What causes precession of the equinoxes?

1 Answer
May 2, 2016


Precession of equinox is due to the precession of the Earth's polar axis about the normal to the ecliptic.


An equinox is the instant when the noon-Sun is right overhead, twice in a year by about 21st March (vernal equinox) and by about 23rd September (autumnal equinox). At this instant, the line of centers of the Earth and the Sun would pass through the location. .

As the poles move around the normal to the ecliptic (Earth's orbital plane) in respective circles, over a period of nearly 258 centuries called Great year, equinox location responds to this motion with the same angular speed about the center of the Earth, upon the equator,

From vernal equinox to autumnal equinox (six months), the shift in the equinox location is about 810 meters. From a vernal equinox to the next vernal equinox, the shift is about 1.62 km. Note that this shift is longitudinal = 50", nearly. .

I have calculated all these from the angular speed of precession# = 360/260=1.385^o#/century