CHAPTER 2 | THE SKY 17
Precession
Over 2000 years ago, Hipparchus compared a few of his star
positions with those recorded nearly two centuries earlier and
realized that the celestial poles and equator were slowly moving
across the sky. Later astronomers understood that this motion is
caused by the top-like motion of Earth.
If you have ever played with a gyroscope or top, you have
seen how the spinning mass resists any sudden change in the
direction of its axis of rotation. Th e more massive the top and
the more rapidly it spins, the more it resists your eff orts
to twist it out of position. But you probably recall that even
the most rapidly spinning top slowly swings its axis around
in a circle... Th e weight of the top tends to make it tip over,
and this combines with its rapid rotation to make its axis
sweep out the shape of a cone in a motion called precession
(■ Figure 2-7a). In later chapters, you will learn that many
celestial bodies precess.
Earth spins like a giant top, but it does not spin upright in its
orbit; it is tipped 23.5° from vertical. Earth’s large mass and rapid
rotation keep its axis of rotation pointed toward a spot near the star
Polaris, and the axis would not wander if it were not for precession.science is to understand nature, not to memorize defi nitions.
Study the diagrams and see how the geometry of the celestial
sphere and its motions produce the sky you see above you.
Th e celestial sphere is an example of a scientifi c model, a
common feature of scientifi c thought (How Do We Know?
2-1). Notice that a scientifi c model does not have to be true to
be useful. You will encounter many scientifi c models in the chap-
ters that follow, and you will discover that some of the most useful
models are highly simplifi ed descriptions of the true facts.
Th is is a good time to eliminate a couple of Common
Misconceptions. Lots of people, without thinking about it
much, assume that the stars are not in the sky during the day-
time. Th e stars are actually there day and night; they are just
invisible during the day because the sky is lit up by sunlight.
Also, many people insist that Favorite Star Polaris is the brightest
star in the sky. It is actually the 51st visually brightest star. You
now know that Polaris is important because of its position, not
because of its brightness.
In addition to causing the obvious daily motion of the sky,
Earth’s rotation conceals a very slow celestial motion that can be
detected only over centuries.The So-Called Scientifi c Method
How can a scientifi c model be useful if it
isn’t entirely true? A scientifi c model is a
carefully devised conception of how something
works, a framework that helps scientists think
about some aspect of nature, just as the celes-
tial sphere helps astronomers think about the
motions of the sky.
Chemists, for example, use colored balls
to represent atoms and sticks to repre-
sent the bonds between them, kind of like
Tinkertoys. Using these molecular models,
chemists can see the three-dimensional
shape of molecules and understand how the
atoms interconnect. The molecular model of
DNA proposed by Watson and Crick in 1953
led to our modern understanding of the
mechanisms of genetics. You have prob-
ably seen elaborate ball-and-stick models of
DNA, but does the molecule really look like
Tinkertoys? No, but the model is both simple
enough and accurate enough to help scien-
tists think about their theories.A scientifi c model is not a statement of
truth; it does not have to be precisely true
to be useful. In an idealized model, some
complex aspects of nature can be simplifi ed
or omitted. The ball-and-stick model of a
molecule doesn’t show the relative strength
of the chemical bonds, for instance. A model
gives scientists a way to think about some
aspect of nature but need not be true in every
detail.
When you use a scientifi c model, it is
important to remember the limitations of
that model. If you begin to think of a model
as true, it can be misleading instead of
helpful. The celestial sphere, for instance,
can help you think about the sky, but you
must remember that it is only a model. The
universe is much larger and much more inter-
esting than this ancient scientifi c model of
the heavens.2-1 Scientifi c Models
Balls represent atoms and rods represent chemical
bonds in this model of a DNA molecule.
(Digital Vision/Getty Images)