Why have star signs shifted by one constellation? (Intermediate)

Can you please tell me why it is that star constellations have now 'shifted' into the next constellation, as such everyone's star signs are wrong. I know it's to do with the 'wobble' of Earth but find it hard to explain to my friend.

We have to start from the fact that Earth is not a perfect sphere but is a bit fatter at the equator and flattened at the poles. If we make a model of the Solar system with planetary orbits pictured as horizontal, Earth would look like a flattened top spinning on an axis that is somewhat tilted relative to the vertical direction. All the other bodies (the Sun, the Moon, the planets) are in this orbital plane and their gravity tries to "straighten up" Earth and bring the fattest part in the same plane with them. This is analogous to how gravity wants to topple a top spinning on a table. In both cases, the spin (technically, angular momentum) of the rotating body prevents it from falling or straightening up, but the outside force still has to produce some effect. For reasons that are non-obvious and have to do with vectors etc. the result of all this is wobble (technically "precession"). This means that while the tilt between Earth's axis and orbit stays the same, the direction of the axes execute a circular motion with a period of 26,000 years.

Now, the seasons are caused by Earth orbiting the Sun on an inclined axis. So when Earth is in the part of orbit where the Northern hemisphere tilts towards the Sun we have the summer here (and you guys have winter down there) and vice versa. The star signs were defined by the constellation that was behind the Sun when one was born. Obviously, the part of the Zodiac (the ring of constellations in the plane of Earth's orbit) towards which the Northern hemisphere leans contains the star sign that applies to June and so on for the whole year. But if the direction of Earth's axis changes, the constellation that appears behind the Sun during a particular season will also change. As Earth's axis wobbles, the star signs move forward through the year, making a full circle once in 26,000 years, which is close to a shift by one sign every 2000 years. The present Zodiac was introduced at Roman times, at the very beginning of Common Era. Since then about 2000 years have passed and the signs moved forward among the dates by about one sign. So if one's "official" sign is Libra, chances are that the Sun was in constellation Virgo at the time of his/her birth. The same applies to all other signs.

Now, this was astronomy. Astrologers who still use the old signs make distinction between "signs" and "constellations", claiming that the former don't have to have anything to do with the latter. I don't believe in astrology so my opinion cannot be objective, but I must say that I think this distinction is quite silly. 

This page was last updated July 18, 2015.

About the Author

Matija Cuk

Matija works on the orbital dynamics of the lesser moons of Jupiter and Saturn. He graduated with his PhD from Cornell in November 2004 and is now working at the University of British Columbia in Canada.

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