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. 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.]
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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 17991825 AD.] |
©
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...
© Dr Shepherd Simpson, Astrological
Historian |
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Historical Astrology
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