It’s a common story with astronomy enthusiasts. You are at a party or with friends when a friend introduces you as a person having an interest in astrology. You smile politely and gently correct them, but in the back of your mind you realise that they didn’t really get it. After all your explaining, you still expect to be called “the horoscope guy” later on. To many people, astrology and astronomy are different sides of the same coin.

Indeed, on a very superficial level, astronomy and astrology are quite similar. They are both concerned with the stars and planets, they both have very ancient pedigrees and are accompanied by a vast body of literature. Both astrology and astronomy are highly prominent in modern culture as any newspaper or magazine will attest. They both deal with future predictions and people involved in a professional level take their expertise very seriously.

However the astronomical and astrological camps are very, very different, and it is very rare to find an astronomer who has any regard for astrology whatsoever. (The opposie is probably not the case, but astrologers don’t particularly like astronomers so much). So what is wrong? Is it a case of snobbishness from the astronomical community? Professional rivalry perhaps? Or a conspiracy theory against the hard-working astrologers?

The answer is somewhat different. Fundamentally, astronomy and astrology are quite different philosophically.

Astronomy is a scientific philosophy. Astronomy is based primarily on the evidence, the facts. Beliefs about what these facts mean come second. All beliefs are tested and if they fail the tests, they are rejected. If they pass all the tests they are accepted as true, or at least provisionally true until new evidence becomes available. In this way, astronomy has been very successful in changing what were once strongly cherished beliefs – the belief that the sun and the planets revolved around the Earth, for instance, or the belief that the universe was timeless, even that time itself was somehow outside of the universe; all these ideas have perished as better data and better knowledge came on the scene.

Not so with astrology. With astrology, the beliefs themselves come first, with facts and evidence coming a poor second place. One of the strongest beliefs in astrology is that the stars and planets affect us in all sorts of ways. They guide our personalities, our moods and our fortunes in life. Now, this is a testable proposition and yet no evidence has ever been found to back up these claims. Furthermore, it is not a particularly plausible proposition given the enormous distances between astronomical bodies and ourselves on Earth and the lack of any coherent mechanism that would link the position and movement of the planets with the human psyche. The basic beliefs behind astrology therefore are magical, miraculous – somehow outside the realm of normal experience and scientific understanding.

Yet the beliefs persist. Plenty of people will tell you that astrology works. As proof they will often claim direct personal experience. The charts indicated that something would happen, and it did – exactly as described. The horoscopes gave a reading of their personalities with breathtaking accuracy. How could this happen?

The answer lies, not so much with the effectiveness of astrology, but with how our brains work. Most of us realise our brains are not perfect, but far less people understand how deep those imperfections extend. We are subject to all sorts of biases. We tend to assign undue significance to ideas we agree with while ignoring contrary ideas. We seek purpose and causality where it does not exist. We forget quickly and what we remember may often be very different from what actually happened. We are highly prone to suggestion. Professional magicians use such weaknesses against us to good (and profitable) use.

It’s not just astrology that is subject to such biases. Bias is commonplace throughout all human experience – politics, business, management, relationships, you name it. Science too. What makes the sciences different however are the extensive set of techniques that are used to eliminate bias. Controls, randomisation, blinding, sampling and peer review are examples. Such techniques, while seemingly arcane, are quite rational and logical in reality. They tend to make the process less subjective and any results tend to have greater weight, particularly if they can be repeated in a number of different settings.

The difference between astronomy and astrology highlights an important difference between science and pseudoscience. One area is founded on facts and evidence, the other is founded on beliefs. There are many fields of endeavour that are based on a set of implausible or untestable beliefs. Homeopathy, for instance, uses a belief that a tiny amount of material can cure chronic complaints and that the more dilute you make the solution, the more powerful the remedy will be. It’s over 200 years since Homeopathy originated, yet homeopaths have never properly challenged these founding beliefs. They assume them to be true and move on from there. In any field of study, when the founding beliefs are deemed to be too precious to be properly challenged, you should be very wary indeed.