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Figure 2.1. Shape of the Earth

Fossils convinced scientists that the Earth was very old. They
learned that the oldest known rocks were formed about 4.2 billion

years ago. Most of people believed that about one hundred billion
years ago the Earth, the Sun, and all the planets of the Solar System
were nothing but a cloud of cold dust particles swirling through empty
space. Gradually, these particles were attracted to each other and came

together to form a huge spinning disk. Then the disk separated into
rings and the furious motion made the particles white-hot. The center
of the disk became the sun, and the particles in the outer rings turned

into large balls of gas and molten liquid that cooled and condensed to
take on solid form. Four or five billion years ago, they turned into the
planets that we know today as the Earth, Mars, Venus, and others.

The Earth’s rocky core formed first, with heavy elements
colliding and binding together. Dense material sank to the center,
while the lighter pieces created the crust. The planet’s magnetic field

probably formed around this time. Gravity captured some of the
lighter elements that made up the planet's early atmosphere.
People in ancient times also believed that the Earth was flat and
stood still. They thought that the Sun, the Moon, and the stars moved

around us. It is now known that the Earth turns on its axis. The planet
spins once every 23 hours, 56 minutes, 4.09 seconds. As the Earth
rotates, it also revolves around the Sun. The journey takes 365 days, 6

hours, 9 minutes, 10 seconds. This is called a sidereal (star) year.
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