1. The Reasons for Seasons
Copyright 2005 by the Rector and Visitors of the University of Virginia
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The Seasons

2. Day Night Cycle
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3. Rotation vs. Revolution
Rotation is the spin of the Earth around its axis.
The Earth rotates once a day (every 24 hours).
Revolution is the orbit of one object around another.
The Earth revolves around the Sun every 365 days.
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4. Tilt of the Earth’s Axis
The axis around which the Earth rotates is tilted by 23.5 degrees
This is what causes seasons.
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5. 4/8/2019
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6. Tilt of the Earth’s Axis
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7. Equinoxes and Solstices
The Vernal (Spring) Equinox (March) Equal day and night
The Summer Solstice (June)
Longest day of the year
The Autumnal (Fall) Equinox (Sept.) Equal day and night
The Winter Solstice (December) Shortest day of the year
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8. Midnight Sun
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9. Earth on June 21
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10. Seasons Misconception
Many people think that the seasons are due to the distance of the Earth from the Sun. However:
The Earth’s orbit around the Sun is nearly a perfect circle. The Earth is slightly closer to the Sun in January and farther from the Sun in July.
If the seasons were due to our distance from the Sun both hemispheres would have the same seasons at the same time.
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11. Orbit of the Earth
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12. The Seasons
In fact, the seasons are due to the tilt of the Earth’s axis. Consider what happens on June 21 when the northern hemisphere of the Earth is tilted toward the Sun:
The sunlight strikes the ground more vertically than in December. The light is spread out over less ground and heats the ground better.
The Sun is above the horizon for a longer period of time.
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13. Earth on June 22
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14. Earth on December 22
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15. A.M. and P.M. At midday, the Sun is on your meridian.
This occurs close to, or at, noon.
A.M. comes from ante meridiem (before midday)
P.M. comes from post meridiem (after midday)
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16. Daylight Saving Time
During the late spring, summer, and early fall, we set out clocks ahead to have an extra hour of daylight at the end of the day.
This change in time is called Daylight Saving Time.
The idea of changing our clocks was first used in the United States during World War I to conserve energy.
Since 1986, the United States has set our clocks one hour ahead on the first Sunday in April and one hour behind on the last Sunday in October.
Spring ahead, fall back.
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17. animations Seasons use code cfp-5012
Cosmic Map

18. Now, see what you know about the seasons!