Categories: Astronomy HistoryStar Constellations

The Zodiac: Origins and the Problem with Precession

Regardless of what one thinks of astrology as a means to predict the future or to define personality types, it must be stated that modern astronomy has its roots deep in several ancient astrological traditions. Contrary to popular belief, however, astrology did not start out as a way of predicting the future and personality types, but ancient practitioners were more concerned with keeping accurate time during the year by following the Sun, Moon, and planets as they moved through the twelve constellations that make up the zodiac.

The Zodiac

The word ‘zodiac’ derives from the Greek meaning “circle of animals”, and describes the circular arrangement of 12 divisions of celestial longitude that are 30° apart, and is centred on the path the Sun follows through the sky, known as the ecliptic. The ancient Greeks did not initiate the search for meaning and purpose in the heavens, though, and together with the Romans were relative late-comers to the astrological party.

Greek Influence

Based on current knowledge, it seems almost certain that astrology among the Indo- European nations originated in the 3rd millennium B.C. In classical Greek times, there were 48 constellations, which we now know were derived from two sources, namely Mesopotamia, which developed the zodiac consisting of 12 signs from around 3200 BC to as recently as 500 BC; and the Mediterranean region as it was at about 2800 BC, where the constellations were primarily used in navigation at sea. These Mediterranean constellations included huge serpents and bears that marked the north celestial pole and equator as it was then, but also included the constellations Auriga, Bootes, Ophiuchus, and Hercules, in addition to some “marine” constellations of the more southerly skies.

Originated at Latitude 36°N in 2700BC?

As mentioned, many surviving records show that the Greeks borrowed the modern zodiac from many cultures that predate their own by millennia, including Mesopotamia, Egypt, Babylon, and Sumeria. Moreover, the available evidence even seems to suggest a point and time for the origin of astrology, which turns out to be sometime around 2700 to 2900BC, and at a latitude of around 36° north. This rather remarkable claim is substantiated by the following pieces of evidence:

1: The Empty Circle; in the southern sky there exists a circle of about 36° in diameter that does not contain any of the 48 original constellations, meaning that for observers at a position north of latitude +36° such an area would not be visible. The absence of constellations in that area of sky therefore implies that the originators of astrology must have lived north of latitude +36°.

2: Tilted Constellations; there is reason to believe that constellations were deliberately “constructed” to align with North, South, East, and West. Today, however, many constellations are tipped, slanted, or otherwise misaligned with the compass with the only logical conclusion pointing to the effect of precession over time. The time at which all the constellations would have been more aligned to ancient observers is estimated to be about 2900 BC, which corresponds roughly with the time when the “Empty Circle” would have been invisible to observers north of latitude 36°.

Taken together, these observations indicate a point of origin for the constellations located near latitude 36° N, which happens to be close to where ancient Sumeria was located, at latitude 32° 9′ 20″ N. Therefore, the Sumerians are currently regarded as the originators, or “inventors” of the constellations.

The Problem with Precession

In principle, the Sun should pass in front of each of the 12 zodiac constellation in turn as its moves along the ecliptic. Viewed from Earth, the stars that mark out any given constellation should form the background against which the Sun’s position can be gauged, but due to the effects of precession, each zodiac sign no longer correspond to the actual dates the Sun appears to be “in” a particular sign, and have not done so for hundreds of years.

This is because the Earth’s axis shifts by a measurable amount during just one orbit of the Earth around the Sun, meaning that over time this difference adds up to appreciable distances. In fact, the signs of the Western zodiac were more or less aligned with their corresponding constellations about 2000 years ago, but during the intervening centuries, precession has caused the solstice points to shift westward relative to the constellations by as much as 30°. This means that in practice, the signs of the zodiac and the constellations are by now misaligned by roughly one month, and they will be misaligned by as much as two months about 2000 years from now.


While astrology may fail us when predictions of love, wealth, health, and good fortune don’t work out, it is nevertheless an excellent tool to explain, and demonstrate the movements of solar system bodies relative to the constellations. Moreover, a study of astrology can be very instructive with regards not only to the cultures that developed it, but also of modern astronomy’s humble beginnings.

Peter Christoforou :

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  • Hi, I´d like to know the source for this picture you used in this blog. I want to use it as well, and I need to know if there is any copyright to it. Thanks!

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