1. The Reasons for Seasons
Edward M. Murphy
Space Science for Teachers
2005
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 an object about its axis.
The Earth rotates once a day (once every 24 hours).
Revolution is the orbit of one object around another.
The Earth revolves around the Sun every days.
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4. The Zodiac
As the Earth revolves around the sun the different constellations of the Zodiac appear in the sky
What are the 12 signs of the zodiac?
Pieces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricorn, Aquarius
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6. Tilt of the Earth’s Axis
The axis around which the Earth rotates is tilted by 23.5 degrees with respect to the ecliptic.
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8. Tilt of the Earth’s Axis
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9. Tropics
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10. Equinoxes and Solstices
The Vernal (Spring) Equinox
(about March 21):
1st day of spring
Every place on earth has 12 hrs day light
Sun is directly over equator at 12 noon
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11. Equinoxes and Solstices
The Summer Solstice (about June 21):
1st day of summer
Longest day of the year
Sun is over tropic of Cancer
Sun does not set above arctic circle
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12. Earth on June 21
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13. Equinoxes and Solstices
The Autumnal (Fall) Equinox (about September 21):
1st day of Fall
Every place on earth has 12 hrs day light
Sun is directly over equator at 12 noon
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14. Equinoxes and Solstices
The Winter Solstice
(about December 21):
1st day of winter
shortest day of the year
Sun is over tropic of Capricorn
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15. Earth on December 21
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16. Motion on the Ecliptic
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18. Standing on the North Pole
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19. Midnight Sun
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20. Standing on the Tropic of Cancer
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21. Standing on the Equator
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22. Seasons Misconception
Many people carry the misconception that the seasons are due to the distance of the Earth from the Sun. However, consider the following facts:
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.
Perihelion (closest to the Sun) is around January 3 when Earth is about 91,405,436 miles from the Sun.
Aphelion (farthest from the Sun) is around July 4 when Earth is about 94,511,989 miles from the Sun.
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23. Seasons Misconception
While it is winter in the Northern hemisphere it is summer in the Southern hemisphere.
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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24. Orbit of the Earth
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26. The Seasons
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27. The Seasons Seasons are due to the tilt of the Earth’s axis. Summer:
The sunlight 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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28. Solar Illumination
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29. Earth on June 22
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30. Earth on December 22
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31. Time:
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32. Day Night Cycle
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33. Why is there Time Zones?
Problem: If the sun travels slowly across the US technically each city will have a different time
Imagine starting in Charlottesville at exactly noon.
As you travel to the west, the Sun will appear further east in the sky (i.e. lower and further from the sun).
Even if you travel only a few miles west, the Sun gets lower in the horizon.
Each city would have its own time.
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34. Time Zones
With the advent of rapid travel by trains in the 19th century, it became necessary to standardize the time for all cities within a certain region.
In November 1883, the railroad companies divided the United States into four time zones.
Everyone in a time zone set their clocks to the same standard time.
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35. Time Zones
In 1884, an international conference was held in Washington D.C. by 26 countries.
The world was divided into 24 times zones, with each zone being roughly 15 degrees wide in longitude.
Divided into 24 time zones one for each hour of the day.
Since there are 24 hours in a day, and 360/15=24, the time in each zone differs from the time in adjacent zones by one hour.
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36. International Date Line
Standard time gets earlier as you travel to the west.
The International Date Line line was established in the middle of the Pacific Ocean.
As you go from east to west, you gain a day as you cross the line.
As you go from west to east, you lose a day as you cross the line.
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37. Time Zones
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38. Daylight Saving Time.. Why?
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. Now is used so people can make better use of time in the afternoons
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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39. Tilt of the Earth’s Axis
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