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By Neil deGrasse Tyson

May 1994

Chapter 12 from Universe Down to Earth

The Sun is among hundreds of billions of stars in the disk of our own Milky Way spiral galaxy. The stars in the Sun’s vicinity comprise what is politely called the solar neighborhood. Some stars emit copious amounts of light while others emit much less. Some are near and some are far. Yet all stars present the illusion that they are firmly embedded in the “dome” of the night sky. Any two stars form a line. Any three stars form a triangle. Imagine the endless shapes and patterns that can be envisioned among the six thousand stars visible to the unaided eye.

Over five millennia ago, in the days before evening television, the Babylonians and Chaldeans kept accurate records of the night sky. They assigned icons of the local mythology to the planets and the various star patterns. Between AD 127 and AD 150, the Greek philosopher Ptolemy catalogued many of the familiar names upon these skeletal star patterns that are otherwise known as constellations. Thrust upon these patterns were not only the names of animals and gods but their quirky behavioral traits as well.

Over five millennia after Ptolemy, in the days of space travel, recombinant DNA, and microwave popcorn there are people who believe that planets and star patterns influence their life’s events in ways derivable from the mythology of sleepless and TV-less Babylonians.

The basis for modern day astrology is quite simple. Its premise is that the relative positions of the Sun, Moon, planets and constellations affect you and the events in your life—especially your social and financial life. Note that in a free, capitalistic society these aspects can be the totality of one’s life. All planets of the solar system orbit the Sun in roughly the same plane. Consequently, it makes sense that when viewed from Earth, the motion of the planets, Moon, and Sun appear to be restricted to a relatively narrow band across the entire sky. The exact path of the Sun is centered in this band and is known as the ecliptic. As Earth orbits the Sun we see the Sun travel eastward along this ecliptic until one year has passed when the Sun returns to its initial position. If Earth’s rotation axis were not tilted, the ecliptic would coincide with the projection of Earth’s equator on the celestial sphere. Our 23½ degree tilt, however, creates what is tongue-twistingly called the “obliquity of the ecliptic.” This obliquity grants two special points to the Sun’s yearly journey across the sky. They are where the ecliptic crosses the projection of Earth’s equator. They are commonly called the first day of spring (vernal equinox) and the first day of autumn (autumnal equinox). The first day of spring is referred by astrologers as the “first point of Aries.” This officially begins the Sun’s journey through the 12 constellations of the zodiac. If your astrological sign is Aries, this means the Sun was passing through the constellation Aries when you were born. Following Aries on the calendar are Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius, and Pisces.

Many astrologers say they can predict your behavior and personality based on your sign in conjunction with the signs that the planets and Moon happen to be in when you were born. It would be wrong to say that the planetary positions in the sky have no effect on human behavior. Planets do reflect light from the Sun and do have gravitational forces. Both factors (the only quantities measurable from Earth) have the potential to affect objects at a distance. But before we jump to conclusions you should know that if we calculate the force of gravity of Mars on you when you were born you will discover that the force of gravity from the obstetrician or midwife who delivered you was 150 times greater than that of Mars.

This leaves light as a last hope for the effects of Mars on your birth. If we presume optimum conditions—you were born on a clear night by an open window in a hospital that happened to have Mars in the sky in view from the delivery table—then we may discuss the effects of its light. We discover here that the six high intensity lamps over your delivery table produce 160 billion times more light on you than the light from Mars. It is not clear why national cults never emerged that would predict your fate based on how the obstetrician was positioned at your birth, or whether GE bulbs were used instead of Westinghouse bulbs.

Astrologers generally agree that the most important astrological effect is the Sun’s location on the zodiac at your time of birth inasmuch as it defines your “sign.” But those who believe in the Sun’s positional influences will have to contend with a sobering fact: the first point of Aries today no longer coincides with the first day of spring. This is a far-reaching twist of the same magnitude as if Alex Haley, author of his well-known genealogy Roots, had later discovered that he was an adopted child. The Sun at the vernal equinox is currently in the constellation Pisces and will soon be in Aquarius. Among people who read their daily horoscope it is not widely known that Earth wobbles on its axis. This is normal behavior for any oblate, spinning, tilted top under the influence of an external gravity so we should not be surprised that the spinning tilted Earth also wobbles on its axis under the influence of the Sun and Moon. The time for one complete Earth wobble is about 25,700 years and its effects are manifested by the drift of the vernal equinox through the entire zodiac over a 25,700 year period. Ptolemy named the constellations about 2,000 years ago. We see that since then, the equinox has traveled nearly one-twelfth the way around the zodiac—one complete sign. What this means is that if you thought you were an Aries, you are really a Pisces; if you thought you were a Pisces, you are really an Aquarius, and so forth. Matters are worsened upon learning that the boundaries of the constellations do not split the zodiac evenly—some astrological signs “last” longer than others. Matters are worsened further upon recognizing that seven of the twelve constellations are feeble skeletal excuses for the animals and objects they are purported to be.

