The Solar System

88 PART 1^ |^ EXPLORING THE SKY

forces and recognize their surprising eff ects. For example, the
moving water in tidal bulges experiences friction with the ocean
beds and resistance as it runs into continents, and that slows
Earth’s rotation and makes the length of a day grow by 0.0023
second per century. Th in layers of silt laid down millions of years
ago where rivers empty into the oceans record tidal cycles as well
as daily, monthly, and annual cycles. Th ose confi rm that only 620
million years ago Earth’s day was less than 22 hours long. Tidal
forces can also aff ect orbital motion. Earth rotates eastward, and
friction with the ocean beds drags the tidal bulges slightly east-
ward out of a direct Earth–moon line. Th ese tidal bulges are
massive, and their gravitational fi eld pulls the moon forward in
its orbit, as shown at the bottom of Figure 5-8. As a result, the

Lunar gravity acting on Earth and its oceans

To sun

To sun

Third quarter

First quarter Neap tides occur when

tides caused by the sun

and moon partially

cancel out.

Spring tides occur when tides

caused by the sun and moon

add together.

Neap tides

are mild.

Spring tides

are extreme.

Diagrams not to scale

Full moon New moon

Gravitational

force of

tidal bulges

Earth’s

rotation

Moon

North Pole

Tidal bulge North Pole

The moon’s gravity

pulls more on the

near side of Earth

than on the far side.

Subtracting off the force

on Earth reveals the

small outward forces that

produce tidal bulges.

Friction with ocean beds

slows Earth and drags its

tidal bulges slightly ahead

(exaggerated here).

Gravity of tidal bulges

pulls the moon forward

and alters its orbit.

moon’s orbit is growing larger by about 3.8 cm a year, an eff ect

that astronomers can measure by bouncing laser beams off

refl ectors left on the lunar surface by the Apollo astronauts.

Earth’s gravitation exerts tidal forces on the moon, and

although there are no bodies of water on the moon, friction

within the fl exing rock has slowed the moon’s rotation to the

point that it now keeps the same face toward Earth.

Newton’s gravitation is much more than just the force that

makes apples fall. In later chapters, you will see how tides can pull

gas away from stars, rip galaxies apart, and melt the interiors of small

moons orbiting near massive planets. Tidal forces produce some of

the most surprising and dramatic processes in the universe.

Astronomy After Newton

Newton published his work in 1687 in a book called Philosophiae

Naturalis Principia Mathematica (Mathematical Principles of

Natural Philosophy), now known simply as Principia (pronounced

Prin-KIP-ee-uh) (■ Figure 5-9). It is one of the most important

books ever written. Principia changed astronomy, changed sci-

ence, and changed the way people think about nature.

■ Figure 5-8

Tides are produced by small differences in the gravitational force

exerted on different parts of an object. The side of Earth nearest

the moon feels a larger force than the side farthest away. Relative

to Earth’s center, small forces are left over, and they cause the

tides. Both the moon and sun produce tides on Earth. Tides can

alter both an object’s rotation and its orbital motion.