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The Reason for the Seasons. Heres Mars… Where would you guess that it is COLDEST on Mars? If you guessed here… …and here… …youre right. But why? And how.

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Presentation on theme: "The Reason for the Seasons. Heres Mars… Where would you guess that it is COLDEST on Mars? If you guessed here… …and here… …youre right. But why? And how."— Presentation transcript:

1 The Reason for the Seasons

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

3 The key is that Planets are…

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

5 Its all about the angle at which the light hits the planet! Venus, lit by the Sun 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 suns energy (heat) over a relatively small area.

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

7 So direct light heats planets up more than indirect light! middle Around the middle of the planet, the suns 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 …and… Down here we have… Im cold

11 This helps, but it doesnt 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 WOULDNT have different seasons…just the same boring weather all year long. In Texas, itd 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… Its… the…

13 The Earth doesnt sit perfectly straight up and down A false view of Earth rotating on its axis. This just isnt right!

14 TILTED #13 Our Axis is TILTED ! 23.5 degrees About 23.5 degrees

15 A more accurate view… Notice the…

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

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

18 What does this mean for Texas? When our Northern Hemisphere tilts toward 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 So…how do we get Winter here??? away That happens when our (Northern) hemisphere tilts away from the sun. Summer down here!

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 youve known for years! The Earth revolves around the Sun! Watch the animation…pay attention to the direction Earths axis points as it orbits the Sun.

21 So its a combination of the TILTed axis and our revolving around the sun!

22 Complete the table 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

23 Seasons Animation animations/animations/01_EarthSun_E2.html animations/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. 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, wouldnt Hawaii experience snow?

25 Notice the distance during the summer!

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

27 Day versus Night Sunlight Daytime Nighttime 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.

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

34 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 Earths axis is TILTED 23.5 o. 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.


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