The Equinoxes

People living in ancient cultures paid a great deal of attention to the sun's motion in the sky; the sun was almost universally considered as the most important deity and was watched with care and interest. It became apparent that the sun executed repetitive motion corresponding to the changing seasons.

It was noted that as winter turned to summer, the sun climbed higher in the sky and as summer gave way to winter, the sun lowered. By keeping careful track of the annual motion, the dates of the highest and lowest points reached by the sun became known with some accuracy, and these times (the solstices) were traditionally marked with celebrations and sacrificial offerings.

Exactly halfway between the times of the highest and lowest sun, the sun stands at an equinox. Two equinoxes occur each year, one marking fall (autumnal equinox) and the other marking spring (vernal equinox). On about September 22, the sun crosses the equator from north to south. When the center of the sun is precisely on the imaginary dotted line marked by the celestial equator, equinox is occurring, this year it happened at 11:46 p.m. on September 22.

On the date of the equinox, day and night are everywhere of equal length. Furthermore, everywhere on earth, on vernal and autumnal equinox, the sun rises precisely in the east and sets precisely in the west. An observer standing at the equator on equinox would see the sun rising in the east and continuing on until it was exactly overhead. On every other day of the year, the sun would drift north or south of the zenith. An observer standing at the north pole, on the other hand, would see the sun circling 'round and 'round the sky slowly becoming lower until, on autumnal equinox, it would appear as a red ball rolling around the horizon. September 22nd would also mark the day of sunrise at the south pole (after six months of darkness). The fall and spring equinoxes are exactly reversed in the southern hemisphere.

The equinox date is so fundamental that it was kept track of with great care by the Babylonians, the Mayans, the Chinese, the Egyptians and by those who built Stonehenge in southern England around 1400 B.C.

The time of passage from equinox to corresponding equinox is called the tropical year; its length is 365.242199 days. It was known to about one minute accuracy by Hipparachus in the 2nd century, B.C. Hipparachus also discovered that the time it appears to take a star to move around the sun and return to the same position in the sky (the sidereal year) differs slightly from the tropical year, being longer by about 20 minutes. This is due to the fact that the earth wobbles as it rotates on its axis like a precessing top, requiring 26,000 years to complete one cycle. This causes the constellations to slowly drift forward in the sky.

Since the system of astrology was set up during the classical Greek period two millennia ago, and no corrections have been applied to bring the system up to date, those who follow astrology and have always believed that their sign is Aries are actually Pisceans.