陈按：这篇文章主要是阐述恒星黄道制与回归黄道制的区别，大家可以参考学习。

当中还有很多插图待补上，大家可以不断关注我博客更新。

by Glenn R. Smith

There are two zodiacs in use today. One is the TROPICAL zodiac used by the western astrologers and the second is the SIDEREAL zodiac used by the Hindu astrologers. At one time both zodiacs were identical but over the centuries they have drifted apart. This drifting phenomenon is called the Precession of the Equinoxes and is caused by the two primary motions of the Earth. The first primary motion of the Earth is its orbit around the Sun. All planets circle around the Sun in the same direction keeping the Sun on the left. If we were to look down upon our solar system from millions of miles above the Sun's north pole, we would see all planets orbiting in a counterclockwise direction. Because all planets share this similarity without exception it can therefore be said that the first primary motion of the Earth is its orbit around the Sun. Further, all planets appear to travel within a narrow plane around the Sun. This plane of orbit is called the ECLIPTIC. Only distant Pluto deviates from this plane by an appreciative amount.
 
The secondary motion of the Earth is its rotation upon its axis. Because the Earth's rotational axis is tilted in respect to its orbit, a second plane is projected into space. This second plane is called the CELESTIAL EQUATOR. Since the CELESTIAL EQUATOR is unique to earth only and is not shared by other planets, the rotation of the Earth around its axis is considered a secondary motion. These two planes derived from the two primary motions are angular to one another and the difference of the angles creates the four seasons on Earth. The places where these two planes intersect are called the equinoxes.
 

When the Sun, as it appears to travel on the ECLIPTIC, crosses the CELESTIAL EQUATOR in a northward direction it is called the Vernal Equinox, the beginning of Spring. When the Sun reaches the northern most point from the CELESTIAL EQUATOR it is called the Summer Solstice, the beginning of Summer. From the Summer solstice the Sun appears to turn southward. The word tropic is from the Greek word tropos which means turning. When the Sun crosses the CELESTIAL EQUATOR in a southerly direction it is called the Autumnal Equinox, the beginning of Autumn. And when the Sun reaches the southern most point from the CELESTIAL EQUATOR it is called the Winter Solstice, the beginning of Winter. And then the Sun appears to turn northward again. The TROPICAL ZODIAC is derived from these apparent turning points of the Sun which are based solely upon the secondary motion of the Earth (Spinning on a tilted axis).
 

The SIDEREAL ZODIAC, on the other hand, is based solely upon the first primary motion of the Earth, that of its orbit around the Sun. The difference between the two zodiacs might appear at first to be like splitting hairs over trivial details were it not for one important thing -- stability. Over the course of thousands of years the stars will not appear any different in relation to the plane of the ECLIPTIC. But even after a few hundred years the stars will shift in relation to the plane of the CELESTIAL EQUATOR. Indeed, the names of the twelve signs of the zodiac are derived from star groupings that are no longer relevant to the CELESTIAL EQUATOR today. The Vernal Equinox which used to occur in the constellation of Aries now occurs in the constellation of Pisces. Yet the Tropical astrologers of today still talk of the commencement of the Vernal Equinox to be the beginning of the Sun's entrance into the sign of Aries.
 

The two equinoctial points of the topical ZODIAC slowly shift in a clockwise direction in relation to the stars thereby creating a distinction with the SIDEREAL ZODIAC. This clockwise shift is called the Precession of the Equinoxes and is due to the Earth third motion which resembles the revolving oscillation of a spinning top. This motion is so slow, however, that it does not project a third plane into space. But it does affect the plane of the CELESTIAL EQUATOR.

If you have watched a spinning top you notice that its axis slowly revolves around the pole of gravity. In the same way, the axis of the Earth's rotation slowly revolves around the Pole of the ECLIPTIC.
  

In a way you could say the pole of the ECLIPTIC represent the shared gravitational center of our solar system. One complete revolution of the Earth's axis around the Pole of the ECLIPTIC takes about 25,714 years. The reason for this precession phenomenon is believed to be due to the fact that the Earth is not a perfect sphere. The Sun and Moon's gravitational effect upon the protuberant equator of the Earth causes the Earth's axis to revolve slowly in relation to the Pole of the Ecliptic.
  

The Earth's first primary motion in orbit around the Sun on the ECLIPTIC takes 365 days, 6 hours, 9 minutes, and 9.5 seconds in relation to any specific star. The Earth's secondary motion creates the plane of the CELESTIAL EQUATOR. The Sun's apparent movement around the ECLIPTIC takes 365 days, 5 hours, 48 minutes, and 46 seconds in relation to the equinox points. This makes a Tropical year shorter than a Sidereal year by about 20 minutes, 23.5 seconds. It has been stated by western scholars that the first person to make notice of the Precession of the Equinoxes was a Greek named Hiparchus. But we have shown, however, in our article on the Six Thousand Year Barrier that the Hindus preceded him by a few thousand years. The Hindu term for precession is AYANAMSHA. Ayana is the Sanskrit word for falling back and amsha is the word for portion. Ayanamsha therefore refers to the portion of measurement between the Tropical and Sidereal zodiacs. One of the criticisms of the Tropical astrologers against the Siderealists is that the measured portion or value of the Ayanamsha appears to vary according to which authority is followed. The Tropical Zodiac is appealing because there is only one value derived from direct observation. Another criticism is that the constellations are placed at irregular intervals in the sky and not in neat thirty degree portions. If the zodiac is to be divided up into equal portions of thirty degrees, they argue, why not do away with the stars altogether and just follow the vernal equinox as the start of the zodiac?

