Originally Posted by vive le beau jeu !
actually it does change a bit... you've got 2 variations:
- precession (rotation of the earth's rotation axis around another axis, perpendicular to the ecliptic plane, with a period of ~26000 years)
- nutation (short-term 'oscillations' of the earth's rotation axis, the largest component having a period of ~18.6 years)
But the Poster wanted to know why the tilt does not change. Precession does not change it relative to orbit (? almost flunked physics) and nutation, only minutely. To change the earth's tilt will require the same thing that created it:
When that happens, we will cease worrying about Nadal's knees, Federer's reflexes, Djokovic's grin, and the location of Murray's watch.