Daylight on Earth and the Reason for the Seasons

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Daylight on Earth and the Reason for the Seasons
School's Out!
Brrrrrrrrrr!
Leaves go down!
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Why do we experience day and night?

• The Earth is

Rotating
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Day vs. Night
When your location points toward the Sun, it is
day. Then the Earth rotates you away from the
Sun, and it is night.
Daytime
Nighttime
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Day and Night as the Earth is rotating on its AXIS
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How long does one rotation take?
24 Hours!
So remember, the earth is rotating much faster
than it is revolving!
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Heres Mars
Where would you guess that it is COLDEST on Mars?
youre right. But why? And how did you know?
If you guessed here
and here
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The key is that Planets are
ROUND
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and sunlight comes in from just one direction
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Its all about CONCENTRATION!
Look at a ray of sunlight hitting the middle of
the planet.
The ray hits this part of the planet directly,
giving it a lot of energy (HEAT!), and focusing
this energy over a small area.
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But at the north south pole?
Here comes a ray from the sun toward the
south pole of Venus
notice how much bigger an area the
ray coversthis means its LESS concentrated!
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So direct light heats planets up more than
slanted light!
YUP!
Around the middle of the planet, the light is
very focused and hits directly, heating that
area up greatly.
But at the top and bottom of the planet, the
light comes in at a slant, meaning it warms those
areas much less.
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Illustrate how the sun hits the planet
Here comes a ray from the sun toward the
south pole of Venus
notice how much bigger an area the
ray coversthis means its LESS concentrated!
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So that explains why
Up here we have
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and
Around here we have
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and
Down here we have
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This helps, but it doesnt explain the seasons!
So far we only know why the hottest part of each
planet is around the middlethe sunlight hits
that part most directly.
If that was all that happened, though, we
WOULDNT have different seasonsjust 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.
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This is the key, so repeat after me
Its
the
TILT
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The Earth doesnt sit perfectly straight up and
down
A false view of Earth rotating on its axis.
This just isnt right!
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Our Axis is TILTED!
TILT
About 23.5 degrees
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A more accurate view
Notice the
TILT
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Remember from earlier
That the middle of the planet gets the most
direct sunlight (heat).
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So one half of the Earth tilts toward the sun!
What does the tilt do?
Slanted down here Winter!
Which hemisphere of Earth (Northern or Southern)
is having Summer Winter in this diagram?
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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!
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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 animationpay attention to the
direction Earths axis points as it orbits the
Sun.
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Sohow do we get Winter in Texas???
That happens when our (Northern) hemisphere tilts
away from the sun 6 months after summer.
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So its a combination of the TILTed axis and our
revolving around the sun!
Yeah!!!
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Complete the table
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1
3
2

• 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.

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Seasons Animation

• http//esminfo.prenhall.com/science/geoanimations/
  animations/01_EarthSun_E2.html

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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, wouldnt Hawaii experience
snow?
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Notice the distance during the summer!
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Which hemisphere in this picture would experience
longer amounts of daylight? Why do you think so?
Which part of the world would experience 24
hours of daylight each day? 24 hours of darkness
each day?
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Which hemisphere in this picture would experience
longer amounts of daylight? Why do you think so?
Which part of the world would experience 24
hours of daylight each day? 24 hours of darkness
each day?
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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

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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

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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

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Three months later

• Position 3 December Solstice
• About December 22nd
• TX tilted away from Sun
• Shortest day, longest night
• Begins Winter

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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

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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 23.5o.
Fact 3 As Earth revolves around the Sun, the
tilt causes different parts of it to receive more
or less sunlight for a few months, causing the
weather and daylight to change.
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The End!