There are two ways of arriving at a figure for the Ayanamsha. One direct and the other indirect. The indirect method involves a formula using an epoch date and an average yearly rate of precession. The epoch date is when the commencement points of the two zodiacs were identical. The average yearly rate is the distance in degrees that the equinoxes shift westward (clockwise) each year. The faults of this method are that the rate of precession changes over the centuries and the exact epoch date is in debate. This is probably why there are so many various Ayanamsha values in use today. The direct method is much easier and more accurate being based upon direct evidence of astronomical data. The direct method uses an observable fixed star and the value of the current position of the Autumnal Equinox.

The ancient Indian astronomical text, Surya-Siddhanta lists a brilliant star CHITRA (Spica, alpha-virginis) as exactly on the 180 degree mark of the Sidereal zodiac. Chitra is therefore located at the exact opposite of the zero-point of the Sidereal zodiac. This is important since the Surya-Siddhanta gives no star for the exact zero degree mark. The star Chitra becomes very significant when you understand that the ancient Hindus used shadow sticks and reflecting pools to measure the position of stars and planets. In order to observe solar movements especially, one would need reference points in the starry sky opposite the Sun for the obvious fact that no reference stars are visible during daylight. Whenever Chitra is on the meridian of observation at local mean midnight it is known that the Sun is in the first degree of Sidereal Aries. Therefore the measurement of the Ayanamsha simply entails measuring the distance between the ECLIPTIC position of Chitra and the point of the Autumnal Equinox. For the year 1950 AD Chitra (Spica, alpha-virginis) was listed as 203d 09m l2s Ecliptic longitude. This is the same as 23d 09m l2s east of the Autumnal Equinox. Thus the Ayanamsha for 1950 AD is 23d 09m l2s.

It is true that the twelve zodiacal constellations are irregularly spaced and not in neatly ordered thirty degree sections. However, if Chitra is selected as the reference star in which the zodiac is measured out one will find the majority of the main stars in each constellation falls within the thirty-degree equal division. To illustrate this point we give below a tabulation of percentages in which the main stars of each sign fall within the respective equal thirty degree divisions of the Sidereal and Tropical measurements. The number given for the main stars are simply those necessary to draw the basic outline of the sign as it has been recognized for thousands of years. Although in modern times different methods are used to connect the dots of the stars in the signs , the standard I have used is the one adopted by Donald H. Menzel in his edition of A Field Guide to the Stars and Planets, 1964 edition. In the tables below the first column gives the name of the zodiacal sign. The second column shows the number of the major stars that would make up the constellation. The third column shows the number of main stars that fall OUTSIDE the particular thirty degree equal division. The forth column has the percentage of total main stars included in the particular thirty-degree division. The last column has the total main stars excluded from the division. Keep in mind that we are NOT basing these tables on the modern constellation boundaries found in many star atlases today. Those mark off square boundaries around the entire celestial sphere. Here we are only concerned with the traditional stick-man figures used for thousands of years. Today they are known as the Rey's constellational standard.
 

SIDEREAL ZODIAC

IT should be noted that there is no better way to divide the zodiac into twelve equal parts of thirty degree and get a higher percentage of main stars from the twelve constellations to fall within them than the method of using Chitra as the main reckoning star of measurement. Also this percentage is constant and does not increase nor decrease in time. In comparison we show how many main stars fall into the Tropical division of the Zodiac for the epoch year given as 1950 AD.
 

TROPICAL ZODIAC

As you can see only 19 percent of the main stars still fall within the Tropical zodiac twelve part equal division of thirty degrees. What's more, this percentage is getting less as time goes on. In about four hundred and twenty eight years from now, the precession of the equinox will have drifted the Tropical zodiac completely out of range of these main stars.

Of course one can say this comparison is unfair since Tropical astrologers regard the vernal equinox as the beginning point of their system and make no attempt at trying to reconcile the drifting of the equinoxes to the stars. But the point being made is that to reject the Sidereal zodiac simply because the constellations are irregular and do not fit into the equally measured thirty degree sections in a one hundred percent way and then adopt a zodiac measurement aloof from stellar considerations is a little like throwing out the baby with the bath-water. Astrology originally meant "knowledge of the stars".
 

Here we see a circle inscribed in the heavens by the oscilating motion of the Earth's North Pole. Near the top we see the current North Star Polaris. Also shown is the point where the North Star was a thousand years ago and 6,500 years ago. The last North Star before Polaris was alpha-draconis or Thuban at around 3,000 BC. We can also see where the North Star will be in the year 8,419 AD and 14,860 AD.

In summary I would like to point out that I am not trying to discredit or criticize the use of the Tropical Zodiac. Its use has clear benefits for calendrics and Earth Cycles. My emphasis, however, is that the Sidereal Zodiac can keep us from becoming too provincial in our thinking. Although respect for the Earth and its cycles is central to Earth based religions, we must not forget that we are also members of a larger universe. The powers derived from Earth energies are interconnected with the universal energies or star power. Recognition and use of the Sidereal Zodiac can lead us to the ultimate in holistic thinking.

                                          Text and images Copyright ?1999 Glenn R. Smith