In the west, since about 2000 years ago, there have been two schools of astrology that appear, to the outsider, to be fundamentally opposed – tropical and sidereal - but astrologers claim that there is no conflict. Astrology is a hugely complicated subject that has grown and developed over 5 millenia in many different traditions. Here I'm not planning to question the astrological interpretations themselves – tripe though they may all be; but instead I'd like to focus on the astronomy involved and what it can tell us about these two different astrological models.
So as you probably know, astrologers seek to make predictions about our lives, and about world events, based on the “influences” of certain heavenly bodies. Of particular importance in developing a person's chart are the relative positions of these heavenly bodies at the time of birth. Depending on which system is used, the position of the sun, the moon, the other planets of the solar system and various distant stars are used and the angles between them are calculated and plotted in a special way to develop the chart. The positions of each of these relevant heavenly bodies are looked up and referenced using a chart called an ephemeris. Of course there are numerous disagreements about how this process should be done, and numerous different, contradictory ephemera. And numerous, different interpretations of the influences. But don't let that put you off this scientific, results-driven science.
Terran orbital mechanics and how we see the universe
If you are already familiar with the language and concepts in planetary geometry, you may want to skip this bit. In order to make it is simple as possible, I'm going to explain everything at a basic level.
The Earth moves in three main ways – it spins round on its axis, it orbits the sun, and it wobbles on its axis. The period we call a day is the time taken for the Earth to do one revolution on its axis. It takes a bit shy of 365 and a quarter days to go all the way around the sun, which is simplistically the period of time we call a year. More on this later. The wobbling on the axis is what is called precession, and is much slower – one whole cycle of the wobble, known affectionately as a great year, takes approximately 25,800 years (no one seems to be able to agree on the exact length of time).
So, when we look from the Earth at the skies, all these three motions play a part in the way we observe the rest of the universe.
The daily spinning of the Earth on its axis means that, through each 24-hour period, looking directly up, we see a whole 360 degree panorama of the universe appearing to rush across the sky from East to West.
The yearly motion of the Earth's orbit round the sun means that over the course of a year, the time of day that we see a particular star or constellation (from a given location on the Earth's surface) gradually rotates around, coming round full circle in a year's time. I'll make that simpler with an example. As I write, here in Los Angeles at 130am, the constellation Perseus is just rising from the Eastern horizon. At the same time of the night in a couple of month's time, it will already be high in the sky. Next spring it will be invisible to the naked eye, as it will only be in the sky during the day. On this day next year, it will once again be rising in the sky at around 130am.
The sun can be seen to “appear” in a series of certain constellations throughout the course of the year. What this means is, that constellation is directly behind the sun, from the Earth's perspective, depending on the time of year. (That is, assuming that there are no light bending effects of the type known to be exerted by dark matter and black holes). In a way, the sun traces a path across the heavens, always “appearing” in the same series of constellations in a constant cycle, year after year. This path is called the ecliptic line. The movement of the moon across the skies is essentially the same is this, tracing roughly the same line, except it cycles around the heavens in about a month instead of a year. The other planets of the solar system can only be seen on or close to this line, since the solar system is basically a flat plane. Their periods and locations in the sky are more complicated.
Most of the stars, galaxies, nebulae and so on can be said to have “fixed” positions relative to the Earth, and to each other. Of course, this is bullshit. They are all moving at thousands of miles per second, as is our own solar system, but since they are all so far away we can't see this movement (without lab equipment) as it appears to happen extremely slowly.
Precession and the different ways of defining a year
So when you talk about a year, what do you mean exactly? The period from New Year's Day to New Year's eve? 365 days? Another cup final day?
In fact there are several distinct and precise ways of defining a year. Two are important here:
a tropical year is the time taken for the Earth to complete one 360 degree orbit around the Sun, being defined by the time from one vernal equinox to the next.
a sidereal year is the time taken for a “fixed star” to return to the same position (at the same place and time).
