5. The Zodiac - Constellations and "Signs"

The “signs” of the Zodiac, as used by astrologers, share their names with twelve of the constellations used by astronomers – but they are definitely not the same thing! In fact, their positions in the sky don’t even coincide; they did a long time ago, when astrology was first invented, but they no longer do, for a reason which I’ll come to. They are now “out of step”, with each astrological “sign” roughly corresponding to the adjacent constellation; when astrologers say the Sun is in the “sign” of Gemini, it’s actually in the constellation of Taurus, and so on. Why this is, I’ll explain shortly.

5.1. The Celestial Sphere

Before I explain the difference between the “signs” and constellations of the Zodiac, I need to introduce a few terms, for the benefit of readers with no knowledge of astronomy. In order to locate objects in the sky, astronomers use a coordinate system, similar to that which we use to describe the location of places on the Earth’s surface. We think of the sky as being the inside of an imaginary sphere, called the Celestial Sphere. Obviously, from any point on Earth and at any given time, an observer can see half of the Celestial Sphere.
The Celestial Equator is the projection of the Earth’s Equator onto the Celestial Sphere. The Celestial Poles are the points on the sphere to which the Earth’s axis of rotation points; therefore, they are the points around which everything else in the sky appears to revolve, from the point of view of an observer on Earth. So if you were standing at the North Pole, the North Celestial Pole would be directly overhead, and the Celestial Equator would run around your horizon. If you were on the Equator, the Celestial Equator would pass directly overhead, and the Celestial Poles would lie on the horizon to the north and south. Anywhere else in the Northern Hemisphere, the sphere is tilted at an angle which depends on your latitude; the angle of the North Celestial Pole above your horizon is equal to your latitude, while the Celestial Equator is tilted with respect to the horizon at an angle of 90° minus your latitude. Obviously, the opposite applies in the Southern Hemisphere.
At the present time, it’s very easy to find the North Celestial Pole, as a bright star – Alpha Ursae Minoris, also known as Polaris or the Pole Star – is conveniently located less than a degree from it. But this wasn’t the case in the distant past, nor will it be in the future – again, for a reason which I’ll come to. Astronomers in the Southern Hemisphere are not so lucky, as there is no bright star to mark the South Celestial Pole.
The positions of objects on the Celestial Sphere are defined in terms of declination and right ascension, which are the equivalent of latitude and longitude respectively. Declination is measured from zero on the Celestial Equator, to +90° and -90° at the North and South Celestial Poles. Right ascension is measured eastwards from an arbitrary zero point, called the Vernal Equinox, which I’ll define shortly.
The Earth’s axis of rotation is not perpendicular to the plane of its orbit; if you think of the orbital plane as the “horizontal”, then the poles are tilted at an angle of 23.5° from the “vertical”. Therefore, the Equator is tilted at the same angle with respect to the orbital plane. This tilt of the axis is what causes the seasons.
The projection of the Earth’s orbital plane onto the Celestial Sphere is called the Ecliptic; this is the path which the Sun appears to follow around the sky, with respect to background stars, during the course of a year, due to the Earth’s orbital motion. It follows that the Ecliptic is tilted at that same angle, 23.5°, with respect to the Celestial Equator.
The equinoxes are the two points at which the Ecliptic crosses the Celestial Equator. The Vernal, or Spring, Equinox is the point at which the Sun appears to cross the Celestial Equator from south to north; the Autumnal Equinox is where it crosses in the opposite direction. The Sun passes these points on 21 March and 21 September, marking the start of spring and autumn in the Northern Hemisphere, and the opposite in the Southern. The Summer and Winter Solstices are the points where the Ecliptic reaches its greatest declinations north and south of the Celestial Equator.
The Vernal Equinox is also known as the First Point of Aries – though it’s actually in the constellation of Pisces! This is an unfortunate hangover from astrology, which is still used in astronomy; it’s named after the astrological “sign” of Aries, and to astrologers, it’s the point at which the Sun enters that “sign”. Centuries ago, the corresponding date was defined as New Year; that’s why astrologers traditionally define Aries as the first “sign” of the Zodiac.

