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Western Astrology Articles by Julene Packer-Louis

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Celestial Mechanics
by Julene Packer-Louis

Mechanics of the Seasons

Look at your globe, or recall one in your mind. Notice the three major horizontal circles on the globe: the Equator in the center, the Tropic of Capricorn south of that, with the Tropic of Cancer to the north. Extend the Equator into space and create a great circle known as the Celestial Equator. Now imagine another circle that passes over the globe at roughly a 23 degree slant with the boundaries being the two Tropic circles. You've now visualized the Ecliptic Plane. Notice that it crosses the Celestial Equator in two places. These places are 00 Aries in the East and 00 Libra in the West. The solstices of 00 Cancer and 00 Capricorn occur at the sun's maximum declination where the Ecliptic meets the tropic of the same name. While the Sun is stationary and Earth is rotating around it, the Sun appears to travel through our sky and defines the ecliptic plane with its apparent path.

When the Sun reaches the intersection of the Celestial Equator & Ecliptic in the east, we experience the vernal equinox. We call that intersection 00 Aries and mark it the beginning of the tropical zodiac. The tropical zodiac corresponds to earth's seasons and therefore connects us to the rhythmic cycles of our planet Earth.

The Difference between the Tropical & Sidereal Zodiacs

The sidereal zodiac follows the fixed stars of the zodiac that populate the ecliptic plane. When you look up in the night sky and see planets in the zodiacal constellations, you are witnessing the sidereal zodiac and the planet's connection to the stars above. When you look in the ephemeris, quite often the planet position will be listed one sign behind what you seeing in the night sky. This is because the ephemeris notes the planet positions in the tropical zodiac. The difference in degrees between the sidereal & tropical zodiacs is called ayanamsha. Currently the ayanamsha is 23 degrees and 56 minutes. So planets in the night sky (sidereal zodiac) are roughly 24 degrees behind the planet position in the ephemeris (tropical zodiac).

Precession of the Equinoxes

When the Greeks developed horoscopy, the tropical and sidereal zodiacs aligned. It was in about 130 B.C. that Hipparchus discovered the equinox was moving backwards from the fixed star background. The vernal equinox was not occuring in the constellation Aries, but was slipping backwards into the constellation Pisces. This happens because of the procession of the equinoxes. Precession occurs because the earth's poles wobble like a top and over 26,000 years traces out a small circle in the heavens. This is what causes the pole stars to change. Currently our pole star is Polaris. In 1,000 BC it was Kochab of Ursa Minor. In 3,000 BC the pole star was Thuban of Draco. We will cycle through a few more pole stars before we make the 26,000 year return to Thuban: Alrai and then Alderamin of Cepheus, followed by Vega of Lyra.

The complete precession cycle of 26,000 years can be broken down into sub-cycles. It takes 2,166.6 years for the vernal equinox to move backwards through one sign of the zodiac. It takes 72.2 years for the vernal equinox to precess through one degree of a sign.

© Julene Packer-Louis 2006