1. Earth-Moon-Sun Cycles

2. Cycle # 1 Day and Night Day  Night  Day  Night

3. We have day and night because the Earth ROTATES on its AXIS. The axis is an imaginary line around which Earth spins, causing day and night, that is drawn from the north geographic pole through Earth to the south geographic pole. The spinning of Earth on its axis is called rotation. It takes 24 hours for Earth to complete one rotation.

4. As the Earth spins, part of the Earth is facing toward the Sun, and part is facing away from the Sun.

5. The side of the Earth facing the Sun has day. The side of the earth facing away from the Sun has night.

6. As the Earth rotates… The side of the Earth in the sunlight spins away and becomes night. The side of the Earth in the dark spins toward the Sun and becomes day.

7. Cycle #2 The Length of Days Long  Longer  Longest  Longer  Long  Short  Shorter  Shortest  Short  Shorter  Shortest  Long  Longer  Longest

8. Why does the length of day change? The Earth moves, or REVOLVES around the Sun. One REVOLUTION takes about 365 ¼ days, or one year, to complete. This changes the way sunlight hits the Earth.

9. Hours of Daylight The number of hours of daylight is different at different times of the year. The number of hours of daylight is different at different latitudes.

10. Earth’s Tilted Axis: The earth's axis is tilted about 23.45 degrees, so the Earth is never straight up and down compared to the Sun. This tilt, along with latitude, effects the length of day.

11. Changing Length of Day & Night The hemisphere that is tilted toward the Sun gets more hours of daylight than the hemisphere tilted away from the Sun. Latitudes closest to the equator have less change in hours of daylight. Latitudes further from the equator have more dramatic changes in hours of daylight.

12. What is the effect of the Earth’s tilt on the length of a day? Part of the year, the Earth's Northern Hemisphere is tilted mostly toward the Sun, six months later, the Northern Hemisphere is tilted mostly away from the Sun.

13. What is the effect of the Earth’s tilt on the length of a day? The same is true for the Southern Hemisphere, at opposite times of year – Part of the year, the Earth's Southern Hemisphere is tilted mostly toward the Sun, six months later, the Southern Hemisphere is tilted mostly away from the Sun.

14. In the Summer and Winter SOLSTICE During the winter the Northern Hemisphere day lasts fewer than 12 hours and the Southern Hemisphere day lasts more than 12 hours. During the winter solstice, the North Pole has a 24-hour night and the South Pole has a 24-hour day.

15. In the Summer and Winter SOLSTICE During the summer solstice the Northern Hemisphere day lasts more than 12 hours and the Southern Hemisphere day lasts fewer than 12 hours. During the summer, the North Pole has a 24-hour day and the South Pole has a 24-hour night.

16. In the Spring and Fall EQUINOX A day lasts 12 hours and a night lasts 12 hours at all latitudes. Sunlight strikes the Earth most directly at the equator.

18. In Summary … Days are longer (more hours of daylight) in summer, shorter in winter. There are greater differences in day length further away from the Equator. There is not much change in day length near the Equator.

19. Cycle # 3 The Seasons Winter  Spring  Summer  Fall  Winter  Spring  Summer

20. Seasons A regular change in temperature that repeats itself every year.

21. Seasons are caused by: The TILT of the Earth. The position of Earth in space. The difference in DIRECT and INDIRECT light from the Sun.

22. Seasons As the Earth revolves around the Sun, the tilt sometimes points toward the Sun, sometimes away from the Sun, and sometimes neither away nor toward the Sun.

24. Tilt-Light-Position: In WINTER: –Part of the Earth is tilted AWAY from the Sun. –Part of the Earth gets INDIRECT energy from the Sun. In SUMMER: –Part of the Earth is tilted TOWARD the Sun. –Part of the Earth gets DIRECT energy from the Sun. The position of the Earth in space determines which part of the planet is tilted toward the Sun.

25. Solstice Time when the Sun reaches its greatest distance north or south of the equator, which occurs June 21 or 22 for the northern hemisphere (longest day of the year) and December 21 or 22 for the southern hemisphere (shortest day of the year).

26. Equinox Twice-yearly time when the Sun is directly above Earth's equator and the length of day equals the length of night worldwide.

27. Seasons: Summer (Northern Hemisphere) Position of the Earth is such that the northern hemisphere is tilted toward the Sun, allowing more of the Sun’s energy to heat the Northern Hemisphere. Summer Solstice (1 st day of summer): –Sun at highest point (over Tropic of Cancer) –Most hours of sunlight (longest day) –About June 21

29. Seasons: Fall (Northern Hemisphere) Position of the Earth is such that the northern hemisphere is tilted neither towards nor away from the Sun. Autumnal Equinox (1 st day of fall): –About 12 hours of sunlight –About September 22

31. Seasons: Winter (Northern Hemisphere) Position of the Earth is such that the Northern Hemisphere is tilted away from the Sun, which lowers the amount of the sun’s energy warming the Northern Hemisphere. Winter Solstice (1 st day of winter): –Sun at lowest point (over Tropic of Capricorn) –Fewest hours of sunlight (shortest day) –About December 21

33. Seasons: Spring (Northern Hemisphere) Position of the Earth is such that the northern hemisphere is tilted neither towards nor away from the Sun. Vernal Equinox (1 st day of spring): –About 12 hours of sunlight –About March 21

35. Summing Up: Winter - Earth tilted away from Sun, fewer hours of sunlight, less direct sunlight (insolation) Spring – Earth neither tilted away nor toward the Sun, equal hours of daylight and night Summer – Earth tilted toward the Sun, more hours of daylight, more direct sunlight (insolation) Fall - Spring – Earth neither tilted away nor toward the Sun, equal hours of daylight and night

36. Conclusion: The seasons are the result of this tilt of the Earth's axis. If the tilt of the Earth's axis was 0° there would be no difference in how the rays from the sun hit its different regions, and there would be no seasons.

37. Review Look closely at where the Sun is hitting the Earth during each season: http://Search.Lycos.com/setup.asp?r=5&src=clear2&query=weather+savvy

38. http://www.enchantedlearning.com/subjects/astronomy/planets/earth/Seasons.shtml

39. http://www.nmm.ac.uk/uploads/gif/seasons-full.gif