5.2. The constellations

Astronomers divide the sky into 88 constellations. These have no scientific significance whatsoever; they are a hangover from thousands of years of tradition, but are still used today, simply as a means of navigating around the sky and defining the locations of objects. The word “constellations” originally meant the traditional star patterns, but today, it refers to arbitrarily defined regions of the Celestial Sphere which contain those patterns, so that every star or other astronomical object can be said to be located in one or another constellation.
Many of the traditional constellations which we still use were known to the classical Greeks and Egyptians, but are believed to have originated much earlier. 48 of them were “standardised” by Ptolemy in the First Century AD; all but one of these are still used. ( One, which was ridiculously big, was later split into three separate constellations. ) The remainder are more “modern” inventions, dating from the Seventeenth and Eighteenth Centuries. Most of these are in the far southern region of the sky, which is never visible from the Mediterranean region, and was therefore unknown to Ptolemy; they were invented when European sailors began to explore the Southern Hemisphere. There are also a number of small and obscure ones in the northern sky, which were invented simply to fill the gaps between the traditional patterns.
Of these “modern” constellations, many variants were in use before the Twentieth Century; many more were invented by various astronomers, but never gained general acceptance. In 1933, the constellations were standardised by the International Astronomical Union, and their boundaries were arbitrarily defined, using lines of right ascension and declination.
Of course, we are concerned here with only twelve of the constellations – those of the Zodiac. These are the ones through which the Ecliptic passes – the path of the Sun in its apparent annual motion around the sky. Strictly speaking, the Ecliptic actually passes through thirteen constellations, not only the twelve of the Zodiac. Using the IAU boundaries, it passes through part of the constellation of Ophiuchus, between Scorpius and Sagittarius; in fact, the Sun spends considerably more time passing through Ophiuchus than Scorpius.
Some astronomers use this fact to try to embarrass astrologers, but it’s really a non-argument – since the astrological “signs” bear no relation to the constellations of the same names, and the IAU constellation boundaries are purely arbitrary anyway.
Some of the constellations of the Zodiac – as opposed to the astrological “signs” – are believed to have originated long before astrology did. There is an interesting theory as to how they may have originated, as markers of the seasons in different historical ( and prehistoric ) epochs, but that’s outside the main scope of this essay. For readers who are interested, this is described in Appendix B.

5.3. The astrological “signs”

While the constellations of the Zodiac are of widely varying sizes, the astrological “signs” are not. The astrological Zodiac divides the Ecliptic into twelve equal parts, each a 30° arc of the circle; these are what astrologers call the “signs”, or “houses”, of the Zodiac. So the Sun, during its apparent annual journey around the Ecliptic, spends an equal length of time in each “sign”. The first “sign”, Aries, begins at the Vernal Equinox, or First Point of Aries. ( The ancients, of course, thought the Sun actually did travel around the sky during the course of a year, as they didn’t know that it’s the Earth which is moving. )
There is another coordinate system, which astronomers use for some purposes, which uses “ecliptic latitude and longitude”, instead of declination and right ascension. The Ecliptic, rather than the Celestial Equator, is the zero line of ecliptic latitude, while ecliptic longitude is again measured from the Vernal Equinox. So the astrological “sign” of Aries covers the range 0°-30° of ecliptic longitude, the “sign” of Taurus covers the range 30°-60°, and so on.
When astrology as we know it was first invented, around 2500 years ago, the “signs” of the Zodiac were named after the already existing constellations, with which they roughly coincided. ( I say “roughly”, because the constellations do not cover equal angles along the Ecliptic. ) However, they no longer coincide, but are gradually moving “out of step” – hence the First Point of Aries is now in the constellation of Pisces. This is due to a phenomenon called precession of the equinoxes – usually just referred to as precession – which I’ll now explain.

5.4. Precession

I mentioned earlier that the fact that the bright star Polaris conveniently marks the North Celestial Pole is only temporary; it didn’t do so in the distant past, nor will it in the future. This is another consequence of precession, which causes the pole to move over a long period of time. In fact, considering the pole is the easiest way to visualise what happens.
I also mentioned earlier the tilt of the Earth’s axis, at an angle of 23.5° from the “vertical”. Precession is a consequence of this tilt, combined with the fact that the Earth is not a perfect sphere, and effects of the gravitational attraction between it and the Sun.
Imagine a spinning gyroscope, such as a child’s spinning top, standing on a solid surface. If its axis of rotation is exactly vertical ( i.e. parallel to the direction in which gravity acts, towards the centre of the Earth ), then everything is nice and stable, and the axis remains vertical. But now imagine what happens if you give the top a slight nudge, so that its axis of rotation tilts away from the vertical – but not far enough to make it topple over! The axis of rotation will then change direction, or precess, periodically; its direction rotates around the vertical at a constant rate, so that the axis traces out a cone.
The physics of why this happens is beyond the main scope of this essay - if you’re interested, it’s explained in detail in Appendix A – but it’s what happens to any rotating object when an external torque ( turning effect of two forces ) is applied.
The same thing happens with the Earth, which is effectively a very big gyroscope! What causes the torque is explained in Appendix A, but the result is that its axis of rotation precesses very slowly, making one rotation every 25800 years. As shown in Fig. 1, the North Celestial Pole traces out a circle on the sky – the Precessional Circle - over that same period, with a radius of 23.5°, and centred on the North Ecliptic Pole. The North Ecliptic Pole ( marked NEP in Fig. 1 ) is the point “vertically” above the Ecliptic plane; this point is fixed with respect to the stars, and is located in the constellation of Draco.

