The Reason for the Seasons

1. The Reason for the Seasons School's Out! Brrrrrrrrrr! Leaves go down!

2. Here’s Mars… Where would you guess that it is COLDEST on Mars? …you’re right. But why? And how did you know? If you guessed here… …and here…

3. The key is that Planets are… ROUND

4. light Half of Jupiter lit by the Sun …and sunlight comes in from just one direction

5. Venus, lit by the Sun It’s all about the angle at which the light hits the planet! Look at a ray of sunlight hitting the middle of the planet. The ray hits this part of the planet directly at a 90 degree angle. It focuses the sun’s energy (heat) over a relatively small area.

6. the direct ray of light But at the north & south pole? Here comes a second ray from the sun toward the south pole of Venus. Notice it hits Earth’s surface at a angle that is much less than 90 degrees. Notice how much LARGER an area the ray covers…this means it’s LESS concentrated (or more spread out).

7. So direct light heats planets up more than indirect light! YUP! Around the middleof the planet, the sun’s rays hit Earth directly, so the energy is more concentrated. There is more energy per unit area and the Earth heats up a relatively greater amount. But at the top and bottom of the planet, the light comes in at a less direct angle, meaning the energy is less concentrated. It is spread over a greater area and therefore does not warm the planet as much.

8. So that explains why… Up here we have…

9. …and… Around here we have…

10. I’m cold …and… Down here we have…

11. This helps, but it doesn’t explain the seasons! So far we only know why the hottest part of each planet is around the middle…the sunlight hits that part most directly. If that was all that happened, though, we WOULDN’T have different seasons…just the same boring weather all year long. In Texas, it’d be about 90 degrees every day of the year with occasional rain. Not bad, but definitely BORING.

12. This is the key, so repeat after me… It’s… the… TILT

13. The Earth doesn’t sit perfectly straight up and down A “false” view of Earth rotating on its axis. This just isn’t right!

14. #13 Our Axis is TILTED! TILT About23.5 degrees

15. A more accurate view… Notice the… TILT

16. COLD medium HOT medium COLD Remember from earlier… …That the middle of the planet gets the most direct sunlight (heat).

17. Direct light = Summer! So one half of the Earth “tilts” toward the sun! What does the tilt do? Indirect down here = Winter! Which hemisphere of Earth (Northern or Southern) is having Summer & Winter in this diagram?

18. What does this mean for Texas? When our Northern Hemisphere tilts toward the Sun, we actually get nearly the most direct sunlight of anywhere on the planet! No wonder we get over 100 degrees so often in July & August!

19. Summer down here! So…how do we get Winter here??? That happens when our (Northern) hemisphere tilts away from the sun.

20. And the final piece to the puzzle? So, the TILT is a big part of it, but something else must cause us to tilt either away or toward the Sun at different times during the year… …and that last bit is something you’ve known for years! The Earth revolves around the Sun! Watch the animation…pay attention to the direction Earth’s axis points as it orbits the Sun.

21. So it’s a combination of the TILTed axis and our revolving around the sun! Yeah!!!

22. Complete the table 4 1 3 A hemisphere is experiencing summer if it is tilted directly toward the sun. A hemisphere is experiencing winter if it is tilted directly away from the sun. 2

23. Seasons Animation • http://esminfo.prenhall.com/science/geoanimations/animations/01_EarthSun_E2.html

24. Misconceptions about the Seasons 1. We experience seasons because Earth is closer to the sun in the summer and farther in the winter. WRONG Actually, the opposite is true. We are closer to the sun in the winter (91million miles away) than we are in the winter (94 million miles away). If this were true, wouldn’t Hawaii experience snow?

25. Notice the distance during the summer!

26. Why do we experience day and night? The Earth is… Rotating

27. Sunlight Sunlight Day versus Night When where you are is pointed toward the Sun, it is day. Then the Earth rotates you away from the Sun, and it is night. Daytime Nighttime

28. Day and Night as the Earth is rotating on its AXIS

29. Northern Hemisphere Summer Southern Hemisphere Winter Hemispheres experiencing summer receive MORE DIRECT RAYS from the sun than hemispheres experiencing winter. This direct light as its tilted towards the sun keeps the hemisphere ILLUMINATED for a greater amount of time during the day.

30. 4 1 3 2 Length of Day in Hemispheres #1 June Day length England (Northern Hemisphere) - 15 hours and 38 minutes Australia (Southern Hemisphere) - 10 hours and 24 minutes

31. 4 1 3 2 The Earth reaches 4 important points in its orbit • Position 1: June Solstice • About June 21st • TX tilted toward Sun • We have our longest day, shortest night • Begins Summer

32. 4 1 3 2 Three months later… • Position 2: September Equinox • About September 23rd • TX tilt in between “toward” and “away” • 12 hrs day, 12 hrs night • Begins Fall

33. 4 1 3 2 #23 Three months later… • Position 3: December Solstice • About December 22nd • TX tilted away from Sun • Shortest day, longest night • Begins Winter

34. 4 1 3 2 Three months later… • Position 4: Vernal Equinox • About March 21st • TX tilt half way between “toward” and “away” • 12 hrs day, 12 hrs night • Begins Spring

35. So, In Summary… If somebody asked you, “Why do we have seasons on Earth?” you could now answer with great confidence… Fact 1: Round planets heat up wherever they get direct sunlight and are cooler at the tops and bottoms. Fact 2: The Earth’s axis is TILTED 23.5o. Fact 3: As Earth revolves around the Sun, the tilt causes different parts of it to receive more or less direct sunlight for a few months, causing the weather and daylight to change.

36. The End!