4. #3 …and sunlight comes in
from just one direction
½ of Jupiter lit by the
Sun
light
5. #4 It’s all about
CONCENTRATION!
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, giving
it a lot of energy (HEAT!),
and focusing this energy
over a small area.
6. #5 But at the north & south
pole?
That same picture of Venus
the direct ray of light
Here comes a ray from
the sun toward the south
pole of Venus…
notice how much bigger an area the ray
covers…this means it’s
LESS concentrated!
7. #6 So direct light heats planets
up more than slanted light!
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.
8. #7 So that explains why…
The best planet ever!
Up here we have…
11. #10 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. #11 This is the key, so repeat
after me…
It’s…
the…
13. #12 The Earth doesn’t sit
perfectly straight up and down
A “false” view of
Earth rotating on its
axis.
This just isn’t right!
16. #15 Remember from earlier…
…That the middle of the planet gets the most direct sunlight
(heat).
HOT
medium
medium
COLD
COLD
17. #16 So one half of the Earth
“tilts” toward the sun!
Yep!
Which hemisphere of Earth (Northern or Southern) is having
Summer & Winter in this diagram?
Direct light = Summer!
Slanted down here = Winter!
18. #17 What’s 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. #18 So…how do we get Winter
here???
That happens when our (Northern) hemisphere tilts away from
the sun.
Summer
down
here!
20. #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 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. #20 So it’s a combination of the
TILTed axis and our revolving!
22. #21 The Earth reaches 4
important points in its orbit
Summer Solstice
•Late June
•TX tilted toward Sun
•Longest day, shortest night
•Begins Summer
23. #22 Three months later…
Autumnal Equinox
•Late September
•TX tilt balanced
•12 hrs day, 12 hrs night
•Begins Autumn/Fall
24. #23 Three months later…
Winter Solstice
•Late December
•TX tilted away from Sun
•Shortest day, longest night
•Begins Winter
25. #24 Three months later…
Vernal Equinox
•Late March
•TX tilt balanced
•12 hrs day, 12 hrs night
•Begins Spring
26. #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…
Yeah, we’re old.
27. #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 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 sunlight for a few months,
causing the weather and daylight to change.
