Transcript File - Ms. Benson Earth Science

The Reason for the Seasons
Science Starter
• What’s your favorite season and why?
What causes seasons?
• There are several misconceptions about
seasons
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, wouldn’t Hawaii experience snow?
Notice the distance during the
summer!
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?
The key is that Planets are…
…and sunlight comes in from
just one direction
light
Half of Jupiter lit by the
Sun
It’s all about the angle at
which the light hits the planet!
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 sun’s energy (heat)
over a relatively small area.
Venus, lit by the Sun
But at the north & south pole?
Here comes a second ray
from the sun toward the
south pole of Venus. Notice
it hits Earth’s surface at a
angle that is much less than
90 degrees.
the direct ray of light
Notice how much LARGER an
area the ray covers…this means
it’s LESS concentrated (or more
spread out).
So direct light heats planets
up more than indirect light!
Around the middle of the planet, the sun’s 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.
So that explains why…
Up here we have…
…and…
Around here we have…
…and…
Down here we have…
I’m cold
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.
This is the key, so repeat after
me…
It’s…
the…
The Earth doesn’t sit perfectly
straight up and down
A “false” view of
Earth rotating on its
axis.
This just isn’t right!
Our Axis is TILTED!
About 23.5 degrees
A more accurate view…
Notice the…
Remember from earlier…
…That the middle of the planet gets the most direct sunlight
(heat).
COLD
medium
HOT
medium
COLD
So one half of the Earth “tilts”
toward the sun!
What does the tilt do?
Direct light = Summer!
Indirect down here = Winter!
Which hemisphere of
Earth (Northern or
Southern) is having
Summer & Winter in this
diagram?
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!
So…how do we get Winter
here???
Winter happens when our (Northern) hemisphere tilts away from
the sun.
Summer
down
here!
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.
So it’s a combination of the
TILTed axis and our revolving
around the sun!
4
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.
The Earth reaches 4 important
points in its orbit
4
1
3
2
Position 1: June Solstice
•About June 21st
•TX tilted toward Sun
•We have our longest day, shortest night
•Begins Summer
Three months later…
4
1
3
2
Position 2: September Equinox
•About September 23rd
•TX tilt in between “toward” and “away”
•12 hrs day, 12 hrs night
•Begins Fall
#23 Three months later…
4
1
3
2
Position 3: December Solstice
•About December 22nd
•TX tilted away from Sun
•Shortest day, longest night
•Begins Winter
Three months later…
4
1
3
2
Position 4: Vernal Equinox
•About March 21st
•TX tilt half way between “toward” and “away”
•12 hrs day, 12 hrs night
•Begins Spring
Why do we experience day and
night?
The Earth is…
Day versus Night
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.
Sunlight
Sunlight
Daytime
Nighttime
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 direct sunlight for a few months,
causing the weather and daylight to change.