last longer or shorter. The Vanilla Sun and Moon appear far larger than in reality, which produces more eclipses; their size can be made realistic in the config.

Caelum is open source under the MIT License and is published for Forge and NeoForge, with files for 1.20.1 and 1.21.1. Shader support is partial: the Sun, Moon and their shadows should match for many shaders, but custom points of light and planets generally do not render with shaders that draw their own sky. Version 3.0.0.0 and later use Keplerian orbits, while earlier versions approximate the Moon with a circular orbit.