CHAPTER 2 | THE SKY 25
Th e cycles of the sky are so impressive that it is not surprising
that people have strong feelings about them. Ancient peoples saw
the motion of the sun around the ecliptic as a powerful infl uence
on their daily lives, and the motion of the planets along the eclip-
tic seemed similarly meaningful. Th e ancient superstition of
astrology is based on the cycles of the sun and planets around the
sky. You have probably heard of the zodiac, a band around the sky
extending 9 degrees above and below the ecliptic. Th e signs of the
zodiac take their names from the 12 principal constellations along
the ecliptic. A horoscope is just a diagram showing the location
of the sun, moon, and planets around the ecliptic and their posi-
tion above or below the horizon for a given date and time.
Centuries ago, astrology was an important part of astronomy, but
the two are now almost exact opposites—astronomy is a science
that depends on evidence, and astrology is a superstition that
survives in spite of evidence (How Do We Know? 2-2). Th e
signs of the zodiac are no longer important in astronomy.Astronomical
Infl uences on Earth’s
ClimateThe seasons are produced by the annual motion of Earth
around the sun, but subtle changes in that motion can have dra-
matic aff ects on climate. You don’t notice these changes during2-4
your lifetime; but, over thousands of years, they can bury conti-
nents under glaciers.
Earth has gone through ice ages when the worldwide cli-
mate was cooler and dryer and thick layers of ice covered polar
regions, extending partway to the equator. Between ice ages,
Earth is warmer and there are no ice sheets even at the poles.
Scientists have found evidence of at least four ice ages in
Earth’s past. One occurred 2.5 billion years ago, but the other
three that have been identifi ed have all occurred in the last
billion years. Th ere were probably others, but evidence of early
ice ages is usually erased by more recent ice sheets. Th e lengths
of ice ages range from a few tens of millions of years to a few
hundred million years. Th e most recent ice age began only
about 3 million years ago and is still going on. You are living
during one of the periodic episodes during an ice age when
Earth grows slightly warmer and the glaciers melt back and do
not extend as far from the poles. Th e current warm period
began about 12,000 years ago.
Ice ages seem to occur with a period of roughly 250 million
years, and glaciers advance and melt back during ice ages in a
complicated pattern that involves cycles of about 40,000 years
and 100,000 years. (Th ese cycles have no connection with global
warming, which can produce changes in Earth’s climate over just
a few decades. Global warming is discussed in Chapter 20.)
Evidence shows that these slow cycles of the ice ages have an
astronomical origin.The So-Called Scientifi c Method
What is the difference between a science and
a pseudoscience? Astronomers have a low opin-
ion of beliefs such as astrology, mostly because
they are groundless but also because they pre-
tend to be a science. They are pseudosciences,
from the Greek pseudo, meaning false.
A pseudoscience is a set of beliefs that
appear to be based on scientifi c ideas but that
fail to obey the most basic rules of science.
For example, in the 1970s a claim was made
that pyramidal shapes focus cosmic forces on
anything underneath and might even have
healing properties. For example, it was claimed
that a pyramid made of paper, plastic, or other
materials would preserve fruit, sharpen razor
blades, and do other miraculous things. Many
books promoted the idea of the special power
of pyramids, and this idea led to a popular fad.
A key characteristic of science is that its
claims can be tested and verifi ed. In this
case, simple experiments showed that any
shape, not just a pyramid, protects a piece of
fruit from airborne spores and allows it to drywithout rotting. Likewise, any shape allows
oxidation to improve the cutting edge of a
razor blade. Because experimental evidence
contradicted the claim and because supporters
of the theory declined to abandon or revise
their claims, you can recognize pyramid power
as a pseudoscience. Disregard of contradictory
evidence and alternate theories is a sure sign
of a pseudoscience.
Pseudoscientifi c claims can be self-fulfi ll-
ing. For example, some believers in pyramid
power slept under pyramidal tents to improve
their rest. There is no logical mechanism by
which such a tent could affect a sleeper, but
because people wanted and expected the claim
to be true they reported that they slept more
soundly. Vague claims based on personal tes-
timony that cannot be tested are another sign
of a pseudoscience.
Astrology is another pseudoscience. It has
been tested over and over for centuries, and it
doesn’t work. Nevertheless, many people believe
in astrology despite contradictory evidence.Astrology may be the oldest pseudoscience.Many pseudosciences appeal to our need to
understand and control the world around us.
Some such claims involve medical cures, ranging
from using magnetic bracelets and crystals to
focus mystical power to astonishingly expensive,
illegal, and dangerous treatments for cancer.
Logic is a stranger to pseudoscience, but human
fears and needs are not.