Just gravity and a bulge.

A star’s apparent position drifts over time, and most of that drift isn’t the star at all. It’s us: Earth moving through space, and its spin axis slowly turning. That drift is really three effects layered together.

One is a trick of the light. Earth races around the Sun at 30 km/s, so starlight reaches us at a slight slant, and the star appears nudged toward our direction of travel. That is aberration.

Another is our frame of reference turning. Earth’s spin axis swings in a slow, steady circle around the ecliptic pole, one lap every 26,000 years, dragging the coordinate grid with it. That is precession.

The third rides on top: smaller, periodic nods of that same axis, chiefly an 18.6-year wobble driven by the Moon. That is nutation.

The simulator builds all three from first principles: give the Sun, Moon, and a spinning, slightly squashed Earth nothing but Newtonian gravity, and the effects emerge on their own, none of it coded in. The star is left idealized, infinitely far and perfectly still, so parallax and proper motion stay out, both negligible for all but the nearest stars. Read the technical details →