Because of precession, being the Earth's wobbling on its axis, these two years are of slightly different lengths. This is because the two measurements are topologically different. A tropical year is a function only of the Earth's orbit around the sun; whereas a sidereal year is measured from the surface of the earth, which has it's own independent movements. The sidereal year system has another degree of freedom, as there are two mechanisms contributing to it.
To illustrate by example: this year, the sun was very close to the star Omega Pisces (i.e. the Earth, the Sun and that star were in approximate alignment) on the vernal equinox. They rose from the horizon at about the same time. Next year, the sun will be about 50 seconds ahead of that star. In 2018, the difference in rising times will be closer to 5 minutes. This is because, since the Earth has wobbled slightly, a given location on Earth, at a given time each tropical year, is pointing in a slightly different direction towards the heavens.
Another way of expressing this is to go back to the apparent “motion” of the sun through the different constellations of the ecliptic. On a particular time and day of the year (lets say the vernal equinox) the Sun's position will gradually shift across the sky, over thousands of years, such that at the end of the precession cycle it will have cycled all the way around the ecliptic and back to it's start point.
Zodiacs and different types of astrology
A zodiac is a map of the ecliptic line that astrologers use. Most western astrologers use a zodiac that is divided into twelve equal sections of 30 degrees each. These are just like grid references on a map of a section of the Earth. The sections are named after the real star patterns – Scorpio, Leo etc. but only loosely. (The real constellations have varying angular widths and there are in fact at least thirteen real constellations along the ecliptic).
The starting point for the zodiac was set a long, long time ago as the “first” star in Aires, Mesarthim. The sun was directly on the line between the Earth and this star on the vernal equinox. In a tropical zodiac, which defines a year as a tropical year, this starting point of the zodiac always occurs on the vernal equinox, March 21st.
However, because of precession, over many years the position of the sun, (relative to the real constellations along the ecliptic) on this date and time changes. Therefore a sidereal zodiac takes into account the precession cycle by shifting the start times of each “sign” according to what is observed from Earth.
The difference means that over time, these two different zodiacs have become out of sync with each other. In fact, while Tropical astrologers have always defined the start of the sign of Aires on March 21st, sidereal astrologers currently define its beginning on April 15th.
(There is a third type that I'll mention only briefly – where the “signs” are of varying length, defined by the movement of the sun through the actual constellations in the ecliptic as defined by the International Astronomical Union in 1930. This zodiac usually has 13 signs (14 in some systems). Even most astrologers think this system is bogus. “Reputable” western astrologers generally agree that there should be twelve signs of equal length.)
What does this mean for astrology?
To the lay observer, it seems that at least one of these systems must be fundamentally flawed. If your birthday is today, then in one system you're a gemini and the other you're a cancer. All that gobshite about this sign means this and that sign means that, its meaningless. At first glance it seems that tropical astronomy is the one that is bunk, because the dates are so out of sync with the real constellations. So if your horoscope is telling you that Jupiter is rising in Aires, whilst a quick glance out the window clearly reveals it to be in Taurus, it makes you question the validity of this amazing ancient divination tool...
But to be fair you need to look a bit deeper than this really. According to most writers, the zodiacs were never meant to be representative of the actual starscape, they were only supposed to serve as a referencing grid system. One way of thinking about tropical astrology is that its zodiac measures cycles of solar time and, therefore, remains linked to the seasons on Earth. OK, well if that's the case, why all the significance and symbolism around the constellations themselves then? Why do we have to hear about Aires, god of war being influential in our relationships and Libra bringing harmonious balance to our lives?
The tropical zodiac preserves the Earth's solar cycle
Imagine each zodiac like a measuring stick against the heavens. The tropical “measuring stick” is fixed to the Earth's Solar cycle. The cycles and patterns of all other heavenly bodies are out of sync with it. This means that the positions of the “constellations” themselves, and the position of the planets in relation to them, should be meaningless in tropical astrology, since the predictions of their locations via a tropical ephemeris are incorrect.
