The Reason for the Seasons

Here’s Mars…

Where wouldyou guess thatit is COLDESTon Mars?

If you guessedhere…

…and here…

…you’reright. Butwhy? Andhow did youknow?

The key is that Planets are…

…and sunlight comes in from just one direction

Half of Jupiter lit by theSun

light

It’s all about the angle at which the light hits the planet!

Venus, lit by the Sun

Look at a ray of sunlighthitting the middle of theplanet.

The ray hits this part ofthe planet directly at a 90 degree angle. It focuses the sun’s energy (heat)over a relatively small area.

But at the north & south pole?

the direct ray of light

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.

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 middlemiddle of the planet, the sun’s rays hit Earthdirectly, 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 comesin 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 planetis around the middle…the sunlight hits that part most directly.

If that was all that happened, though, we WOULDN’T havedifferent 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 ofEarth rotating on itsaxis.

This just isn’t right!

#13 Our Axis is TILTEDTILTED!

About 23.5 degrees23.5 degrees

A more accurate view…

Notice the…

Remember from earlier…

…That the middle of the planet gets the most direct sunlight(heat).

HOT

medium

medium

COLD

COLD

So one half of the Earth “tilts” toward the sun!

What does the tilt do?

Which hemisphere of Earth (Northern or Southern) is havingSummer & Winter in this diagram?

Direct light = Summer!

Indirect down here = Winter!

What does this mean for Texas?

When our Northern Hemisphere tilts towardtoward the Sun, weactually get nearly the most direct sunlight of anywhere on theplanet! No wonder we get over 100 degrees so often in July &August!

So…how do we get Winter here???

That happens when our (Northern) hemisphere tilts awayaway fromthe sun.

Summerdown here!

And the final piece to the puzzle?

So, the TILT is a big part of it, but something else mustcause us to tilt either away or toward the Sun at differenttimes during the year……and that last bit is something you’ve known for

years!

The Earth revolves aroundrevolves around the Sun!the Sun!

Watch the animation…payattention to the direction Earth’s axis points as itorbits the Sun.

So it’s a combination of the TILTed axis and our revolving

around the sun!

Complete the table

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

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!

Why do we experience day and night?

The Earth is…

Day versus Night

Sunlight

Sunlight

Daytime

Nighttime

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.

Day and Night as the Earth is rotating on its AXIS

Northern Hemisphere SummerSouthern Hemisphere Winter

Hemispheres experiencing summer receive MORE DIRECT RAYS from the sun than hemispheres experiencing winter. This direct light as its tilted towards the sun keeps the hemisphere ILLUMINATED for a greater amount of time during the day.

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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#23 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?” youcould now answer with great confidence…

Fact 1: Round planets heat up wherever they get direct sunlightand 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 differentparts of it to receive more or less direct sunlight for a few months,causing the weather and daylight to change.