The Reason for the Seasons

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The Reason for the Seasons
School's Out!
Brrrrrrrrrr!
Leaves go down!
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1 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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2 The key is that Planets are
ROUND
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3 and sunlight comes in from just one direction
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4 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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5 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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6 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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7 So that explains why
Up here we have
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8 and
Around here we have
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9 and
Down here we have
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10 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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11 This is the key, so repeat after me
Its
the
TILT
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12 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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13 Our Axis is TILTED!
TILT
About 23.5 degrees
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14 A more accurate view
Notice the
TILT
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15 Remember from earlier
That the middle of the planet gets the most
direct sunlight (heat).
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16 So one half of the Earth tilts toward the
sun!
Yep!
Slanted down here Winter!
Which hemisphere of Earth (Northern or Southern)
is having Summer Winter in this diagram?
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17 Whats 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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18 Sohow do we get Winter here???
That happens when our (Northern) hemisphere tilts
away from the sun.
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19 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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20 So its a combination of the TILTed axis and
our revolving!
Yeah!!!
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21 The Earth reaches 4 important points in its
orbit

• Summer Solstice
• Late June
• TX tilted toward Sun
• Longest day, shortest night
• Begins Summer

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22 Three months later

• Autumnal Equinox
• Late September
• TX tilt balanced
• 12 hrs day, 12 hrs night
• Begins Autumn/Fall

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23 Three months later

• Winter Solstice
• Late December
• TX tilted away from Sun
• Shortest day, longest night
• Begins Winter

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24 Three months later

• Vernal Equinox
• Late March
• TX tilt balanced
• 12 hrs day, 12 hrs night
• Begins Spring

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25 And this cycle continues over and over
completing every 365 ¼ days, which we call a
year. By our best estimation, this process has
happened approximately
4,500,000,000 TIMES
Yeah, were old.
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26 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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27 and
THAT is the reason for the seasons!
The End!