"I don't believe in astrology; I'm a Sagittarius and we're skeptical."
--Sir Arthur C. Clarke
Astrology and astronomy came from the same roots. The motion of the planets against the stars attracted attention and interest. Astronomy diverged from astrology when someone began trying to figure out what was actually going on. This was the beginning of a long history of progress in knowledge, otherwise known as science. Two millennia ago the Earth was considered to be the center of everything. We know different now. Astronomy has made enormous progress. Distances to nearby stars were first measured in the 19th century. Understanding of galaxies began to develop in the 1920s. Astronomical knowledge exploded in the last half of the 20th century. There's progress in every area. The new X-ray and infrared telescopes are opening new windows for studying the universe.

Astrology is still casting horoscopes and trying to predict your future, just as it was 2,000 years ago. There is no real progress of knowledge.

We can now ask some questions of astrology. From a scientific viewpoint these are legitimate questions.

  • The Greeks were not aware of Uranus, Neptune, or Pluto and seemed not to have any problem making predictions. Uranus was discovered in 1781, Neptune in 1846 and Pluto in 1930. Does this mean that horoscopes cast before these discoveries are wrong?

  • Why don't different astrologers agree in their predictions? Look at two different newspapers -- the predictions may overlap or differ wildly.

  • Can astrologers work backwards? Given your state of life now, can they tell when you were born, your sign?

  • What is the influence? Electromagnetic? Gravitational? The obstetrician present at my birth had more gravitational influence on me than Mars did. Electric and magnetic forces tend to cancel because the charges and poles have opposite signs. Electromagnetic radiation (light) from Mars is blocked by walls. Strong and weak nuclear forces are short range -- limited to the nucleus 1/100,000 of the atom. Some new force? Why can't it be detected?

  • Does the influence decrease with distance (inverse square like gravity?) If not, then what about undiscovered solar planets? What about extra-solar planets? Asteroids? Kuiper belt objects? Comets in the Oort Cloud? Satellites (natural and artificial)?

  • Twins, triplets, etc. are born in the same place and almost the same time. Often their lives diverge. Is timing of birth to the nearest millisecond important? Who knows this much information about their birth? Parturition is a gradual process. How can anyone predict anything that depends so precisely on time? This is indistinguishable from randomness.

  • Astrological signs give people the property of the sign. Taurus the bull is strong, but bull-headed (obstinate). Look at these stars. Do you see a bull?

  • The Earth's rotational axis precesses with a 26,000 year period. The Sun is no longer in your astrological sign on the date of your birth.

Astrology - Science or Pseudoscience?

    Statement by Russian scientists (equally applicable here).

    Astrology is quite old, dating back to ancient Babylon. In the earliest form, astrology consisted of watching the motions of the planets and trying to read omens from them. By the first century CE, astrology was Hellenized (Greekified) and had taken the general form we know today. This included the development of natal horoscopes, which used a plot of the positions of the Sun, Moon, and visible planets to divine the future of a child. A complex (and mind-bending) set of rules described various possible combinations of planets alignments and their influences. Some alignments could produce great kings, others great generals.

    In principle, plotting a natal horoscope is simple. Positions of the Sun, Moon and planets are looked up in an ephemeris (listing of positions) and plotted on a horoscope form. The appropriate symbols for the Zodiac signs and Solar System objects are used in the plot.

    You can get a detailed horoscope blank from this site. Do so and examine it. It appears VERY scientific. Look at the amount of data required. Resort to an ephemeris is necessary. In fact, there are special astrological ephemerides for the purpose.

    The blank contains a LARGE number of places where data must be filled in. They start with date, time and place of birth. From that the local Sidereal time is computed (sidereal time is time measured according to the stars instead of the Sun. The sidereal day is about 24 hours 56 minutes long by the clock). Planet positions are looked up and added, along with their aspects (relationship in the sky). Positions are plotted on a circular diagram which has places for the 12 Zodiac signs and the 12 houses. If it looks complicated, that's because it is!

    Once all of this is plotted, the astrologer interprets the meaning of the configuration. This is where the hand-waving starts. Different astrologers will interpret the chart differently.

    You saw two horoscope columns from two different newspapers. These columns are syndicated nationally; each will appear in MANY newspapers. We chose the Pisces horoscope and found that they disagree. In fact, they rarely are in sync with each other. Two different astrologers, two different horoscopes.

