The Solar System

20 PART 1^ |^ EXPLORING THE SKY

Because of its rotation, Earth has a slight bulge around its
middle, and the gravity of the sun and moon pull on this bulge,
tending to twist Earth’s axis “upright” relative to its orbit. If Earth
were a perfect sphere, it would not get twisted. Notice that gravity
tends to make the top fall over, but it tends to twist Earth upright in
its orbit. In both cases, the twisting of the axis of rotation combines
with the rotation of the object and causes precession. Earth’s axis
precesses, taking about 26,000 years for one cycle (Figure 2-7b).
Because the locations of the celestial poles and equator are
defi ned by Earth’s rotational axis, precession slowly moves these
reference marks. You would notice no change at all from night to
night or year to year, but precise measurements can reveal the

slow precession of the celestial poles and the resulting change in

orientation of the celestial equator.

Over centuries, precession has dramatic eff ects. Egyptian

records show that 4800 years ago the north celestial pole was near

the star Th uban (alpha Draconis). Th e pole is now approaching

Polaris and will be closest to it in about the year 2100. In about

12,000 years, the pole will have moved to within 5° of Vega (alpha

Lyrae). Next time you glance at Favorite Star Vega, remind your-

self that it will someday be a very impressive north star. Figure

2-7c shows the path followed by the north celestial pole.

The Cycles of the Sun

Earth’s rotation on its axis causes the cycle of day and night,

but it is its motion around the sun in its orbit that defi nes the

year. Notice an important distinction. Rotation is the turning of

a body on its axis, but revolution means the motion of a body

around a point outside the body. Consequently, astronomers are

careful to say Earth rotates once a day on its axis and revolves

once a year around the sun.

Because day and night are caused by the rotation of Earth,

your time of day depends on your location on Earth. You can see

this if you watch live international news. It may be lunchtime

where you are, but for a newscaster in the Middle East, it can

already be dark. In ■ Figure 2-8, you can see that four people in

diff erent places on Earth have diff erent times of day.

The Annual Motion of the Sun

Even in the daytime, the sky is fi lled with stars, but the glare of

sunlight fi lls Earth’s atmosphere with scattered light, and you can

see only the brilliant sun. If the sun were fainter, you would be

able to see it rise in the morning in front of the stars. During the

day, you would see the sun and the stars moving westward, and

the sun would eventually set in front of the same stars. If you

watched carefully as the day passed, you would notice that the

sun was creeping slowly eastward against the background of stars.

2-3

■ Figure 2-7

Precession. (a) A spinning top precesses in a conical motion around the
perpendicular to the fl oor because its weight tends to make it fall over.
(b) Earth precesses around the perpendicular to its orbit because the grav-
ity of the sun and moon acting on Earth’s equatorial bulge tend to twist it
upright. (c) Precession causes the north celestial pole to move slowly among
the stars, completing a circle in 26,000 years.

Precession

Vega

Polaris

Thuban

AD 14,000

Path of

north

celestial

pole

Earth’s

orbit

Precession

To Polaris

a

c

b

23.5°

Rotati

on

3000 BC

Sunlight

Midnight

North

Pole

Earth’s

rotation

Sunset

Sunrise

Noon

■ Figure 2-8

Looking down on Earth from above the North Pole shows how the time of

day or night depends on your location on Earth.