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!
Everybody
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) – Winter
Smaller (farther) - Summer
If distance from
the Sun isn’t
responsible,
what is?
Look at Earth
The 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.
Summer
Earth
tilts
towards
the Sun.
Source: Virtual Solar System
http://inkido.indiana.edu/a100/projecthelp.html
Winter
Earth
tilts away
from the
Sun.
Source: Virtual Solar System
http://inkido.indiana.edu/a100/projecthelp.html
Autumn/Spring
Earth does
not tilt
towards or
away from
the Sun.
Source: National Schools’ Observatory
http://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 Planetarium
http://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
st
June 21 .
Key Season
Dates
st
21
June
also has the
most daylight hours of
any day in the year.
Key Season
Dates
st
21
June
– 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
st
September 21 .
Key Season
Dates
st
September 21 –
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
st
December 21 .
Key Season
Dates
st
21
December
also has
the fewest daylight
hours of any day in the
year.
Key Season
Dates
st
21
December
– The
Winter Solstice (the
Sun is farthest south in
the sky).
Key Season
Dates
First day of
Spring – around
st
March 21 .
Key Season
Dates
st
March 21 –
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 increase
When 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.