How is precession calculated?

Question

How is precession  calculated?

Hunter Arthur · Accepted Answer

Precession can be calculated from the longitudinal shift or time interval between two consecutive equinoxes. At the equinox, the location will be on the line of centers of Earth and Sun.

The location will be on the line connecting the centers of the Earth and the Sun around March 21 (vernal equinox), when the Sun will be exactly overhead at noon. This occurs twice a year, around the vernal and autumnal equinoxes of March 21 and September 23, respectively, at one of the two locations where the ecliptic, or Earth's orbital plane, meets the equator. The annual variation in both longitude and time for this event allows us to approximate the precession period known as the Great Year = 258 centuries, roughly, by measuring the time it takes to complete one full rotation around the equator. The equinox moves in unison with the motion of either pole in relation to the Earth's center, forming a path that is not exactly a circle because of the polar axis' multi-period nodding. For this Great Year = 25800 years, my reverse calculations show that Annual change in equinox-longitude = #(2pi)/25800=0.0002435 radian=0.01395^o=50''#, nearly. The equator's lateral shift = (Earth radius) X (change in radians for longitude) = 0.0002435 radians * (6738 km) = 1.641 km

Hailey Alvarez · Answer

undefined Precession is calculated using the formula:

$ P = \frac{2\pi}{T} $

where:
- $ P $ is the precession rate in radians per unit time,
- $ \pi $ is the mathematical constant pi (approximately 3.14159),
- $ T $ is the period of the precessional motion in units of time.