The Solar System

CHAPTER 2 | THE SKY 21

The Seasons

Earth would not experience seasons if it rotated upright in its

orbit, but because its axis of rotation is tipped 23.4° from the

perpendicular to its orbit, it has seasons. Study The Cycle of

the Seasons on pages 22–23 and notice two important

principles and six new terms:

Because Earth’s axis of rotation is inclined 23.4°, the sun moves

into the northern sky in the spring and into the southern sky

in the fall. Th at causes the cycle of the seasons. Notice how the

vernal equinox, the summer solstice, the autumnal equinox, and

the winter solstice mark the beginning of the seasons. Further,

notice the very minor eff ects of Earth’s slightly elliptical orbit

as it travels from perihelion to aphelion.

Earth goes through a cycle of seasons because of changes in

the amount of solar energy that Earth’s northern and south-

ern hemispheres receive at diff erent times of the year. Because

of circulation patterns in Earth’s atmosphere, the northern

and southern hemispheres are mostly isolated from each

other and exchange little heat. When one hemisphere receives

more solar energy than the other, it grows rapidly warmer.

Notice that the seasons in Earth’s southern hemisphere are

reversed with respect to those in the northern hemisphere.

Australia and other lands in the southern hemisphere experience

1

2

It would move a distance roughly equal to its own diameter
between sunrise and sunset. Th is motion is caused by the motion
of Earth in its nearly circular orbit around the sun.
For example, in January, you would see the sun in front of
the constellation Sagittarius (■ Figure 2-9). As Earth moves along
its circular orbit, the sun appears to move eastward among the
stars. By March, you would see it in front of Aquarius.
Although people often say the sun is “in Sagittarius” or “in
Aquarius,” it isn’t really correct to say the sun is “in” a constella-
tion. Th e sun is only 1 AU away, and the stars visible in the sky
are at least a million times more distant. Nevertheless, in March
of each year, the sun crosses in front of the stars that make up
Aquarius, and people conventionally use the expression, “Th e
sun is in Aquarius.”
Th e apparent path of the sun against the background of stars
is called the ecliptic. If the sky were a great screen, the ecliptic
would be the shadow cast by Earth’s orbit. Th at is why the eclip-
tic is often called the projection of Earth’s orbit on the sky.
Earth circles the sun in 365.25 days and, consequently, the
sun appears to circle the sky in the same period. Th at means the
sun, traveling 360° around the ecliptic in 365.25 days, travels
about 1° eastward in 24 hours, about twice its angular diameter.
You don’t notice this apparent motion of the sun because you
can’t see the stars in the daytime, but it does have an important
consequence that you do notice—the seasons.

■ Figure 2-9

Earth’s orbit is a nearly perfect circle, but it is shown in an inclined view in this diagram and consequently looks oval. Earth’s motion around the
sun makes the sun appear to move against the background of the stars. Earth’s circular orbit is thus projected on the sky as the circular path of
the sun, the ecliptic. If you could see the stars in the daytime, you would notice the sun crossing in front of the distant constellations as Earth
moves along its orbit.

Capricornus

Sagittarius

Scorpius

Libra

Virgo

Projection of Earth’s

orbit — the ecliptic

Leo

Gemini Cancer

Taurus

Aries

Pisces

Aquarius

Sun

January 1

Earth’s orbit

March 1

View from Earth

on March 1

View from Earth

on January 1

Sun

Sun