|
Edited on Thu Jan-27-11 06:06 PM by Ricochet21
Calculation aspects
Precessional movement as seen from 'outside' the celestial sphere. The rotation axis of the Earth describes over a period of about 25800 years a small circle (blue) among the stars, centred around the ecliptic northpole (blue E) and with an angular radius of about 23.4°: the angle known as the obliquity of the ecliptic. The orange axis was the Earth's rotation axis 5000 years ago when it pointed to the star Thuban. The yellow axis, pointing to Polaris is the situation now. Note that when the celestial sphere is seen from outside constellations appear in mirror image. Also note that the daily rotation of the Earth around its axis is opposite to the precessional rotation. When the polar axis precesses from one direction to another, then the equatorial plane of the Earth (indicated with the circular grid around the equator) and the associated celestial equator will move too. Where the celestial equator intersects the ecliptic (red line) there are the equinoxes. As seen from the drawing, the orange grid, 5000 years ago one intersection of equator and ecliptic, the vernal equinox was close to the star Aldebaran of Taurus. By now (the yellow grid) it has shifted (red arrow) to somewhere in the constellation of Pisces. Note that this is an astronomical description of the precessional movement and the vernal equinox position in a given constellation may not imply the astrological meaning of an Age carrying the same name, as they (ages and constellations) only have an exact alignment in the "first point of Aries", meaning once in each ca. 25800 (Great Sidereal Year).The Earth, in addition to its diurnal (daily) rotation upon its axis and annual rotation around the Sun, incurs a precessional motion involving a slow periodic shift of the axis itself: approximately one degree every 72 years. This motion, which is caused mostly by the Moon's gravity, gives rise to the precession of the equinoxes in which the Sun's position on the ecliptic at the time of the vernal equinox, measured against the background of fixed stars, gradually changes with time.
In graphical terms, the Earth behaves like a spinning top, and tops tend to wobble as they spin. The spin of the Earth is its daily (diurnal) rotation. The spinning Earth slowly wobbles over a period slightly less than 26,000 years. From our perspective on Earth, the stars are ever so slightly `moving' from west to east at the rate of one degree approximately every 72 years. One degree is about twice the diameter of the Sun or Moon as viewed from Earth. The easiest way to notice this slow movement of the stars is at any fixed time each year. The most common fixed time is at the vernal equinox around 21 March each year.
In astrology, an astrological age has usually been defined by the constellation or superimposed sidereal zodiac in which the Sun actually appears at the vernal equinox. This is the method that Hipparchus appears to have applied around 127 BCE when he calculated precession. Since each sign of the zodiac is composed of 30 degrees, each astrological age might be thought to last about 72 (years) × 30 (degrees) = about 2160 years. This means the Sun crosses the equator at the vernal equinox moving backwards against the fixed stars from one year to the next at the rate of one degree in seventy-two years, one constellation (on average) in about 2160 years, and the whole twelve signs in about 25,920 years, sometimes called a Platonic Year. However the length of the ages are decreasing with time as the rate of precession is increasing. Therefore no two ages are of equal length.
Now. You got it?
|
Printer-friendly format
Email this thread to a friend
Bookmark this thread
(1000+ posts)
Send PM |
Profile |
Ignore