Essentially, a tropical zodiac is little more than a solar clock. Predictions made from it can only really relate to cyclical effects of the Earth-Sun system. Some attempts to justify astrology point to the possibility of the seasons having an influence on foetal development, as a possible mechanism by which time of birth could help predict personality. Of course, even if this were true, it would only explain a tiny minority of the claims astrologers make.
The tropical zodiac does allow comparison of the solar system bodies, however. Astrologers are very concerned with the angles between Earth and the different planets. Using this system, calculations of these angles will be correct, although it does depend on where you get your data. Of course, prediction of where in the sky the planets actually appear will be skewed. But that doesn't matter to a tropical astrologer as long as they are in the correct sign of the chart.
Also, position on the earth isn't relevant either. This is because, over time, a particular place on Earth moves slightly for the same date each year. It doesn't observe the same part of the "measuring stick".
Say a prediction is made about the influence of Sagittarius on someone born in New York. In tropical astrology the same prediction might be made year after year for children born at that place and time of year, but after hundreds of years, Sagittarius would instead be rising over Oregon at that time, and Capricorn would be the actual part of the zodiac “influencing” the birth (with whichever planets are currently in that constellation).
The age of Aquarius is only possible in Sidereal astrology
This is because the so called ages come about due to the precession cycle. The Age of Aquarius is supposed to begin when the Vernal Equinox shifts into the part of the zodiac known as Aquarius. So if you hear any astrologers who follow the tropical system (most of the ones in the western world), talking about the dawning of a new age, they are talking complete horseshit.
The sidereal zodiac preserves the cycle of the ecliptic
On the flipside, sidereal astrology maps the ecliptic with its zodiac in way fixed to a given location on the Earth. It effectively charts and maps the constellations themselves. The “measuring stick” is fixed to the distant stars. When these guys talk about the positions of constellations and planets in the sky, over a particular place at a particular time, they really mean it. However, this is at the expense of the influence of the cycles of the seasons. This zodiac is out of sync with the Earth-solar cycle. Also, it must be noted that the precise mechanics of precession are not perfectly understood, so the length of a precession cycle is not precisely known. It may oscillate and never be the same from one cycle to the next, which means there will be inaccuracies in the sidereal zodiac also. But as long as it is is based on (and updated according to) actual observation, this doesn't matter too much.
Most writers agree that Sidereal astrology was around long before it's tropical cousin, and sidereal (and complex versions thereof) is still the most common form in the East. Indian writers refer to tropical astrology simply as “Western”. Some say that Tropical astrology is simply a more esoteric and mystical form, whereas sidereal is more practical and factual. Some think that tropical is better for predicting psychology, whereas sidereal is better for predicting events.
However, the best conclusion to come to, is that right from the off, tropical astrology is complete crap. One huge mindfuck of a mistake that people have been labouring under for the last 2000 years, and are still in denial about it.
Think about it. In ancient times, people believed that gods ruled the world and that the movements of stars and planets had a real effect on peoples' lives. It's entirely understandable that they would then invent a system for predicting the movement of the stars and planets, in the hope that it could be used to predict events. Before the days when humans really understood anything about astronomy, this sort of logic is reasonable, if a little flawed.
|Was Ptolemy on drugs when he made this?|
But then, someone came along and said, “Ah ha! Well instead of using the real movements of the stars and planets, why don't we simply refer to a fantasy one instead? We can use inaccurate information about the heavens to make even more inaccurate predictions about our lives!”
Thinking about it, Ptolemy was probably high as a kite when he came up with the first tropical zodiac system. Looking over it the next day, and realising he'd made a big fuck up, he probably invented all this new age stuff about linking the zodiac to the cycles of the Earth, just so he could avoid losing his status and employment. Not that I'm cynical about these things or anything.