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CONCEPTS Applied

Sunrise, Sunset
Make a chart to show the time of sunrise and sunset for
a month. Calculate the amount of daylight and darkness.
Does the sunrise change in step with the sunset, or do
they change differently? What models can you think of that
would explain all your findings?

ROTATION
Observing the apparent turning of the celestial sphere once a
day and seeing the east-to-west movement of the Sun, Moon,
and stars, one could certainly get the impression that it is the
heavenly bodies and not Earth doing the moving. You can-
not sense any movement, and there is little apparent evidence FIGURE 16.8 As is being demonstrated in this old woodcut,
that Earth indeed moves. Evidence of a moving Earth comes Foucault’s insight helped people understand that Earth turns. The
from at least three different observations: (1) the observation pendulum moves back and forth without changing its direction of
that the other planets and the Sun rotate, (2) the observation movement, and we know this is true because no forces are involved.
of the changing plane of a long, heavy pendulum at different We turn with Earth, and this makes the pendulum appear to change
latitudes on Earth, and (3) the observation of the direction of its plane of rotation. Thus, we know Earth rotates.
travel of something moving across, but above, Earth’s surface,
such as a rocket.
Other planets, such as Jupiter, and the Sun can be observed
to rotate by keeping track of features on the surface such as the
Great Red Spot on Jupiter and sunspots on the Sun. While such
observations are not direct evidence that Earth also rotates, they
do show that other members of the solar system spin on their
axes. As described earlier, Jupiter is also observed to be oblate,
flattened at its poles with an equatorial bulge. Since Earth is also
oblate, this is again indirect evidence that it rotates, too.
The most easily obtained and convincing evidence about
Earth’s rotation comes from a Foucault pendulum, a heavy mass
swinging from a long wire. This pendulum is named after the
FIGURE 16.9 The Foucault pendulum swings back and forth
French physicist Jean Foucault, who first used a long pendu-
in the same plane while a stool is turned beneath it. Likewise, a
lum in 1851 to prove that Earth rotates. Foucault started a long,
Foucault pendulum on Earth’s surface swings back and forth in the
heavy pendulum moving just above the floor, marking the plane same plane while Earth turns beneath it. The amount of turning
of its back-and-forth movement. Over some period of time, the observed depends on the latitude of the pendulum.
pendulum appeared to slowly change its position, smoothly
shifting its plane of rotation. Science museums often show this
shifting plane of movement by setting up small objects for the would appear to turn as you rotate with the turning stool. This
pendulum to knock down. Foucault demonstrated that the pen- is what happens on Earth. Such a pendulum at the North Pole
dulum actually maintains its plane of movement in space (iner- would make a complete turn in about 24 hours. Moving south
tia) while Earth rotates eastward (counterclockwise) under the from the North Pole, the change decreases with latitude until,
pendulum. It is Earth that turns under the pendulum, causing at the equator, the pendulum would not appear to turn at all.
the pendulum to appear to change its plane of rotation. It is dif- At higher latitudes, the plane of the pendulum appears to move
ficult to imagine the pendulum con tinuing to move in a fixed clockwise in the Northern Hemisphere and counter clockwise
direction in space while Earth, and everyone on it, turns under in the Southern Hemisphere.
the swinging pendulum (Figure 16.8). More evidence that Earth rotates is provided by objects that
Figure 16.9 illustrates the concept of the Foucault pendu- move above and across Earth’s surface. As shown in Figure 16.10,
lum. A pendulum is attached to a support on a stool that is Earth has a greater rotational velocity at the equator than at
free to rotate. If the stool is slowly turned while the pendu- the poles. As an object leaves the surface and moves north or
lum is swinging, you will observe that the pendulum main- south, the surface has a different rotational velocity, so it ro-
tains its plane of rotation while the stool turns under it. If you tates beneath the object as it proceeds in a straight line. This
were much smaller and looking from below the pendulum, it gives the moving object an apparent deflection to the right of

410 CHAPTER 16 Earth in Space 16-6