From Abracadabra to Zombies
In a nutshell: Astrology is the idea that the stars, planets, and other objects in the sky shape who you are and what your life will be like. The science doesn't favor this idea.
Astrology is a kind of fortune telling based on the positions of stars, planets, and other objects in the deep sky (called celestial objects). Astrologers believe that the position of celestial objects affect what kind of personality you have and also cause such things as forest fires, floods, volcanoes, earthquakes, hurricanes, and tornadoes.
Astrology is sometimes confused with astronomy, which is the scientific study of planets, the Sun, stars, comets, galaxies, and other things outside of the Earth's atmosphere. Astronomers know that celestial objects like the Moon and the Sun affect the ocean tides but have nothing to do with what kind of personality you have. Astronomers also know that the position of celestial objects has nothing to do with such things as forest fires, floods, volcanoes, earthquakes, hurricanes, and tornadoes.
Astronomers make predictions about things like eclipses and when and where in the sky comets will appear. Their predictions are based on thousands of years of observations and laws of nature that have been discovered. One of the things astronomers have found is that the Earth revolves around the Sun. We all know, however, that the Earth seems to be standing still while the Sun seems to be moving across the sky around the Earth.
Thousands of years ago in Babylon (now in modern Iraq) astrologers charted out the path that the Sun seems to make around the Earth. They divided the path into twelve equal parts. The stars in some of the twelve parts of the sky looked like the outlines of animals to them. The chart is called a zodiac (Greek for circle of animals). I was born when the Sun was in a part of the sky where the stars looked sort of like the outline of a bull's head to those who were charting the zodiac. According to Sun sign astrology, I am a Taurus. The bull has a reputation for being stubborn. Sun sign astrologers believe that those born under the sign of the bull will have bull-like qualities. Scientific skeptics think this idea is a bunch of bull. When I'm asked what sign I was born under, I say "St. Joseph's Hospital."
Astronomers know that the stars are moving within our galaxy (the Milky Way), and that the billions of galaxies that make up our universe are also moving through space. Astronomers know that the stars we see are vast distances from us and that some are much farther away than others. A bright star that is closer to Earth might be much smaller than a dim star that is much farther away. The stars that are seen from Earth as the outlines of Leo the lion or Pisces the fish may be billions of miles away from each other.
In addition to Sun sign astrology, there are several other astrologies, but none of them are scientific. All systems of astrology make unscientific claims about the effects on people or things on Earth by the positions of planets, comets, and other things in the sky. Some of these systems are very complicated and involve making charts that connect the positions of many different celestial objects. With many charts covering many different possibilities, it is easy to find one that matches something on Earth like a forest fire, an earthquake, or a tragedy in some famous person's life.
It is also easier than most people think to find evidence in support of a strong belief. We pay attention to what agrees with our beliefs and ignore what goes against them. If you believe that people born under the sign of Taurus are stubborn, you might pay more attention to stubborn behavior by someone born under that sign. You might even call that person stubborn for not doing something, while calling another person firm for not doing the same thing. You might not pay attention to or remember a Taurus when she isn't stubborn. Psychologists call this natural bias we have to confirm our beliefs confirmation bias.
Astrologers sometimes make accurate predictions that are really nothing more than lucky guesses. Many astrologers believe that comets, the alignment of planets, or an eclipse are omens that something terrible is going to happen on Earth. Since something terrible happens on this planet every day, it is always easy to find something after the fact that the astrologer can say was predicted. Before the disastrous earthquake and tsunami that hit Japan on March 11, 2011, some astrologers predicted that something awful was in store for us because on March 19, 2011, the Moon would be the closest it's been to Earth in 18 years. Even though the so-called "supermoon" happened a week after the earthquake and tsunami, some astrologers claimed the two events were connected.
Sometimes astrologers predict events that are actually likely to happen anyway, such as predicting an earthquake in California. (During a recent 10-year period, California averaged over 37,000 earthquakes a year. That's about 100 earthquakes a day.) Sometimes astrologers "predict" events that have already happened. After a fire at a movie studio in Hollywood, one astrologer claimed that Mars is in Leo and approaching conjunction with Ketu (a point where the paths of the Sun and Moon intersect). Leo, she believes, has "a fiery nature." It is common for astrologers to believe that planets have personalities. Mars, named after the Roman god of war, is associated with such things as aggression and ambition.
Many astrologers make predictions based on sympathetic magic, not science. Sympathetic magic is the superstitious belief that like affects like. Sympathetic magic is the basis for most forms of fortune telling. The fortune teller tries to predict the future by looking at lines, shapes, and patterns in stars, tea leaves left in a cup (if they look like a heart, love may be involved), the palm of the hand (the longer the lifeline, the longer the life), or even the guts of animals. These lines and shapes are believed to be magically connected to past, present, and future things. There is no scientific support for such a belief.
You might wonder why astrology and other forms of fortune telling are so popular since they're not based on scientific evidence. There are many reasons ancient superstitions have such holding power on the imagination.
Astrology and other forms of fortune telling are popular because they give people a sense of order and control in their lives. They give us a sense that things happen for a reason. They connect us to the "heavens" and other mysterious forces. They give us a sense of being connected to everything in the universe. Most of all, though, fortune telling relieves some of the stress we might have about the uncertainty of the future. It gives us a false sense of security that we can predict the future. Finally, it's fun. We have fun trying to make sense out of our fortunes, whether we get them from a fortune cookie, a newspaper horoscope, or a family tea-leaf reader.
Humans evolved over millions of years. Our brains evolved to be very good at finding patterns and connections. Seeing the connections between things helped humans survive. Maybe we see too many connections. Maybe we see patterns where there aren't any. The chances for survival of the human race would not have been very good if we never thought there might be a connection between two things until we made absolutely sure we were right. Had we evolved to be super-skeptics who must have absolute certainty before we act, the tigers in the bush might have eaten our ancestors as they wondered if it was just the wind blowing the leaves.
So, we might say that superstitions like astrology survive because the human race had a better chance of surviving with them than without them.
For more scientific information on planets, stars, and other objects beyond the sky see:
Amazing Space (uses the Hubble Space Telescope's discoveries to inspire and educate about the wonders of our universe)
Understanding the Universe (an exploration with the American Museum of Natural History).