Fig. 1

So now we see why Polaris has not always been the Pole Star. About 4500 years ago, when the Pyramids were built, Alpha Draconis, or Thuban, was the Pole Star, and was about as close to the true pole as Polaris is now. ( Indeed, we know that Thuban was very important in the culture of ancient Egypt. ) Around AD 7500, Alpha Cephei will be the Pole Star; and so on.
As the direction of the Earth’s axis changes, so too does the plane of its Equator. So while the Ecliptic is fixed with respect to the stars, the Celestial Equator is not. Remember that the Vernal Equinox is one of the points at which the Ecliptic and Celestial Equator intersect; the equinox moves slowly along the Ecliptic, at the rate of about 50 arc seconds per year, going right round the circle in that same period of 25800 years.
Now remember that the astrological “signs” of the Zodiac are defined as equal 30° sectors of the Ecliptic, measured from the Vernal Equinox. So as the equinox moves with respect to the stars, the “signs” move with it; they become “out of step” with the constellations, at an average rate of one constellation per ( 25800 / 12 ), or 2150 years. So now we see why the “signs” are now “out of step” by roughly one constellation – the “sign” of Aries corresponds to the constellation of Pisces, and so on – because that’s roughly how far the equinox has moved in the time since the “signs” were named.
In Section 3.3, I said that astrology as we know it could not have originated much earlier than about 1000 BC. Had it done, then Taurus would have been the first “sign”, rather than Aries, and the “signs” and constellations would now be “out of step” by two.
Incidentally, the phenomenon of precession has been known for nearly as long as astrology has been around. It was discovered by the Greek astronomer Hipparchus, as long ago as 125 BC – though its cause wasn’t understood until the time of Sir Isaac Newton.

5.5. How do astrologers deal with it?

Just as in other aspects of their nonsense, astrologers are divided with regard to how to deal with the problem of precession, and the non-alignment of their “signs” with their eponymous constellations.
Some of them say that it isn’t a problem at all. They say – correctly – that there is actually no relationship between the “signs” and constellations. The “signs” are defined only as 30° sectors of the Ecliptic; they were simply named after the constellations with which they coincided at one particular time in history, and the fact that they no longer coincide is of no consequence.
Others, however, make a huge song and dance about it! They claim that the passage of the Vernal Equinox from one constellation into another heralds the beginning of a new “age”, which brings momentous changes to the world; needless to say, they can’t tell us anything about what those changes will be! So the era in which astrology was invented was the “Age of Aries”, and we are now in the “Age of Pisces”. This will be followed, around AD 2600, by the “Age of Aquarius”, when the equinox crosses the boundary into that constellation.
It’s easy to see that this is meaningless drivel, since the boundaries between the constellations, as defined by astronomers, are completely arbitrary; like the star patterns themselves, they have no physical meaning whatsoever. Astrologers, however, make up their own definitions of their “ages”, which don’t depend on the IAU boundaries. Needless to say, they can’t agree; some say the “Age of Aquarius” is several centuries in the future, while others claim it has already begun! As usual, astrology isn’t even self-consistent!
As an aside, it has been suggested that some stories in the Bible are connected with these so-called “ages”; remember that precession was known by the time the New Testament was written. In the Old Testament, the story of Moses destroying the Golden Calf might symbolise the transition from the “Age of Taurus” to that of Aries; Moses himself is said to have carried a ram’s horn, which he used as a trumpet to summon his followers – perhaps symbolising the latter age.
Even the story of Jesus, or part of it, might symbolise the “Age of Pisces”, as it contains several references to fish and fishermen. There is even a passage where Jesus tells his disciples that he will be with them “until the end of the age”, and then says, “Look for the man carrying water, and follow him” – which could be a reference to the future “Age of Aquarius”!

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