Isaac Newton postulated that there is a force of attraction between any two bodies in the Universe. Your News story 'Gravity passes a little test' (Nature 446, 31–32; 2007) points out that “Isaac Newton's inverse-square law of gravity has given faultless service ever since”. I have found a curious way to use Newton's law to draw attention to the difference between science and anti-science for a general audience.
I teach an introductory science class at my university, which typically enrols many non-science majors. During a lecture on the gravitational force, I imply that if planets such as Mars exert a force on any object, including humans, then perhaps there is something to astrology's idea that celestial bodies exert a force of influence on our lives. I encourage my students to undertake a test I have designed for this notion.
I present the students with 12 randomly numbered horoscopes from the previous day, with the corresponding signs of the zodiac removed. I ask each student to record the horoscope that best describes the day she or he had, and the astrological sign (for example, Aries) corresponding to her/his birthday. My scientific hypothesis is that planets may exert a force on our bodies, but it is purely random — 1 out of 12 (8.3%) — whether a horoscope foretells the events of one's life.
I am pleased to report that, as Shawn Carlson has noted, “astrology failed to perform at a level better than chance” (Nature 318, 419–425; 1985). The results from my classes are: 8.0% (n = 163 students), 8.4% (n = 155), 7.0% (n = 143), 8.0% (n = 138) and 8.0% (n = 100). In other words, as John Maddox has commented “astrology is a pack of lies ... There is no evidence that the positions of the planets can affect human behaviour” (Nature 368, 185; 1994).
I encourage science teachers to try this approach when they are presented with an opportunity, as this exercise inspires genuine scientific inquiry. For example, students have countered that a certain astrologer may not be qualified to read the stars. I have addressed this question by using horoscopes from different newspapers (Chicago Tribune, Columbus Dispatch, Los Angeles Times and Washington Post) and online sources (Astrology.com, DailyHoroscopes.com). Other students have noted that small groups, with only two students of a particular sign, may obtain a result that is significantly greater than 8.3%. This presents an opportunity to discuss the value of an adequate sample size.
Finally, it is worth reporting that my students are so engaged by this exercise that they actually want to use Newton's law of universal gravitation to calculate force values. In case you are curious, Mars, at its present distance of 264 million kilometres from Earth, is exerting a force of approximately 50 nanonewtons on your being.