How does the precession of the earth's axis shorten the tropic year?

If you put a top on the smooth bottom of the bowl and let it rotate quickly, when the top rotates around its own axis, its own axis is still rotating around another axis. This movement is called precession.

Like a top, the earth keeps spinning around its own axis. Under certain stress conditions, the rotation axis of the earth will also precess. Because of the precession of the earth's rotation axis, the direction of the earth's rotation axis and the swing direction of the equatorial plane perpendicular to the rotation axis are constantly changing, so the intersection of the celestial equator and the ecliptic on the celestial sphere is also constantly changing. This phenomenon is called the westward shift of the bisector. This westward movement is very slow, and it takes about 25,800 years to move around the ecliptic. Due to the westward movement of vernal equinox, there is a certain time difference between the time when the sun orbits the ecliptic relative to the starry sky (sidereal year) and the time when the sun orbits vernal equinox (tropic year, that is, the time when the earth orbits the sun once). The annual return will be shortened, and the solar terms in the southern hemisphere will also move westward. This is called precession.

I hope it can help you solve the problem.