    Ancient astrologers, or magi (plural of magus), could make predictions that worked out fairly well. If you use existing knowledge wisely you can make some good predictions. For example, if the king was ailing, the astrologer could predict that the king will die next year and have a good chance of being right. Magi were NOT stupid!

  • 5 - people
  • 4 - people
  • 3 - people
  • 2 - people
  • 1 - people

Behavior with Distance

    How does this force behave with distance? All known forces behave according to the inverse square law, which is a result of the 3-dimensional space we live in. What this means is that the force is proportional to 1/r². For grins, we will compare the gravitational forces exerted on a newborn baby by the planet Mars (at closest approach to Earth) and the doctor delivering the baby. Here are the basic data needed.
    • Mass of Mars = 6.42 x 1023 kg
    • Mass of doctor = 80 kg
    • Distance of Mars = 5.4 x 1010 m (at closest approach)
    • Distance of doctor = 0.25 m
    Newton's law for gravity is: F = (G x m1 x m2)/r². If we divide the force from Mars by the force from the doctor, we'll get a ratio and a simpler computation. It reduces to:
            6.42 x 1023
            (5.4 x 1010)2
    Use a calculator to reduce this and it becomes approximately 221/1280, or a ratio of about 0.172. This means that the gravitational force from the doctor is just under 6 times the gravitational force from Mars, and that's with Mars at its CLOSEST approach to Earth!

    You can see how the MUCH larger distance to Mars reduces its gravitational influence. That inverse square law makes a difference. Astrology may claim that the planetary influences don't diminish with distance, although no force that acts this way has ever been seen. If the force does not diminish with distance, don't we have to account for the influences of the newly discovered extrasolar planets, more than 100 of which have been found in the last 10 years? Do we need to consider asteroids? Of ALL sizes?

Precession of the Equinoxes

    In the ancient world, the 12 signs of astrology were defined by the 12 constellations of the Zodiac. Those constellations lie along the Sun's annual path through the sky. The sun sign of Scorpius lay in the constellation Scorpius, and the same for the other signs. This is no longer true. In the 2nd century BCE, Hipparchos discovered a very slow change in the sky. We call it precession, and it is a wobbling motion of the Earth's axis caused mostly by the Moon's gravity. This wobble takes about 26,000 years for one cycle, which is why you haven't noticed it.

    This wobble causes a number of effects in the sky. Although we have a very nice North Star (Polaris) now, it is only temporary and will not last. In ancient Egyptian times, 4,800 years ago, the star Thuban in Draco was the north star. The pole happens to be near Polaris right now, but it is slowly and steadily moving, and in 2,000 years will be well away from Polaris.

    Another significant effect precession causes is the constant westward shift of seasonal positions in the sky. For example, the Sun just passed (on 23 Sep) the autumnal equinox, where the Sun's path crosses the celestial equator to mark the beginning of autumn. That point now lies in the constellation Virgo, but 2000+ years ago it was in the constellation Libra, one sign to the east of Virgo. This provides a problem for astrology - the sun sign of Libra now lies over Virgo. This will get worse, sliding one sign every 2,200 years. Astrology must now ignore the constellations as they no longer correspond to the sun signs. So what are the sun signs, anyway?


    Sometimes the star pattern of a constellation doesn't look like the name.

    The constellation Bootes, the herdsman, looks much more like an ice cream cone than a herdsman. Finally, Sagittarius the Archer doesn't look anything like a centaur holding a bow and arrow but makes a perfectly fine teapot.

Impel, not Compel

    Finally, we introduced a statement common in astrology - "The stars IMPEL, they do not COMPEL." This means that the stars and planets produce influences but do not FORCE you to follow those influences. You have "free will" and can choose other paths. If you ask Google for

    "stars impel" "not compel"

    you will get a large number of hits where you can read about this idea. To us, this looks like one of the characteristics of pseudoscience - the built-in excuse for failure. The astrologer can't lose! If the prediction is realized, then astrology has succeeded; if it fails, then you chose something else. Astrology wins either way.

    To find out more about astrology, try You can find definitions of the planetary aspects, the twelve houses and the twelve signs.

    For your own edification, you might try comparing astrology to the other characteristics of pseudoscience we discussed in the previous class.

Video: Cosmos - Episode 3 - Harmony of the Worlds, by Carl Sagan (0:00:40)


Bizarro by Dan Piraro 26 August 2007