When Did Astronomy and Astrology Become Separate Theories?

Question posed by Chaz.

If I was feeling pedantic I'd pick at the use of the word 'theory' in the question. 'Astronomy' is more of a name given to a collection of theories that can be used to describe and make predictions about the things that happen in the Universe. 'Astrology'... well, astrology just isn't a theory or a collection of theories at all.

Chaz is absolutely right, though, in that the two disciplines are separate ones, and also right in the implication that they once weren't. For a bit more on the difference between the two, delve way back into the earlier days of Blogstronomy and check out this post. If you're more web-comically minded, you could do worse than ogle this guest post, too.

But anyway, astrology and astronomy were once pretty much one and the same: astrologonomers* would look at the stars, make observations of the way things moved, use those to make predictions about how they were going to move, and then use those to make predictions about things that might happen down here on Earth. Nowadays we** are well aware that this last step is where the nonsense and gibberish steps in, and it's only astrologers who try to do this bit any more, more often than not using the data collected by astronomers as a starting point.

The split wasn't a quick one, though. Many sources point towards the late middle ages (from about the 14th Century ACE) as being a time when astrology and astronomy started to realise there were issues developing in their relationship, but it wasn't really until modern science properly started to take off towards the end of the 16th Century that astrology began to be rejected as metaphysics next to astronomy's more empirical nature.

Over the next two hundred years or so, astrology began to take a bit of a beating from the intellectual and scientific elite, although some of the most famous names in astronomy from this time, including Brahe, Kepler and Galileo, were astrologers by profession. It appears likely that Newton and Huygens were not followers of astrological principles, however.

Following Newton (towards the end of the 18th Century), the Enlightenment and technological revolutions affected scientific thought and measurement respectively: better measurements could be made (particularly with advances in timekeeping technology), and predictions could be made which could then be tested more thoroughly. In the case of astrological predictions, many were found to be false; more sophisticated thinking led to a greater understanding of the distinction between coincidence and cause-and-effect***.

By the end of the 18th Century astrology, as a superstition or a form of divination, had suffered something of a crowbar separation from the science of astronomy. Even today, though, the two are confused by many people, probably largely due to the similarity of the names and the fact that they're both involved with the motion of things in the sky. Beyond that, however, they have very little in common.

* I made that word up.
** Well, most of us.
*** Although this is something many people still struggle with today.


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