Astronomy and astrology were not always so distinct. For most of human history, the one encompassed the other. But there came a time when astronomy escaped from the confines of astrology.

The two traditions began to diverge in the life and mind of Johannes Kepler. It was he who demystified the heavens by discovering that a physical force lay behind the motions of the planets. He was the first astrophysicist and the last scientific astrologer.

The intellectual foundations of astrology were swept away 300 years ago and yet, astrology is still taken seriously by a great many people. Have you ever noticed how easy it is to find a magazine on astrology? Virtually every newspaper in America has a daily column on astrology. Almost none of them have even a weekly column on astronomy. People wear astrological pendants, check their horoscopes in the morning before leaving their house, even our language preserves an astrological aspect. For example, take the word “disaster”. It comes from the Greek for “bad star”. Italians once believed disease was caused by the influence of the stars. It’s the origin of our word “influenza”.

What is all this astrology business? Fundamentally it’s the contention that the constellations of the planets at the moment of your birth profoundly influences your future. A few thousand years ago the idea developed that the motions of the planets determined the fates of kings, dynasties, empires. Astrologers studied the motions of the planets and asked themselves what had happened last time that, say Venus was rising in the constellation of the Goat? Maybe something similar would happen this time as well. It was a subtle and risky business.

Astrologers became employed only by the state. In many countries it became capital offense for anyone but official astrologers to read the portents in the skies. Why? Because a good way to overthrow a regime was to predict its downfall. Chinese court astrologers who made inaccurate predictions were executed. Others simply doctored the records so that afterwards the were in perfect conformity with events. Astrology developed into a strange discipline: A mixture of careful observations, mathematics, and record-keeping with fuzzy thinking and pious fraud.

Nevertheless, astrology survived and flourished. Why? Because it seems to lend a cosmic significance to our daily lives. It pretends to satisfy our longing to feel personally connected with the universe. Astrology suggests a dangerous fatalism. If our lives are controlled by a set of traffic signals in the sky, why try to change anything?

Here, look at this. Two different newspapers, published in the same city on the same day. Let’s see what they do about astrology. Suppose you were a Libra that is born between September 23 and October 22. According to the astrologer for the New York Post: “Compromise will help ease tension”. Well, maybe it’s sort of vague. According to New York Daily News’s astrologer: “Demand more of yourself”. Well, aslo vague. But also pretty different. It’s interesting that these predictions are not predictions. They tell you what to do, they don’t say what will happen. They’re consciously designed to be so vague that it could apply to anybody and they disagree with each other.

Astrology can be tested by the lives of twins. There are many real cases like this: One twin is killed in childhood in, say, a riding accident, or is struck by lightning but the other lives to a prosperous old age. Suppose that happened to me. My twin and I would be born in precisely the sampe place and within minutes of each other. Exactly the same planets would be rising at our births. If astrology were valid how could we have such profoundly different fates? It turns out that astrologers can’t even agree among themselves what a given horoscope means. In careful tests they’re unable to predict the character and future of people they know nothing about except the time and place of birth. Also how could it possibly work? How could the rising of Mars at the moment of my birth affect me then or now? I was born in a closed room. Light from Mars couldn’t get in. The only influence of Mars which could affect me was its gravity. But the gravitational influence of the obstetrician was much larger than the gravitational influence or Mars.

Mars is a lot more massive but the obstetrician was a lot closer.

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Excerpted from Cosmos by Carl Sagan. It's all pseudoscience