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Precession, World Ages and the Great Year
by Simon at 16:22 31/12/08 (Blogs::Simon)
(For some reason, I had this saved as draft and never got around to publishing it - it was originally written at 16:50 on 02/01/2007. Anyway, here it is...)

It's all Johnny T's fault - mentioning Mithras has got me thinking about calendars again.

It's actually something that I've wondered about for many years - just whereabouts exactly is the original First Point of Aries? Or put another way, when did each of the World Ages (eg the Ages of Taurus/Aries/Pisces etc) start?

Some definitions...

0) Celestial Sphere - imagine the Earth is at the centre of a sphere and the stars are fixed to the inside of that sphere. The Sun, Moon and planets trace out their paths on this sphere as seen from Earth.

1) Ecliptic - the annual path of the Sun around the celestial sphere, as seen from Earth.

2) Celestial Equator - the projection onto the celestial sphere of the Earth's equator.

(1) and (2) are inclined to each other by about 23.5° which is the Earth's axial tilt.

3) Equinoxes - the points on the celestial sphere where the Ecliptic crosses the Celestial Equator. When the Sun is at either of these points, which happens in the northern hemisphere spring (Vernal Equinox) and 6 months later in the fall (Autumn Equinox), the day and night are of equal length and the Sun is overhead at noon when viewed from the Earth's equator.

4) North and South Celestial Poles - the points on the Celestial Sphere to which the Earth's axis is aligned - in the northern hemisphere the North Celestial Pole is very close by the 'Pole Star' in Ursa Minor (the Little Bear) but there's no particularly bright star near the equivalent point in the southern hemisphere.

5) Precession - the Earth's axis doesn't stay pointed in the same direction over time. The Earth is like a spinning top or coin that's wobbling. As a result, the positions of the North and South Celestial Poles change over time with respect to the background stars, taking 25920 years to go around in a circle just once.

As the Earth's axis is precessing around, the projection of Earth's equator on to the Celestial Sphere is also. This means that the intersection points between the Celestial Equator and the Ecliptic move along the Ecliptic path at a defined rate. The rate is 360° in 25920 years - or 1° every 72 years.

Once upon a time, the intersection points between the Ecliptic and the Celestial Equator were somewhere in the constellations of Aries (Vernal Equinox) and Libra (Autumn Equinox) - you can see why Libra is the "scales" or "balance".

As a result of precession these intersections have moved around the Ecliptic so that the background constellation for the Vernal point is now Pisces, with Virgo for the Autumnal one.

Vernal Equinox point 2007Vernal Equinox point 2007

Autumnal Equinox point 2007Autumnal Equinox point 2007

As far as I know, the earliest reference to the term "First Point of Aries" as the Vernal Equinox is from Claudius Ptolemy (c. 140AD) - which is interesting because at that time the Vernal and Autumnal Equinox points had already precessed around to the next constellations along - Pisces and Virgo respectively. Ptolemy himself realised this, and commented that the astrological influences that he was describing were to be ascribed to each successive 30° of the sky following wherever the Vernal point might be. At least it indicates that the term 'First Point of Aries' predates him.

So, on with the quest... is it possible to establish a useful starting point for any "World Age".

Ancient stories are peppered with references to key "precessional" numbers - 72 being a particular favourite so I think it's fair to assume that the phenomenon of the gradual movement of the Equinoxes along the Ecliptic had been noticed very early on in humanity's existence. There are also references to things like the wheel of the sky becoming unbalanced and also a number of interesting clues in religious texts.


GIF image (14 K) Vernal Equinox point 2007
GIF image (14 K) Autumnal Equinox point 2007
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