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Stars and Signs
the Constellations vs the Sidereal and the Tropical Zodiac
Debbie has written in with a question that is fundamental to the understanding of astrology. Kevin Burk, leading astrologer and relationship counsellor, presents this excellent article in response, on the difference between the constellations and the signs, the sidereal versus the tropical zodiacs and the meaning of the precession of the equinoxes.
Kevin Answers: Debbie, thank you for an excellent question. This question, in fact, is so fundamental to the understanding of astrology and is the source of so many misunderstandings and misconceptions. Answering this question, however, is going to require some astronomy as well as some astrology, and the clarification of some terms.
Let's start with the definitions; your terminology in your question was a bit confused, and I know that you're not alone in this.
DEFINITIONS AND GLOSSARY OF TERMS
Celestial Sphere. The ancient understanding of the universe was quite different than our modern understanding of it. First of all, the ancients believed that the Earth was the center of the universe, and that everything revolved around the Earth. They also believed that the Earth was surrounded by a vast black sphere that contained all of the stars, called the Celestial Sphere. Even though this is obviously not the way things really are, projecting an imaginary Celestial Sphere onto the night sky makes it possible for astrologers and astronomers to measure, track and calculate the relative positions of the stars and planets as they appear from the Earth.
Geo-Centric. Geo-Centric literally means "the Earth in the Center" and this is the approach that is used by most astrologers, and also by astronomers when measuring and observing the stars and planets. On a practical level, it simply means that the sky is being observed from the Earth, and that measurements are based on spherical geometry and the use of the Celestial Sphere.
Great Circle. A Great Circle is any circle that divides a sphere (or in particular the Celestial Sphere) into two equal halves. The equator is a Great Circle, dividing the sphere of the Earth into two halves. All lines of Longitude are also Great Circles (connecting the North and South poles). Lines of Latitude (except for the equator), however, are not Great Circles.
Fixed Stars. When the ancients observed the night sky, they noticed that some of the stars seemed to move or wander from night to night. These, of course, were the planets. The way that they were able to determine that the planets moved, however, was because they noticed that the rest of the stars in the sky stayed in the same positions night after night. These are the fixed stars, and they are used as reference points in order to measure the relative positions and movement of the planets.
Constellations. Constellations are groups of fixed stars, that have become associated with a figure, and often with a myth. The Big Dipper, the Little Dipper, and Orion are probably the best-known and most easily recognized constellations in the night sky (at least in North America). Different constellations are visible at different times from different locations on the Earth. There are literally hundreds of constellations. Among these are the constellations of Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces. (As we will discover soon, these constellations are not the same as the Signs of the Zodiac that share these names.)
Ecliptic. The Ecliptic is the Great Circle that describes the apparent path of the Sun around the Earth (but which is really the orbit of the Earth around the Sun). The Ecliptic extends approximately 8-9° of arc above and below (North and South of) the actual path of the Earth/Sun. The other planets in the solar system are always visible within this band of sky. The longitudinal (East-West) position of celestial bodies (i.e. planets, asteroids, etc.) is measured along the ecliptic.
Signs. The Signs are units of measurement each equal to 30 degrees of arc along the ecliptic.
Zodiac. The Zodiac refers to the different names for the Signs dividing the ecliptic. The Signs of the Zodiac are named after twelve of the Constellations that intersect the ecliptic.
Vernal Point. The point measured along the ecliptic which represents the apparent position of the Sun at the moment of the Vernal (Spring) Equinox. It is defined as the first degree of Aries in the Tropical Zodiac
THE PRECESSION OF THE EQUINOXES
When you mentioned that the scientist said that "the Earth shifts on its axis and that makes the constellations go into the Zodiac at the wrong dates", what is being described is the precession of the equinoxes. Just to clarify, though, the constellations don't move – remember, they are made up of fixed stars. The Sun is what appears to enter the different Signs of the Zodiac. As to the part about the dates and the Zodiac, we'll get to that shortly.
The Earth doesn't so much "shift" on its axis as it "wobbles" The Earth's axis is tilted at an angle of approximately 23.5° to the plane of the ecliptic. This tilt is what produces the seasonal variations. The Earth is also not a perfect sphere; it bulges in the middle near the Equator. This unequal distribution of mass causes the Earth to "wobble" around its rotational axis like a gyroscope. What this means is that the Earth's axis makes its own rotation, with the North and South Poles slowly describing a circle around the ecliptic pole (which is the pole exactly perpendicular to the plane of the ecliptic; the North and South poles, remember are tilted 23.5 degrees away from this plane). How slowly? Well, a complete cycle takes about 25,800 years. The precession can also be seen in terms of the "North Star". Currently the North Pole of the Earth is aligned with the fixed star Polaris. This was not the case 3,000 years ago; and by the year 14,000 A.D., the North Star will be Vega, not Polaris.
This rotation of the Earth's axis occurs at something like 1° every 71.5 years (about 50 seconds of arc per year). The "wobble" and the precession of the equinoxes were known to the Ancient Egyptians, although the first official "discovery" of it was made by an Ancient Greek astronomer, Hipparchus, who was born sometime around 190 B.C. It was noted because the Sun was in a slightly earlier position at the time of the Spring Equinox each year (as measured against the fixed stars). Because the movement slips backwards through the zodiac, it is called precession (as opposed to a forward-movement which would be called progression).
Now 1° every 71.5 years doesn't sound like too much, but it certainly adds up over 2,000 years or so, and this is where we get into the different Zodiac systems.
The ecliptic is a circle, and the thing about a circle is that it doesn't have a beginning or an end. If you want to be able to measure something along a circle, you have to establish some sort of a reference point. The Zodiac as we know it today was first used by the Ancient Greeks over 2,000 years ago. Their year began with the Spring Equinox, and so it made sense to pick that point – that is, the point in the sky where the Sun appeared to be at the time of the Spring Equinox – as the reference point and then divide the ecliptic into 12 equal segments from there. At the time, the Spring Equinox occurred when the Sun was in the band of the ecliptic that also included part of the Constellation of Aries. The first 30 degree division of the ecliptic was named "Aries", and the remaining 11 segments were likewise named after the well-known and easily-recognized constellations that roughly corresponded in sequence. The Greeks never used the actual constellations to measure the positions of the planets, however, because the constellations did not divide the ecliptic into equal segments.
The type of astrology practiced at the time was entirely based on cycles. Each of the Signs of the Zodiac were associated with the type of qualities and energy that were experienced during the corresponding time of the year. The foundation of the interpretations of the Signs was seasonal. The Greeks were well aware of the precession of the equinoxes; however, as their system of astrology was based on the seasonal cycles, it did not concern them. Because this Zodiac begins with the Vernal Point, marking the Spring Equinox, when the length of the day equals the length of the night, this Zodiac is called the Tropical Zodiac, or the Seasonal Zodiac.
In part two of Stars and Signs, Kevin explains the difference between the Tropical and Sidereal Zodiacs, taking a sidelong glance at the Age of Aquarius and some myths and misconceptions.