Kepler's law applies to all celestial bodies moving around the center of the universe. It has universal significance in the field of macro-low-speed celestial movement. For high-speed celestial motion, Kepler's law provides an equation for returning to a low-speed state.
In other words, Kepler's second law and its inferences apply not only to all planets orbiting the sun, but also to planets-centered satellites, and to the situation where a single planet or satellite moves in an elliptical orbit.
It is only suitable for celestial bodies moving at a macroscopic low speed. No strict proof was given when it was put forward, but it laid the foundation for the proof of many laws.