This article first appeared as a Vic Skeptics discussion pamphlet.
The full range of our discussion pamphlets can be downloaded here: http://www.skeptics.com.au/resources/educational/

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The basic proposition of Western Astrology is that your personality and fate are influenced by the apparent positions and motions of heavenly bodies.

The Twelve Signs of the Zodiac

In Astrological theory, the interesting part of the sky can be divided into twelve roughly equal regions, named for the major groupings of stars that inhabit them. These constellations, all viewable from the Northern Hemisphere, each occupy the Eastern horizon at sunrise for approximately one twelfth of the year, and each in turn forms the background against which the sun rises. They are the Sun Signs or Signs of the Zodiac. Your birthday coincides with one of these signs. If you were born on (say), April 5th, the Constellation of Aries was the sun’s background in your dawn sky, so you are an Aries person. Your behaviour and fortunes throughout your life are affected to a degree by this accident of birth.

Astrologers agree on the dates of the Signs of the Zodiac. (To be an Aries, for example, you must have been born between 21st March and 20th April inclusive, in any year.) They also claim that the positions of the moon and other planets at the time of your birth relative to these same twelve constellations are necessary to fine-tune their predictions.

Some Problems for Astrology

1. No-one has ever demonstrated any physical phenomena which explain how astronomical bodies might affect outcomes in the way that astrologers claim they do. We’ve known since Newton’s time that ALL astronomical bodies attract each other by a force called gravity. We also know that according to the laws of gravity, the effect of distant objects like the planets on us is tiny. Farther objects like stars exert negligible attraction to the Earth. Do the laws of Astrology follow different rules?

2. Serious statistical studies of personality and behaviour do not show any tendency for similar types of people to group according to their birthdates.

3. Why should prognotications based on dates linked to the apparent motion of constellations visible in the northern hemisphere have any validity for those of us who live south of the equator?

4. The dates upon which Astrology is based do not agree with the modern sky. They were relevant 2000 years ago, and  in the Julian, rather than our modern Gregorian Calendar. A small, continuous change to the alignment of the Earth’s axis of rotation has shifted the positions of the constellations appreciably over those 2000 years. It’s called “Precession”, and is rather like the wobble on a spinning top as it slows down.

The number of days that the Sun actually rises in each sign has changed. Virgo has increased from 31 to 45 days, while Scorpio has declined from 30 to just 7 days. Worse still, the Sun now “rises” annually in not twelve, but thirteen different constellations. For eighteen days of the year the Sun rises in Ophiuchus! Modern astrologers are divided between traditionalists who ignore this fact, claiming that only the twelve 2000 year old signs are valid, and “new-agers” who have enthusiastically embraced “The Serpent Holder” as a powerful new influence on human lives. The situation is complicated further if the moon and planets are considered. Astrologers would like the moon and planets to revolve around the Sun in the same plane. They very nearly do. However, slight inclinations in their orbits cause them to pass not only through the thirteen constellations mentioned thus far, but sometimes into Cetus (The Whale) and (rarely) Orion (the Hunter). By this count, the number of Signs is fifteen, rather than twelve.

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Why Be Concerned?
It is often claimed that astrology is just “a bit of fun” and that “no-one seriously believes in it.” Groups like the Skeptics who have concerns over the proliferation of professional astrology into the 21st century are often told to “lighten up.” A brief glance at ads in New Idea or similar magazines might suggest that;
 A lot of people are taking astrologers seriously, and are consulting them.
 At the rates they are charge, astrologers very much wish to be taken seriously. Here’s a list of astrologers, psychics and mediums who were advertising their telephone consultation services in Australian magazines during the month of August 2005. An interesting fact is that the last four on the list had actually themselves well and truly “crossed  over” at the time their services were being advertised.  Many claimed to be “Australia’s Best / Most Accurate Astrologer”.

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We are entitled to ask a couple of questions of those that make a living at astrology. Is it not reasonable to ask each astrologer to provide documentary evidence to support such a claim; consisting of a list of ALL correct AND incorrect predictions made by that person over the last twelve months, so that we can confidently take note of their percentage of accurate predictions?

It does not matter how assertively astrologers are in trumpeting their skill. Nor does it matter whether they honestly believe that they have a special gift. Until they can comply with this simple request for data, should they be taken seriously?

How many astrologers specifically and unambiguously predicted the greatest natural event of recent times, the December 26th 2004 Tsunami before it occurred? We’d suggest none.
Two links:
http://www.astrology.com/ (“The Leading Astrology Website since 1995”)
http://skepdic.com/astrolgy.html (A skeptical look at Astrology with several follow-up links)
Two classroom exercises:
http://skeptics.cafe/2007/02/21/classroom-astrology-exercises/