An amusing addition to the above hijinks is that the zodiac contains fourteen constellations, not twelve. The Sun, after leaving the constellation Scorpius, enters the constellation Ophiuchus. It then stays in Ophiuchus for a longer period of time than Scorpius, the sign that is advertised to precede Sagittarius. The confusing conclusion is that most Scorpions are actually Ophiuchans and all Scorpions and Ophiuchans are currently Librans. The fourteenth constellation in the set is Cetus. It is a large constellation that dips into Pisces. The Sun passes through Cetus briefly as it ambles through Pisces but you are not normally informed of this in the horoscope pages.

Some astrologers (typically the expensive ones) are actually aware of these astronomical truths. A common response, when confronted with the facts, is the assertion that the effects of the stars were set 2,000 years ago, and still apply today. I once tested a daily (syndicated) horoscope from the newspaper on the 50 students in one of my introductory astronomy classes. Rather than have the students read their horoscope and decide whether it applied to that day’s dilemmas, I picked one of the twelve horoscopes at random and read it to the class. I then asked all students to declare whether it was “unlikely”, “possible”, or “likely” that I had just read the their own horoscope. Fully one third (17) of the class declared that the horoscope was “likely” to be their own. The class was astonished to learn that the horoscope I read belonged to none of these people. Of the ten people who responded “unlikely”, the horoscope actually belonged to three of them. Controlled experiments such as this one, consistently demonstrate that daily horoscopes would do no worse if they were laid on the page at random, yet horoscope casting in the United States remains the most lucrative industry among the pseudo-sciences.

A subject of fascination and confusion for many people is the effect of the full Moon on human behavior. It is commonly thought that more babies are born during full moons than during any other phase. It is also thought by many people that the full moon has some mystical effect on the human psyche that forces people to behave strangely, to commit crimes, or to transmutate into a howling and hairy canine. Literature abounds with stories of werewolves and other moon-induced human behavior. I have heard some people explain these phenomena with the tidal effects of the Moon on the brain: …since the oceans are water and are duly influenced by the lunar tides, then the large water content of the human body should also be affected.

Before we jump to cosmic conclusions, consider the following: 1) If the oceans were 100 percent nitroglycerin (or 100 percent anything else) they would still exhibit tides. 2) Tidal forces of the Moon are, indeed, large during full moon. But they are also large two weeks later during the new moon. This phase of the moon cannot be observed. Nobody sees it. Nobody writes lycanthropic stories about it. 3) Tidal forces of the Moon are measured by the difference in gravity between the side of Earth closest to the Moon and the side of Earth farthest from the Moon. If your skull were 7,000 miles across (the size of Earth) then the lunar tides would, indeed, give you an oblong-shaped head, with untold consequences on your mental facilities. But since your skull is only about eight inches across, the tidal force that you “feel” is quite small. Indeed, the weight of a down-filled bed pillow placed upon your head will produce a force that is seven trillion times larger than that of the Moon’s tidal force on your head. The next time somebody tries to blame a bio-cosmic-lunar connection for irresponsible behavior, perhaps we should first blame the influence of creative literature. And then possibly the pillow.

Let us return to the birth rates during the full moon. The average human gestation period is about 267 days1, which is an excellent match with the 265½ days in nine cycles of lunar phases. What this means is that babies who are born during a full moon are very likely to have been conceived during a full moon. And nobody will argue the romantic effects of a moon-lit evening.

In a free society, intellectual enlightenment is your best defense against misguided claims in the name of science. Only then can society, as a whole, cultivate a scientifically literate public.

  • 1 You get this from a commonly invoked, yet convoluted formula: Add seven days to the first day of your last menstrual cycle before becoming pregnant. Count backwards three months, and then add a year. If we assume a 28 day menstrual cycle, and conception during ovulation, then this formula will give you 267 days.