Reasons for the seasons notes

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Reasons for the seasons notes

  1. 1. Reasons for the Seasons
  2. 2. True or False? <ul><ul><li>Seasons are caused by the variation in distance between the Sun and the Earth. </li></ul></ul><ul><ul><li>FALSE </li></ul></ul>
  3. 3. Seasons Misconception 1 <ul><li>Many people carry the misconception that the seasons are due to the distance of the Earth from the Sun. </li></ul><ul><li>Consider the following: </li></ul><ul><li>The Earth’s orbit around the Sun is nearly a perfect circle (slightly elliptical). </li></ul><ul><li>The Earth is a little bit closer to the Sun in January </li></ul><ul><li>A little bit farther from the Sun in July. </li></ul>
  4. 4. Rotation vs. Revolution <ul><li>Rotation is the spin of an object on its axis. </li></ul><ul><li>The Earth rotates once a day (24 hours) </li></ul><ul><li>Revolution is the orbit of one object </li></ul><ul><li>around another. </li></ul><ul><li>The Earth revolves around the Sun every </li></ul><ul><li>365.24 days. </li></ul>
  5. 6. Tilt of the Earth’s Axis <ul><li>The Earth rotates on its axis which is tilted 23.5 degrees with respect to the ecliptic . (the plane of the Earth’s orbit.) </li></ul><ul><li>The sun appears to move across the sky 15 o per hour </li></ul><ul><li>Think about how many degrees usually separate time zones (15 o ) </li></ul>
  6. 7. Apparent Motion: Earth’s orbit: Seasons
  7. 8. Tilt of the earth on its axis <ul><li>The earth is tilted 23.5º </li></ul>
  8. 9. Closest/Farthest From the Sun <ul><li>Perihelion ( closest to the Sun ) is around January 5, 2012 when Earth is about 91,405,436 miles from the Sun. </li></ul><ul><li>Aphelion ( furthest from the Sun ) is around July 5, 2012 when Earth is about 94,511,989 miles from the Sun. </li></ul>
  9. 11. Earth: Northern and Southern Hemispheres <ul><li>When it is winter in the Northern hemisphere </li></ul><ul><li>… it is summer in the </li></ul><ul><li> Southern hemisphere . </li></ul><ul><li>If the seasons were due to our distance from the Sun then both hemispheres would have the same seasons at the same time . </li></ul><ul><li>And we know that’s NOT true. </li></ul>
  10. 12. So what causes the seasons? <ul><li>The tilt of the Earth (towards or away from the Sun) affects: </li></ul><ul><li>Insolation : the number of hours of in coming sol ar radi ation </li></ul><ul><li>Angle of Incidence : angle at which the sun’s rays hit the earth. </li></ul>
  11. 13. Note : distance still has nothing to do with it!
  12. 14. Apparent Motion: Earth’s orbit: Seasons <ul><li>When your hemisphere is tilted towards the Sun, it is summer for you. </li></ul><ul><li>Why? </li></ul><ul><li>Your location will experience more hours of sunlight at a higher angle. </li></ul>
  13. 15. <ul><li>When your hemisphere is pointed away from the Sun, it is winter for you. </li></ul><ul><li>Why? </li></ul><ul><li>Your location will experience fewer hours of sunlight at a lower angle. </li></ul>
  14. 16. Check this out! <ul><li>http://astro.unl.edu/naap/motion1/animations/seasons_ecliptic.swf </li></ul><ul><li>And this… </li></ul><ul><li>http:// science.sbcc.edu/physics/flash/LengthofDay.swf </li></ul>
  15. 17. The Earth is curved and this causes differing angles of incidence
  16. 18. Note the relationship between angle and area. higher angle of incidence = higher intensity = higher temperatures over time
  17. 20. Seasonal Example: <ul><li>On June 21st in the Northern hemisphere: </li></ul><ul><li>1. sunlight strikes the ground more vertically than in December. The radiation is spread out over less ground and heats the ground more efficiently . </li></ul><ul><li>2. the Sun is “out” for a longer period of time. </li></ul><ul><li>Therefore, temperature increases. </li></ul>
  18. 22. Summer Solstice <ul><li>On the summer solstice: </li></ul><ul><li>the Sun appears directly overhead (at 90 o ) to an observer at 23.5 degrees north latitude ( Tropic of Cancer) </li></ul>
  19. 23. Winter Solstice <ul><li>On the winter solstice : </li></ul><ul><li>the Sun appears directly overhead (90 o )to an observer at 23.5 degrees south latitude ( Tropic of Capricorn ) </li></ul>
  20. 24. <ul><li>SOLSTICES </li></ul><ul><ul><li>SUMMER, JUNE 21 </li></ul></ul><ul><ul><li>WINTER, DECEMBER 21 </li></ul></ul>Where are the overhead rays of the sun on these days?
  21. 25. Autumnal Equinox <ul><li>On the autumnal equinox : </li></ul><ul><li>The Sun will appear directly overhead at the equator . </li></ul><ul><li>The Sun sets at the North Pole, ending 6 months of day and beginning 6 months of night. </li></ul><ul><li>The Sun rises at the South Pole ending 6 months of night and beginning 6 months of day. </li></ul>
  22. 26. Vernal Equinox <ul><li>On the vernal equinox : </li></ul><ul><li>The Sun will appear directly overhead at the equator . </li></ul><ul><li>The Sun rises at the North Pole, ending 6 months of night and beginning 6 months of day. </li></ul><ul><li>The Sun sets at the South Pole ending 6 months of day and beginning 6 months of night. </li></ul>
  23. 27. <ul><li>EQUINOXES </li></ul><ul><ul><li>VERNAL, MARCH 21 </li></ul></ul><ul><ul><li>AUTUMNAL, SEPTEMBER 21 </li></ul></ul>
  24. 28. <ul><li>Important points : </li></ul><ul><li>Seasons are NOT due to variation in the Earth-Sun distance. </li></ul><ul><li>We have seasons because Earth’s axis is tilted 23.5 degrees. </li></ul><ul><li>This tilt causes variation in the amount of in coming sol ar radi ation ( insolation ) any particular location receives throughout the course of a year. </li></ul><ul><li>The tilt also affects the angle of incidence , the angle of the sun’s rays. </li></ul>

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