How did the precession period of 25920 come from? calculate

According to the difference of 20 minutes and 24 seconds between the tropic year and the sidereal year in orbit around the earth, the precession period should be 25782.8 years.

The time (sidereal year) of one orbit of the earth is 6: 9: 10 second on 365, so 20 minutes and 24 seconds is 50.266 arc seconds.

360 ÷ 50.266 arc seconds/year =25782.8 years.