# How is precession calculated?

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

The equator's lateral shift

= (Earth radius) X (change in radians for longitude)

= 0.0002435 radians * (6738 km) = 1.641 km

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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.

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When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

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