1. 1 The Reason for the Seasons Page 520 For this activity, students should copy the underlined information for their notes.

2. 2 What Causes Seasons? Seasons are caused by the tilt of the Earth’s axis as it moves around the sun—the Earth’s axis does not move! It only appears to move as the Earth changes position as it orbits the Sun.

3. 3 The Reason for the Seasons Seasons happen because the Earth is tilted on its axis at a 23.5 º angle. The tilt affects the amount of sunlight—solar energy—the Earth receives as it moves around the sun.

4. 4 Why does the Earth Tilt? No one knows why the Earth's axis is tilted by 23.5º. Some astronomers think that about 5 billion years ago, when the Earth was still very young, it was struck by a Mars-sized planet. If it tilted too much, the seasons would be very extreme—like on the planet Uranus where the winter lasts for 42 years in total darkness!

5. 5 Our 4 Seasons Spring Summer Winter Fall Areas near the equator have summer all year long since they get 12 hours of daylight every day of the year. Vacation anyone?

6. 6 Summer in the Northern Hemisphere—where you live During our summer months, the N. Hem. has warmer temperatures & longer days because it is tilted toward the sun. Seasons in the S. Hem. are always the opposite of our seasons (we live in the N. Hem.) due to the Earth’s tilt.

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8. 8 Winter in the Northern Hemisphere During our winter months, the N. Hem. is tilted away from the sun, temperatures are lower & days are shorter. Seasons in the S. Hem. are always the opposite of our seasons due to the Earth’s tilt.

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10. 10 Seasons and The Equator The equator receives generally the same amount of energy from the sun all year round.  Areas near the equator don’t really have seasons.  At the equator the Sun’s rays stay at the same angle all year. This means that the length of day temperature doesn't vary as much. So countries near the equator only have seasons based on weather or what fruit is in season—pineapple season, rainy season, etc.

11. 11 Seasons and Weather Different seasons bring different types of weather. This weather is caused by the uneven heating & cooling of the Earth’s atmosphere.

12. 12 Solstices The solstices are days when the Sun reaches its farthest points north and south of the equator.

13. 13 Winter Solstice The winter solstice occurs on December 21 or 22. It marks the beginning of winter. This is the shortest day of the year in terms of hours of daylight.

14. 14 Summer Solstice The summer solstice occurs on June 20 or 21 and marks the beginning of summer. This is the longest day of the year in terms of daylight.

15. 15 Equinoxes Equinoxes are days in which day & night are equal in length. Equinoxes  12 hours of daylight & 12 hours of darkness

16. 16 Spring/Vernal Equinox The vernal equinox/spring equinox occurs in late March—around March 20 or 21 st. Beginning of spring in the N. Hem. & the beginning of fall in the S. Hem.

17. 17 Fall/Autumnal Equinox The autumnal equinox/fall equinox occurs in late September on the 21 or 22. Beginning of fall in the N. Hem. & the beginning of spring in the S. Hem.

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19. 19 QUIZ 1.Name the four seasons. 2. Are the Earth's seasons caused by the differences in the distance from the Sun throughout the year or the tilt of the Earth's axis? 3. What is the tilt of the Earth's axis (in degrees)? 4. During which season do the Sun's rays hit the Earth at the most direct angle? 5. During which season are the days the shortest?

20. 20 Quiz Continued 6. What is the name of the shortest day of the year (and the beginning of winter)? 7. What is the name of the longest day of the year (and the beginning of summer)? 8. What is the name of a day in which the day and night are of equal duration? 9. How many times each year do we have days in which the day and night are of equal duration? 10. What are the names of each of these days?

21. 21 ANSWERS 1. Name the four seasons. summer, autumn (fall), winter, spring 2. Are the Earth's seasons caused by the differences in the distance from the Sun throughout the year or the tilt of the Earth's axis? The tilt of the Earth's axis 3. What is the tilt of the Earth's axis (in degrees)? 23.45 degrees 4. During which season do the Sun's rays hit the Earth at the most direct angle? summer 5. During which season are the days the shortest? winter

22. 22 Answers Continued 6. What is the name of the shortest day of the year (and the beginning of winter)? winter solstice 7. What is the name of the longest day of the year (and the beginning of summer)? summer solstice 8. What is the name of a day in which the day and night are of equal duration? an equinox 9. How many times each year do we have days in which the day and night are of equal duration? 2 times 10. What are the names of each of these days? the vernal equinox and the autumnal equinox

23. 23 Earth Viewer http://www.fourmilab.ch/earthview/vpla net.html http://www.fourmilab.ch/earthview/vpla net.html