Why do we have Why do we have seasons?seasons?

October 17/18, 2011October 17/18, 2011

Our SeasonsOur Seasons

In the United States our 4 seasons begin In the United States our 4 seasons begin on the 22on the 22ndnd day of the following months: day of the following months:

• Spring - Spring - MarchMarch• Summer - Summer - JuneJune

• Autumn -Autumn - September September • Winter - Winter - DecemberDecember

The Reason for the SeasonsThe Reason for the Seasons

1.1. TiltTilt - - The Earth is tilted 23.5 The Earth is tilted 23.5 degrees as it revolves around the sun degrees as it revolves around the sun 1.1. The North Pole always points in the same The North Pole always points in the same

directiondirection

2.2. RevolutionRevolution - The Earth - The Earth revolves around the Sun –revolves around the Sun –• The Earth completes one The Earth completes one

revolution every 365 ¼ days; 1 revolution every 365 ¼ days; 1 revolution in 1 yrrevolution in 1 yr

Falls Church is in the mid-latitude Falls Church is in the mid-latitude region of the northern hemisphere; region of the northern hemisphere; We have hot summers and cold We have hot summers and cold winters because of the winters because of the tilt of the tilt of the Earth's axisEarth's axis

Needed background informationNeeded background information Allentown, Pennsylvania is only 202 miles Allentown, Pennsylvania is only 202 miles

northeast of Falls Church, Virginia – They have northeast of Falls Church, Virginia – They have similar latitude and insolation characteristics.similar latitude and insolation characteristics.

The Reason for the SeasonsThe Reason for the Seasons

Because Earth’s axis Because Earth’s axis is tilted it leans:is tilted it leans:

• Towards the SunTowards the Sun (Summer)(Summer)

or or 6 months later6 months later

• Away from the Away from the SunSun (Winter) (Winter)

• In between these In between these postions, Spring postions, Spring and Autumn occurand Autumn occur

FALSE! Changes in the distance between Earth and the sun cause

the seasons In fact, the Earth is farthest from the sun (aphelion) in July,

and is closest to the sun (perihelion) in January!

FALSE! The seasons are caused by the Earth’s axis flipping back and

forth. Can’t be true because we observe:

Circumpolar stars Earth’s north axis always pointing towards Polaris, a change in seasons

TRUE!TRUE! The Earth's movement around The Earth's movement around

the sun causes the seasonsthe sun causes the seasons•In the summer the axis points In the summer the axis points

slightly toward the sunslightly toward the sun•In the winter, the axis points In the winter, the axis points

slightly away from the sunslightly away from the sun http://www.collinseducation.com/resources/ict%20activity/earth_FULL.swfhttp://www.collinseducation.com/resources/ict%20activity/earth_FULL.swf

Summer TimeSummer Time The sun is higher in the skyThe sun is higher in the sky and and

the energy that we receive from the the energy that we receive from the sun is sun is more directmore direct

The sun remains in the sky for a The sun remains in the sky for a longer period of timelonger period of time

Higher sun & longer days mean –Higher sun & longer days mean –• direct energydirect energy• greater amounts of energy being greater amounts of energy being

absorbed by the Earth’s surfaceabsorbed by the Earth’s surface• Temperatures must go upTemperatures must go up

Winter TimeWinter Time The sun is lower in the skyThe sun is lower in the sky and its and its

energy distribution is more spread outenergy distribution is more spread out The sun is visible for a much The sun is visible for a much

shorter time during the dayshorter time during the day

Lower & fewer hours of sun means –Lower & fewer hours of sun means –• indirect energyindirect energy• less energy received/absorbedless energy received/absorbed• Temperatures must go downTemperatures must go down

The Sun’s Changing PositionThe Sun’s Changing Position The sun is in the sky higher (in elevation) for The sun is in the sky higher (in elevation) for

longer in the summer, SOlonger in the summer, SO• It takes longer to rise and set from horizon to It takes longer to rise and set from horizon to

horizon and has more time to heat up Earthhorizon and has more time to heat up Earth

What happens when the sun changes positionsWhat happens when the sun changes positions??

The sun changes from providing direct to The sun changes from providing direct to indirect sunlightindirect sunlight

Direct during summer Direct during summer • • Indirect during winterIndirect during winter

What happens What happens to shadowsto shadows when the sun when the sun changes positionschanges positions??

Summer: shorter shadowsSummer: shorter shadows• Sun is higher in the skySun is higher in the sky

Winter: longer shadowsWinter: longer shadows• Sun is lower in the skySun is lower in the sky

                                                                                                      

ENERGY AND THE EARTH’S SHAPE: Because the Earth is in the shape of a sphere, there will be parts of the Earth that receive direct energy from the sun and other regions of the world that receive indirect energy. Notice the Earth’s axis in this drawing. The axis is the imaginary line about which the Earth spins. It is straight up and down. If this is how the Earth went around the sun each year, the seasons would always remain the same.

The SolsticesThe Solstices

""SolsticeSolstice" " • ""solsol" from the Latin " from the Latin

meaning meaning sunsun• ""sistere,sistere," from the " from the

Latin meaning Latin meaning cause to stand stillcause to stand still

The longest The longest (summer) or (summer) or shortest (winter) shortest (winter) day of the yearday of the year

Summer solstice

Winter solstice

Summer Solstice Summer Solstice (Northern Hemisphere)(Northern Hemisphere) The summer solstice is the first day The summer solstice is the first day

of summer (about the 21st June) of summer (about the 21st June) The longest day of the year in the The longest day of the year in the

Northern Hemisphere Northern Hemisphere When the North Pole is leaning more When the North Pole is leaning more

directly toward the sun than it does directly toward the sun than it does on any other dayon any other day

It is the point when the sun shines It is the point when the sun shines directly over head of the tropic of directly over head of the tropic of Cancer, the northernmost reach.Cancer, the northernmost reach.

Winter SolsticeWinter Solstice

The winter solstice (20th/21st The winter solstice (20th/21st December) is the first day of winter December) is the first day of winter

The shortest day of the yearThe shortest day of the year The winter solstice in the Northern The winter solstice in the Northern

Hemisphere occurs when the North Hemisphere occurs when the North Pole is leaning away from the sunPole is leaning away from the sun

It is the point when the sun shines It is the point when the sun shines directly over head of the Tropic of directly over head of the Tropic of Capricorn, the southernmost reach.Capricorn, the southernmost reach.

The EquinoxThe Equinox When the North Pole is pointing When the North Pole is pointing

neither directly toward nor directly neither directly toward nor directly away from the sun away from the sun

Equal periods of light and darknessEqual periods of light and darkness The halfway mark between summer The halfway mark between summer

and winter solstices and winter solstices • March 20March 20/21/21

First day of Spring; North pole begins to turn First day of Spring; North pole begins to turn back towards from the sunback towards from the sun

• September September 2020/21/21 First day of Fall; North pole begins to turn First day of Fall; North pole begins to turn

away from the sunaway from the sun

Reviewing what you learned:Reviewing what you learned:

Read pages 80-83 in your textbook. Read pages 80-83 in your textbook. In your notebook, answer the section In your notebook, answer the section

review questions for section 4.3 #1-3 review questions for section 4.3 #1-3 on page 83, and question #18 on on page 83, and question #18 on page 87. page 87.