You don’t have to look very far to find references to astrology in modern culture. Despite the fact that the pseudoscience has been out of date for centuries, ask just about anyone and they’ll be able to tell you their astrological sign. Magazines and newspapers worldwide regularly print horoscopes, which claim to be able to predict the future and offer advice based on the time of year in which you were born. Although it may be true that few people take such things seriously, the fact that these publications can print these things without losing all credibility implies that fewer people still ever seriously question this superstition.

The claims made by astrologers can be boiled down to this:

The arrangement of the planets, Moon, and Sun in the sky relative to the time and place of your birth largely determine your personality and the events of your life.

On the face of it, this might seem mildly plausible. We already know that the position of the Sun changes with the length of day and night, and the Moon controls the tides, so it might seem plausible that such bodies could somehow be related to our lives in other ways. If you’ve ever read a horoscope aimed at you or a description of people with your “starsign”, you’ve probably been able to draw parallels between the claims made therein and actual facts about your life and personality. Astrology can be an easy trap to fall into, if you don’t know any better.

Like most superstitions, the claims of astrology can seem plausible at first glance, when intuition tells us that they might be analogous to things we already know to be true. However, just like most superstitions, when you stop to think about astrology it becomes clear that it is implausible, and that its apparent results are entirely consistent with a universe in which astrology is not true.

The main claim of astrology is that, from the arrangement of the solar system at the time of a person’s birth, it is possible to make accurate predictions about that person’s personality. In order for this to be true, some sort of information from these celestial bodies must reach us here on Earth, manifested in some sort of biologically useful way.

If this signal can affect our lives in such a significant way, it seems reasonable to assume that we are somehow able to detect it. The ways in which we can currently detect planets, the Sun, and the Moon from Earth are by observing their gravity and electromagnetic radiation (e.g. visible light) either emitted or reflected by them. If none of these signals are responsible for the effects claimed by astrologers, then there would need to be some very strong evidence that astrology has any predictive power. Like any other claim, if no known mechanism can support it then the evidence needed to show that such a phenomenon exists in the first place must be extraordinary.

The fact that the Moon controls the tides on Earth is often used as an example when trying to make the idea of such objects affecting us personally seem more plausible. You’ve probably heard, for example, of “lunacy” – more extreme behaviour becoming more common during a full moon. These ideas are similar to the ideas behind astrology, that the worlds in the sky can affect our lives on the ground. In order to understand why the Moon’s control over the tides does not mean that it also has control over us, we need to understand exactly what it is that gives the Moon any influence over the tides in the first place.

The answer to that is, of course, gravity. This is usually also given as an example of how the Moon might affect us too – if the Moon can affect the water in the sea could it not also affect the blood in our bodies? While this question is an interesting one, the answer to it is “no”. Why is that the answer? The short answer is because we are too small.

The long answer is because the tides are caused by what is known as the “tidal force”. This is caused by the fact that the gravitational influence of an object decreases with distance. This means that the gravitational influence of the Moon on the Earth is stronger on the side closer to the Moon than it is on the side farther from the Moon, and it is this difference which causes the tides. The water closest to the Moon is pulled more strongly, whereas the water farthest from the Moon is pulled comparatively weakly, causing the “bulge” that we experience as the tides.

The caveat to this effect is that it does not apply to objects that are very far away, or to objects that are not very large. This is because, for such objects, the distance between their proximal and distal ends is very small in comparison to the distance between the objects themselves. So, whereas the Moon does have a gravitational influence on all of us, it is not comparable to the tidal force.

When thinking about the gravitational influence of such large and distant things as the Sun and Moon (hell, even the Earth), for all intents and purposes human beings can be treated as single points. What I mean by this is that the gravitational effects of these objects acting upon us is pretty much exactly constant at each point on our bodies. This means that any small contribution from a distant planet (like Mars, for example) would be negligible next to, say, a change in altitude here on Earth. So, if the Sun, the Moon, and the planets do affect our lives and personalities, it seems it cannot be through their gravity.

Aside from gravity, what other information from the rest of our solar system reaches Earth? Well, we can see the planets in the sky just like we can see the more distant stars, which happens because they reflect some of the Sun’s light back at us. Could this light, perhaps, effect people’s personalities? Considering that light varies much more depending on more local circumstances (location, time of day, artificial lights, etc.) than it does depending on the positions of the planets, it seems very unlikely that their light could have any noticeable effect. The same concept applies to other types of electromagnetic radiation; there simply isn’t a strong enough signal from any of the planets to be considered biologically important, especially when compared with background levels.

As far as I can tell, neither of these signals could plausibly affect us secondarily either (for example by affecting the Earth, which then affects us). It seems, then, that in order for astrology to be true there must be some currently undetected biologically useful signal that can reach us here on Earth while we are being born (as it is the circumstances of birth that astrologers use to make their predictions) that comes from the planets, Moon, and Sun. Given that such a signal has not been detected, and there seems to be no other reason to expect one to exist, surely the evidence behind astrology must be overwhelming, right?

The most easily accessible astrological predictions are the horoscopes printed in various newspapers and magazines. It’s generally well known that these are exceedingly generic, which is justified by the fact that each of them must be applicable to 1/12 of the entire population of the Earth.

Looking at some examples on the New Zealand Herald site they all seem far too generic and non-committal to be suitable for testing. The flip side of this, of course, is that they are also far too generic and non-committal to be useful for prediction. If it’s impossible to test whether or not a prediction was correct, then how could the prediction have been useful?

Looking at other sources, such as www.horoscope.com, the predictions given seem to lend themselves to becoming self-fulfilling prophecies. For example, from their horoscope for Aries on the 14th of August, 2012:

Your imagination should be flying high today, Aries, and your creative juices flowing freely. Exceptional ideas for projects involving writing, music, or painting could pop into your head during the day. Start on one and list the others so you can refer to them later. Don’t be surprised if you find your intuition increasing as well. Make the most of it all, and have fun.

It’s fairly easy to see how reading (and believing) such a prediction at the beginning of the day will make it more likely to come true. In the same way as the prediction “you will think of purple elephants” will inevitably lead to purple elephants forcing their way into your thoughts.

I reasoned that, if I wanted to properly test the predictive powers of astrology, I’d have to find a more specific prediction. Instead of looking at predictions that supposedly apply to over half a billion people, I thought I’d try to get one that was tailored specifically to me. I purchased a personalised astrology report from the Australian company Astrologic Answers. Here is the information they used to create this personalised report:

My first name – Mark

My date of birth – 11th June 1989

My hour of birth – 0600, or 6am

My minute of birth, which I approximated at 30 after asking my mother about it. They claim that this is only required “if known”, so presumably the report should still be accurate if I am a few minutes off the mark.

My city of birth – Auckland

My country of birth – New Zealand

Whether or not this report is meant for a child (no), and my gender (male)

Technically they also had my email address, which is at this domain, so they could have also looked at this blog. As far as I’ve been able to tell from the statistics reported by WordPress, though, they didn’t do this.

From these data, they could supposedly “calculate” a report about 20 pages long about my:

Life Goals Home Education and Communication Relationships Motivation Career Creativity and Originality Challenges Purpose and Joy

They also provided two examples of what the final report might look like: David Helfgott and Gillian Helfgott. The formatting of the report for Gillian Helfgott appears more similar to my own, so I presume it’s the more recent of these examples.

After a couple of days, I received my personalised report in my inbox, along with the “Art of Astrology” ebook included in that particular deal, each page of which seems to contain enough false statements to warrant a blog post all of its own. I’ll discuss the content of my personalised report, and its implications about the accuracy and usefulness of astrological predictions, in my next post.