Transcript seasons PowerPoint

Why Do We Have
Seasons?
We’re here to answer that question.
Some believe we have seasons because
we are farthest from the Sun in winter
and closest to the Sun in summer.
That’s Wrong!
If distance from the Sun isn’t
responsible, what is?
The Earth’s axis is titled 23.5°.
Source: Dr. Mike Eracleous’ website, Penn State University
It’s the tilt, baby!
The Earth revolves around the Sun.
When you 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 and away from the Sun.
In the Northern Hemisphere summer,
the Earth tilts towards the Sun.
As the Earth revolves around the Sun,
different parts of the Earth tilt
towards and away from the Sun.
In the Northern Hemisphere winter,
the Earth tilts away from the Sun.
As the Earth revolves around the Sun,
different parts of the Earth tilt
towards and away from the Sun.
In the Northern Hemisphere fall/spring,
the Earth tilts neither towards or away.
Source: NC State Climate Education for K-12
The tilt affects the Earth in two
important ways.
First, 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.
The tilt affects the Earth in two
important ways.
First, the Earth receives either direct
or indirect sunlight (heat energy).
What do we mean by direct and
indirect sunlight (heat energy)?
We receive direct sunlight (heat energy)
when the Sun is high in the sky.
The sunlight (heat
energy) covers a small
area.
We receive indirect sunlight (heat
energy) when the Sun is low in the sky.
The sunlight (heat
energy) covers a large
area.
Look at this diagram. It shows direct and
indirect sunlight (heat energy) side-by-side.
Source: ASD Planetarium
http://www.astronomy.org/programs/seasons/index.html
Why is Summer Hot?
Dates of the Seasons
Summer – June 21st – September 20th
June 21st – Summer Solstice
Summer Solstice – Date Sun appears
highest in the sky.
Fall – September 21st – December 20
September 21st – Autumnal Equinox
Equinox – Equal nights. Daylight and
nighttime hours both last 12 hours.
Dates of the Seasons
Winter– December 21st – March 20th
December 21st – Winter Solstice
Winter Solstice – Date Sun is lowest in
the sky.
Spring – March 21st – June 20th
March 21st – Vernal Equinox
Throughout the year, the Sun appears
to change positon in the sky.
The Sun appears to make a figure 8 in the
sky.
This is called an analema.
Click here for analema II video.
The second factor that affects our
seasons is the number of daylight
hours.
25
15
10
Sunrise
Sunset
5
Date
1-Dec
1-Nov
1-Oct
1-Sep
1-Aug
1-Jul
1-Jun
1-May
1-Apr
1-Mar
1-Feb
0
1-Jan
Time
20
When the Earth is tilted towards the
Sun (summer), more of the Northern
Hemisphere is in daylight.
The North Pole has daylight for 24
hours.
The South Pole has darkness for 24
hours.
When the Earth is tilted away from the
Sun (winter), less of the Northern
Hemisphere is in daylight.
The North Pole has darkness for 24 hours.
The South Pole has daylight for 24 hours.
Daylight hours (winter) are shorter because
most of the Northern Hemisphere spends
more time in darkness.
Daylight hours (spring/fall) are equal
to nighttime hours throughout the
world.
One last season fact!
Seasons are reversed in the Northern
and Southern Hemispheres!
When it is summer in the Northern
Hemisphere, it is winter in the
Southern Hemisphere.
When it is winter in the Northern
Hemisphere, it is summer in the
Southern Hemisphere.
Summer
Winter
Fall
Spring
Winter
Summer
Spring
Fall
Review
Review II
Review III
The Presentation
ends here.