Precession of the Earth



Definition: [Astrological Ages] Precession means literally, "the act of preceding." The more specific meaning in astrology is "the motion of a spinning object - in which the body wobbles so that the axis of rotation sweeps out a cone." An everyday example of this is a spinning top, it spins but also the axis around which it spins goes around in a larger circle. The expression first appears in Ptolemy's [c 130 - 170 AD] Al Magest and is from the Latin, praecesse, which means to overtake or to outpass.

Precession of the Earth: In the astrological Movement of the Ages the object which is spinning around on its axis is the Earth, and the axis that it spins about is the imaginary line which runs between the Earth's True North and True South Poles: the Earth's axis. It is the Earth's axis which precesses. Viewed over a long period of time - and ignoring the Earth's orbit around the Sun - the Earth would look like a spinning top.

The time taken for the Earth's axis to sweep out a complete cone, as viewed from above - one complete wobble of the Earth's axis if you like - is approximately 25 925 years [2002 AD], otherwise called a Great Year, or a "Platonic Year".

The currently accepted value for the luni-solar rate of Precession is the IAU76 value, corrected by data from NASA's Lunar Laser Ranging system [J Chapront et al, 2002 AD]. The IUA76 value is 50.290966"/cy and the Chapront correction is -0.302"/cy [arc seconds per century]. This gives a Precessional Constant of 49.989 "/cy and a Platonic Year of 25 925 years.

precession of the earth


Precession of the Earth. Each wobble of the axis would in reality take nearly 26 000 years. One consequence of the wobble is that the Earth's axis points to different places in the heavens during the course of its wobble - the arrow above points to a different place - so with time the Pole Star that we see changes. See the Changing Pole Star for more information. [Not shown above is either the Earth spinning about it's own axis, nor the Earth orbiting around the Sun.]

Why does the Earth Precess?

There are three reasons for this:

Reason number 1. The Earth's axis is tilted at 23. degrees to the plane of the ecliptic. If the Earth and Moon were not tilted at all relative to the plane of the Ecliptic then there would be no precession. The Earth's axis would forever be at right angles to the Ecliptic.

Reason number 2. The Earth has 'a bulge.' It's a bit larger [43 km] in diameter at the Equator than it is between the poles. If the Earth had no spin, gravity would act the make the Earth a perfect sphere, but the Earth's spin throws its mass slightly outwards at the Equator.

Reason number 3. The gravity of the Sun and Moon then act on this bulge and their action is to try to 'drag' this bulge down on to the plane of the Ecliptic. [The same gravity action of the Sun and Moon is responsible for the tides.] The Earth's Equator doesn't lie on the Ecliptic, it lies at 23° 26' degrees to it [2004 AD]. It is the act of gravity trying to drag the bulge onto the plane of the Ecliptic that makes the Earth's axis precess. This last bit is pretty hard to visualise. [And was discovered relatively recently by Pierre Laplace, Traité de Méchanique Céleste 1799–1825 AD.]

the tidal gravitational drag of the moon and sun on the earth causing the earth's axis to wobble


The Moon and the Sun dragging on the Earth. This is an attempt to show the dragging action of the Sun and the Moon on the Earth. This drag of the Earth's equatorial bulge towards the Ecliptic does not succeed. Instead it makes the Earth's axis wobble around in a circle.

What Consequences does the Precession of the Earth Have? Along with a small effect due to Nutation, the precession of the Earth causes the Movement of the Vernal Equinox, the Precession of the Equinoxes, the Changing Pole Star and the Movement of the Ages. It is the cause of Astrological Ages.

Key Concepts...

2:   Key Concepts... 
2a:  Precession of the Earth 
2b:   Nutation of the Earth 
2c:  Movement of the Vernal Equinox Point 
2d:  Precession of the Equinoxes
2e:  The Changing Pole Star  

© Dr Shepherd Simpson, Astrological Historian


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