24.2 The Sun and the Seasons
Earth is constantly
moving in two ways.
A Earth rotates around
its axis once a day.
B Earth revolves around
the sun once a year.
Axis of rotation
Tilt of 23.5°
Direction of
rotation
Axis passes
through poles
Earth’s orbit
Sun
24.2 The Sun and the Seasons
What are two ways in which Earth moves?
Earth moves in two major ways: rotation and
revolution.
24.2 The Sun and the Seasons
Earth spins like a top around an imaginary
line, or axis, that connects the north and
south geographic poles.
• The spinning of Earth on its axis is called
rotation.
• Earth’s rotation causes day and night as the
sun appears to move from east to west across
the sky.
• It takes one day for Earth to complete one
rotation.
24.2 The Sun and the Seasons
As Earth rotates on its axis, it also travels
around the sun.
• Revolution is the movement of one body in
space around another.
• It takes Earth one year, about 365¼ days, to
make a complete revolution around the sun.
24.2 The Sun and the Seasons
Earth’s Latitude Zones
How is Earth’s surface divided into zones
based on latitude?
Latitude measures distance in degrees north or
south of the equator.
Scientists use lines of latitude to mark out
three different types of regions within which
temperatures are generally similar: the
tropic, temperate, and polar zones.
24.2 The Sun and the Seasons
Earth’s Latitude Zones
Different parts of Earth receive different
amounts of sunlight, depending on their
latitude.
Regions near the equator receive more direct
sunlight than the poles, so Earth is generally
warmer near the equator and colder toward
the poles.
24.2 The Sun and the Seasons
Earth’s Latitude Zones
The angle at which
sunlight strikes
Earth’s surface
varies with latitude.
As a result, it is
generally warmer
near the equator
than near the
poles.
Polar zone
Polar zone
24.2 The Sun and the Seasons
Earth’s Latitude Zones
Between the latitudes of 23.5° south and
23.5° north is the tropic zone, where Earth is
generally warm.
From 23.5° north to 66.5° north and from
23.5° south to 66.5° south are the temperate
zones, which are generally cooler than the
tropics.
From 66.5° north to the North Pole and from
66.5° south to the South Pole are the polar
zones, which are generally cold.
24.2 The Sun and the Seasons
The Seasons
What causes the seasons?
The seasons are caused by the tilt of Earth’s
axis as it moves around the sun.
24.2 The Sun and the Seasons
The Seasons
Earth’s axis of rotation is not straight up and
down.
• The axis tilts at an angle of about 23.5° relative
to a line perpendicular to its orbital path.
• As Earth orbits the sun, the north end of Earth’s
axis points in the same direction, but the
orientation of Earth’s axis changes relative to
the sun over the course of a year.
24.2 The Sun and the Seasons
The Seasons
Solstices
Because Earth’s axis is tilted, the latitude at which
the noon sun appears directly overhead changes
in a yearly cycle.
A solstice occurs on the two days each year
when the sun is directly overhead at latitude 23.5°
north or 23.5° south.
24.2 The Sun and the Seasons
The Seasons
In the Northern Hemisphere, the summer
solstice marks the beginning of summer,
around June 21.
The Northern Hemisphere is tilted toward the
sun, so it receives more sunlight than the
Southern Hemisphere.
On this day, daylight hours are longest in the
Northern Hemisphere and shortest in the
Southern Hemisphere.
24.2 The Sun and the Seasons
The Seasons
Six months later, about December 21, the
winter solstice marks the beginning of winter
in the Northern Hemisphere.
The Northern Hemisphere is tilted away from
the sun. so the Northern Hemisphere
receives less sunlight than the Southern
Hemisphere.
Daylight hours are shortest in the Northern
Hemisphere but longest in the Southern
Hemisphere.
24.2 The Sun and the Seasons
The Seasons
Equinoxes
Halfway between the solstices are two days,
called equinoxes, which mark the beginning of
spring and autumn.
At an equinox, neither hemisphere is tilted toward
the sun, and the lengths of daylight and of
darkness are approximately equal.
24.2 The Sun and the Seasons
The Seasons
The vernal equinox, about March 21 in the
Northern Hemisphere, marks the start of
spring.
The autumnal equinox, about September 22
in the Northern Hemisphere, marks the start
of autumn.
On the equinoxes, the noon sun is directly
overhead at the equator.
Sunlight reaches the Northern and Southern
Hemispheres in equal amounts.
24.2 The Sun and the Seasons
The Seasons
The seasons are caused by the tilt of Earth’s
axis as it revolves around the sun.
24.2 The Sun and the Seasons
Assessment Questions
1. What causes the seasons?
a.
b.
c.
d.
Earth’s orbit about the sun
Earth’s rotation about its axis
the atmosphere
the tilt of Earth’s axis as it moves around the sun
24.2 The Sun and the Seasons
Assessment Questions
1. What causes the seasons?
a.
b.
c.
d.
Earth’s orbit about the sun
Earth’s rotation about its axis
the atmosphere
the tilt of Earth’s axis as it moves around the sun
ANS: D
24.2 The Sun and the Seasons
Assessment Questions
2. Which of the Earth’s latitude zones receives the
most direct sunlight?
a.
b.
c.
d.
the tropic zone
the temperate zone
the polar zone
They all receive the same amount of direct sunlight.
24.2 The Sun and the Seasons
Assessment Questions
2. Which of the Earth’s latitude zones receives the
most direct sunlight?
a.
b.
c.
d.
the tropic zone
the temperate zone
the polar zone
They all receive the same amount of direct sunlight.
ANS: A