Unit 3 Lesson 1 Earth’s Days, Years, and Seasons
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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 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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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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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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Unit 3 Lesson 1 Earth’s Days, Years, and Seasons
What determines the length of a day?
• How does Earth’s rotation make the sun appear to
move across the sky?
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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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Unit 3 Lesson 1 Earth’s Days, Years, and Seasons
What determines the length of a year?
• As Earth rotates on its axis, it revolves around the
sun.
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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 23.5°. 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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Unit 3 Lesson 1 Earth’s Days, Years, and Seasons
What conditions are affected by the tilt
of Earth’s axis?
• At each position of Earth’s orbit, which
hemisphere is experiencing longer hours of
daylight?
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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 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
What conditions are affected by the tilt
of Earth’s axis?
• Compare the area over which the sun’s rays are
spread at the equator and at the poles.
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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 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 equinox, sunlight shines
equally on both poles.
• During the March 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 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 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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Unit 3 Lesson 1 Earth’s Days, Years, and Seasons
What causes seasons?
• At which point in Earth’s orbit does the Northern
Hemisphere experience each season?
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