Page 28 - 4670

The crust is made up of rock and soil and constitutes the top 25 miles
(40 kilometers) of the mantle.
Geophysicists use seismic waves to learn about the interior of
the Earth. Below its mantle lies the core. It is about 4,400 miles (7,100

kilometers) across. The core has a solid inner core and a molten outer
core.

Figure 2.5. The Earth’s rotation. Perihelion and Aphelion

Fossils study proved that the Earth was very old. Scientists now

measure the Earth’s age from radioactive elements. By knowing how
long these take to decay, geologists can calculate the age of the rocks
or minerals they are in. Scientists got to know that rocks were formed

billions of years ago and believe the Earth itself to be about 4.6 billion
years old. People in ancient times believed that Earth stood still, but
they were mistaken. The planet spins once every 23 hours, 56 minutes,

4.09 seconds. In 1851 the French scientist Jean-Bernard-Léon
Foucault proved this with a pendulum. He attached a heavy, pointed
weight to a long rope that was suspended from a tall ceiling over a
layer of sand. As the pendulum swung back and forth, the point

marked its motion in the sand on the floor. The markings followed
different directions. The Foucault pendulum showed that the Earth
must be turning.

It is the Earth’s rotation that causes day and night. It makes air
currents turn to the right in the Northern Hemisphere and to the left in

28