The Solar System

PART 1 | EXPLORING THE SKY

North

Pole

A satellite above

Earth’s atmosphere

feels no friction and

will fall around

Earth indefinitely.

Earth satellites eventually

fall back to Earth if they

orbit too low and experience

friction with the upper

atmosphere.

Earth satellites eventually

fall back to Earth if they

orbit too low and experience

friction with the upper

atmosphere.

Straight line motion

Motion of the moon

toward

Earth

Curved path of

moon’s orbit

A Geosynchronous Satellite

Earth

The satellite remains

fixed above a spot on

Earth’s equator.

At a distance of 42,250

km (26,260 miles) from

Earth’s center, a satellite

orbits with a period of

24 hours.

The satellite orbits

eastward, and Earth

rotates eastward

under the moving

satellite.

You can understand orbital motion by thinking of
a cannonball falling around Earth in a circular
path. Imagine a cannon on a high mountain aimed
horizontally as shown at right. A little gunpowder
gives the cannonball a low velocity, and it doesn’t
travel very far before falling to Earth. More
gunpowder gives the cannonball a higher
velocity, and it travels farther. With enough
gunpowder, the cannonball travels so fast it
never strikes the ground. Earth’s gravity pulls
it toward Earth’s center, but Earth’s surface
curves away from it at the same rate it falls.
It is in orbit. The velocity needed to stay in a
circular orbit is called the circular velocity.
Just above Earth’s atmosphere, circular
velocity is 7780 m/s or about 17,400 miles
per hour, and the orbital period is about 90
minutes.

1

1a

1b

Ageosynchronous satellite orbits
eastward with the rotation of Earth
and remains above a fixed spot — ideal for
communications and weather satellites.

According to Newton’s first law of motion, the moon

should follow a straight line and leave Earth forever.

Because it follows a curve, Newton knew that some force must

continuously accelerate it toward Earth — gravity. Each second

the moon moves 1020 m (3350 ft) eastward and falls about 1.6

mm (about 1/16 inch) toward Earth. The combination of these

motions produces the moon’s curved orbit. The

moon is falling.