Why Do We Have Seasons?

That’s easy!The Earth is

closer to the Sun in

Summer.

That’s easy!The Earth is

farther from the Sun in

Winter.

That’s easy!Everybod

y knows that!

Wrong!

Didja’ know…The Earth is

closest to the Sun in Winter & farthest in

Summer.

Here’s the Sun thru the Year.

You can actually see the apparent

size of the Sun change.Larger (closer) –

WinterSmaller (farther) -

Summer

If distance from the Sun isn’t

responsible, what is?

Look at EarthThe

Earth is not

straight “up and down.”

The Earth’s axis is titled

23.5°.

Source: Dr. Mike Eracleous’ website, Penn State University

Why?4.5 billion years ago,

Earth was hit by an object the size of

Mars.

Why?This formed

the Moon and knocked the Earth on its side 23.5°.

Why?It also resulted

in the Earth tilting 23.5°.

So why do we have seasons?It’s the tilt,

baby!

Fact: The Earth revolves around the Sun

(click the picture).

Combine the Earth’s tilt and its revolution around the Sun, you get

seasons!

As the Earth revolves around

the Sun, different parts of the Earth tilt towards the

Sun.

SummerEarth tilts

towards the Sun.

Source: Virtual Solar Systemhttp://inkido.indiana.edu/a100/projecthelp.html

WinterEarth tilts away from the Sun.

Source: Virtual Solar Systemhttp://inkido.indiana.edu/a100/projecthelp.html

Autumn/SpringEarth

does not tilt

towards or away

from the Sun.

Source: National Schools’ Observatoryhttp://www.schoolsobservatory.org.uk/news/arch/sky0906.shtml

The tilt affects the

Earth in two important

ways.

The Earth receives

either direct or indirect sunlight

(heat energy).

Direct sunlight (heat energy) is received in the

Summer.Indirect sunlight (heat energy) is received in the

Winter.

Look at this diagram.

Source: ASD Planetariumhttp://www.astronomy.org/programs/seasons/index.html

Look at the diagrams.

Warm – covers

small area.

Look at the diagrams.

Cool – same amount of

sunlight (heat energy) covers

larger area.

Key Season Dates

First day of Summer –

around June 21st.

Key Season Dates

June 21st also has the most daylight hours of any day

in the year.

Key Season Dates

June 21st – The Summer Solstice

(the Sun is farthest north in

the sky).

Why is summer sometimes called “the dog days of

summer? Click below to find out!

Key Season Dates

First day of Autumn– around

September 21st.

Key Season Dates

September 21st –

Autumnal Equinox.

Key Season Dates

Equinox – “equal

nights.”

Key Season Dates

Daylight & nighttime

both last 12 hours.

Key Season Dates

First day of Winter – around

December 21st.

Key Season Dates

December 21st also has the

fewest daylight hours of any day

in the year.

Key Season Dates

December 21st – The Winter

Solstice (the Sun is farthest south

in the sky).

Key Season Dates

First day of Spring –

around March 21st.

Key Season Dates

March 21st – Vernal

Equinox.

Can you really balance an egg only

on the equinox? Click on the egg to see!

The Sun appears to move thru the sky during the

year.This is called an

analema.

Summer

Autumn/Spring

Winter

North South

The second way the tilt affects the

seasons is the number of daylight hours.

The clip on the next slide shows how

daylight changes thru

the year.

When the Earth is tilted towards

the Sun (summer), more of the Northern Hemisphere is in

daylight.

Source: http://www.astro.psu.edu/~mce/A001/lect03.html

At the North Pole, there is 24 hours of sunlight in

the summer.

As you move south, the daylight

hours decrease until you reach the

equator. Daylight hours last 12 hours.

Continue moving south and

daylight hours continue to decrease

When you reach the South Pole, nights last 24

hours (it’s winter).

Source: http://www.personal.psu.edu/faculty/j/e/jea4/earth/climate2.html

At the North Pole, night

lasts 24 hours in the

winter.

As you move south, the daylight

hours increase until you reach the

equator. Daylight hours last 12 hours.

Continue moving south and daylight hours continue to

increaseWhen you reach the South Pole,

daylight hours last 24 hours (it’s

summer.

Didja notice…Seasons are

reversed!Summer in the North – Winter in

the South.Spring in the North – Autumn in the

South.

Source: Windows on the Universe

And, that’s a quick look at the reasons

for the seasons.