Just gravity and a bulge.

The stars aren’t quite where they look. Earth wobbles like a spinning top and swings around the Sun, and both effects nudge every star’s apparent position: a little over a year, a lot over thousands of years.

This is a from-scratch simulation of that drift. Give it Newtonian gravity and a spinning, slightly squashed Earth, and three classical effects fall out on their own: precession, nutation, and aberration. Nothing about them is written into the code. They emerge from the physics.