Transcript The Reason for the Seasons

The Reason for the Seasons
#1 Here’s Mars…
Where would
you guess that
it is COLDEST
on Mars?
If you guessed
here…
…and here…
…you’re
right. But
why? And
how did you
know?
#2 The key is that Planets are…
#3 …and sunlight comes in
from just one direction
light
½ of Jupiter lit by the
Sun
#4 It’s 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.
Venus, lit by the Sun
#5 But at the north & south
pole?
Here comes a ray from
the sun toward the south
pole of Venus…
the direct ray of light
notice how much bigger an area the ray
covers…this means it’s
LESS concentrated!
That same picture of Venus
#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.
#7 So that explains why…
Up here we have…
The best planet ever!
#8 …and…
Around here we have…
The best planet ever!
#9 …and…
Down here we have…
I’m hot
The best planet ever!
#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.
#11 This is the key, so repeat
after me…
It’s…
the…
#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!
#13 Our Axis is TILTED!
About 23.5 degrees
#14 A more accurate view…
Notice the…
#15 Remember from earlier…
…That the middle of the planet gets the most direct sunlight
(heat).
COLD
medium
HOT
medium
COLD
#16 So one half of the Earth
“tilts” toward the sun!
Yep!
Direct light = Summer!
Slanted down here = Winter!
Which hemisphere of Earth (Northern or Southern) is having
Summer & Winter in this diagram?
#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!
#18 So…how do we get Winter
here???
That happens when our (Northern) hemisphere tilts away from
the sun.
Summer
down
here!
#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.
#20 So it’s a combination of the
TILTed axis and our revolving!
#21 The Earth reaches 4
important points in its orbit
Summer Solstice
•Late June
•TX tilted toward Sun
•Longest day, shortest night
•Begins Summer
#22 Three months later…
Autumnal Equinox
•Late September
•TX tilt balanced
•12 hrs day, 12 hrs night
•Begins Autumn/Fall
#23 Three months later…
Winter Solstice
•Late December
•TX tilted away from Sun
•Shortest day, longest night
•Begins Winter
#24 Three months later…
Vernal Equinox
•Late March
•TX tilt balanced
•12 hrs day, 12 hrs night
•Begins Spring
#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.
#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.
#27 and…
THAT is the reason for the seasons!