Unit 3 Lesson 1
Earth’s Days, Years, and Seasons
Unit 3 Lesson 1 Earth’s Days, Years, and Seasons
Spinning in Circles
What determines the length of a day?
• Each planet spins on its axis. The axis is an
imaginary line that goes through the center of the
planet from the north pole to the south pole.
• The spinning of a body, such as a planet, on its
axis is called rotation.
• The time it takes a planet to complete one full
rotation on its axis is called a day.
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Earth’s rotation on its axis 1 rotation = 1 day
Earth’s rotation is used to
measure the length of a
day on Earth
Unit 3 Lesson 1 Earth’s Days, Years, and Seasons
What determines the length of a day?
• Earth rotates in a counterclockwise motion around
its axis when viewed from above the North Pole.
• As a location on Earth’s equator rotates from west
to east, the sun appears to rise in the east, cross
the sky, and set in the west.
• Only one-half of Earth faces the sun at any given
time.
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Counterclockwise
rotation from
west to east
Unit 3 Lesson 1 Earth’s Days, Years, and Seasons
What determines the length of a day?
• People on the half of Earth facing the sun
experience daylight, and this period is called
daytime.
• People on the half of Earth facing away from the
sun experience darkness, and this period is called
nighttime.
• Earth completes one rotation on its axis in 24
hours, or one day.
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Daytime – half of Earth facing the Sun
Nighttime – half of Earth facing away
from the Sun
Unit 3 Lesson 1 Earth’s Days, Years, and Seasons
What determines the length of a year?
• As Earth rotates on its axis, it also revolves
around the sun.
• The motion of a body that travels around another
body in space is called revolution.
• Earth completes a full revolution around the sun in
365 ¼ days, or about one year. We have divided
the year into 12 months, each lasting 28 to 31
days.
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Earth’s revolution around the Sun
1 revolution = 1 year
Start / End Point
Unit 3 Lesson 1 Earth’s Days, Years, and Seasons
Tilt-a-Whirl
What conditions are affected by the tilt
of Earth’s axis?
• Earth’s axis is tilted at a 23.5°angle. During each
revolution, the North Pole may be tilted toward
the sun or away from the sun.
• When the North Pole is tilted toward the sun, the
Northern Hemisphere has longer periods of
daylight than the Southern Hemisphere has.
• When the North Pole is tilted away from the sun,
the opposite is true.
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The Northern Hemisphere
experiences the opposite
season from the Southern
Hemisphere
Both the tilt of Earth’s
axis and Earth’s
spherical shape play a
role in Earth’s
changing seasons
Unit 3 Lesson 1 Earth’s Days, Years, and Seasons
What conditions are affected by the tilt
of Earth’s axis?
• The angle at which the sun’s rays strike each part
of Earth’s surface changes as Earth moves in its
orbit.
• When the North Pole is tilted toward the sun, the
sun’s rays strike the Northern Hemisphere more
directly, and this region is warmer.
• When the North Pole is tilted away from the sun,
the sun’s rays strike the Northern Hemisphere less
directly, and this region is cooler.
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Unit 3 Lesson 1 Earth’s Days, Years, and Seasons
What conditions are affected by the tilt
of Earth’s axis?
• The spherical shape of Earth also affects how the
sun warms up an area.
• At a point near the equator, the sun’s rays hit
Earth’s surface more directly, so temperatures are
higher there.
• At a point near one of the poles, the sun’s rays hit
Earth’s surface less directly, so temperatures are
lower there.
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Unit 3 Lesson 1 Earth’s Days, Years, and Seasons
The angle at which the Sun’s rays strike Earth and
the spherical shape of Earth affects how the Sun
warms up an area
The equator receives more direct rays from the Sun
because the Sun’s rays are at a right angle and are
focused in a small area. At the poles the rays are
spread out over a larger area. Copyright © Houghton Mifflin Harcourt Publishing Company
Unit 3 Lesson 1 Earth’s Days, Years, and Seasons
What conditions are affected by the tilt
of Earth’s axis?
• The number of daylight hours on any given day of
the year varies with location.
• Areas around Earth’s equator receive about 12
hours of light a day.
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Unit 3 Lesson 1 Earth’s Days, Years, and Seasons
What conditions are affected by the tilt
of Earth’s axis?
• Areas on Earth’s surface that are tilted toward the
sun have longer than 12-hour days, as they travel
a longer path through the lit part of Earth.
• Areas on Earth’s surface that are tilted away from
the sun have shorter than 12-hour days, as they
travel a shorter path through the lit part of Earth.
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Unit 3 Lesson 1 Earth’s Days, Years, and Seasons
What conditions are affected by the tilt
of Earth’s axis?
• Which place has the longest days? Which has the
shortest?
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Unit 3 Lesson 1 Earth’s Days, Years, and Seasons
What conditions are affected by the tilt
of Earth’s axis?
• During summer in the Northern Hemisphere,
areas north of the Arctic Circle receive 24 hours of
daylight.
• At the same time, areas south of the Antarctic
Circle receive 24 hours of darkness.
• In winter in the Northern Hemisphere, the polar
areas experience the reverse conditions.
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Unit 3 Lesson 1 Earth’s Days, Years, and Seasons
Seasons Change …
What causes seasons?
• Most locations on Earth experience seasons. Each
season is characterized by a pattern of
temperature and other weather trends.
• We experience seasons due to changes in the
intensity of sunlight and the number of daylight
hours as Earth revolves around the sun.
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Unit 3 Lesson 1 Earth’s Days, Years, and Seasons
What causes seasons?
• At an equinox, sunlight shines equally on the
Northern and Southern hemispheres. Half of each
hemisphere is lit, and half is in darkness.
• As Earth moves along its orbit, the sunlight
reaches more of one hemisphere than the other.
• At a solstice, the area of sunlight is at a
maximum in one hemisphere and at a minimum in
the other hemisphere.
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Unit 3 Lesson 1 Earth’s Days, Years, and Seasons
What causes seasons?
• During the September (autumnal/fall) equinox,
sunlight shines equally on both poles.
• During the March (vernal/spring) equinox, half of
each hemisphere is lit, and the sunlight is
centered on the equator.
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Unit 3 Lesson 1 Earth’s Days, Years, and Seasons
What causes seasons?
• At the December (winter) solstice, the North Pole
leans away from the sun and is in complete
darkness, and the South Pole is in complete
sunlight.
• At the June (summer) solstice, the North Pole
leans toward the sun and is in complete sunlight,
and the South Pole is in complete darkness.
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2014 Equinoxes and Solstices
• Vernal / Spring Equinox – March 20th
• Summer Solstice – June 21st
• Autumnal / Fall Equinox – September 23rd
• Winter Solstice – December 21st