TEK 8.7A – the student knows the effects resulting from cyclical movements of the Sun, Earth, and Moon. The student is expected to model and illustrate how the tilted Earth rotates on its axis, causing day and night, and revolves around the sun causing changes in seasons.

Rotation - Earth spinning on its axis

Earth’s rotation on its axis causes day and night
 It takes 24 hours for Earth to rotate once on its axis
 Earth rotates in a counterclockwise direction
 Earth rotates at about
1,600 kilometers per hour
Rotation Animation Day and Night Song

Revolution
The movement of one object around another object
 Earth travels (revolves) around the sun
 One complete revolution around the sun equals a year
 It is a curved path (elliptical) followed by Earth as it moves around the Sun

Earth’s orbit is about 365 ¼ days (Leap year every four years)

Revolution

Now You Try!
 Stand in place, and make one complete turn (You have just rotated)
 Take a trip around your chair (You have just revolved
 Do you think you can rotate and revolve at the same time? Try it now!

 Think about what a spinning top, a race car on an oval track, and a globe have in common

Turn to your neighbor and explain…

Earth in Space
Earth causing rotates
On its axis
Day and night revolves
Around the sun causing
Years

So now you understand day and night –
What about seasons?
A Year in 40 seconds

Tilt A Whirl!
 Earth has seasons because its axis is tilted as it revolves around the sun

If Earth’s axis were straight up and down, temperatures would remain constant year round (no seasons)

•
Seasons are caused by the tilt in Earth’s axis
•
The tilt causes uneven heating by the sun (seasons)

Why is it warmer near the equator than near the poles?
 At the equator - sunlight hits Earth’s surface more directly
 Near the north and south poles - sunlight hits
Earth’s surface at an angle (energy from the sun is spread out over a greater area)

The Earth’s Tilt
The Earth's axis is tilted by 23.5
°

Earth’s axis always points in same direction
(towards Polaris - the North Star)

Earth’s Tilted Axis

Earth’s axis is tilted away from the sun for part of the year and toward the sun for part of the year
 When tilted toward the sun – summer
 When tilted away from the sun - winter
Note: Summer and winter are not affected by changes in Earth’s distance from the sun.

Summer in the Northern
Hemisphere

Winter in the Northern
Hemisphere

As the Earth moves around the
Sun, this axis always stays pointing in the same direction .

Look at the direction of the axis during each season.

Direct vs. Indirect Light

 When the northern hemisphere is leaning away from the sun, the rays coming from it hits this part of the Earth at a smaller (slanted) angle compared to other parts of the world
 These are called indirect rays
 Indirect rays - same amount of energy spread over a larger area of the earth therefore it receives less energy (heat)

Summer Sun (Direct)
Winter Sun (Indirect)

Winter Sun (Indirect)
Summer Sun (Direct)



Direct sunlight gets more heating
Indirect sunlight gets less heating

Misconception Alert!
 Seasons are NOT caused by the changing distance the Earth is from the Sun.
 The Earth travels in an elliptical orbit, so the distance does vary – but not enough to cause the change in temperature associated with seasons!

When the southern hemisphere is experiencing summer, the northern hemisphere has winter (what do you know about Australia?)

6 months later, the situation reverses, and it is now winter in the southern hemisphere since that part of the earth is now leaning away from the sun

The energy that hits the Earth by the
Sun changes over of the year
The angle the Sun is above the horizon determines how much heat and light strike each square meter of ground

This means that in summer, the ground receives more energy (more heat) per square meter than in the winter.
More energy is received by the ground during the summer (high temperatures) and less energy during the winter (lower temperatures).

 If you had $100.00 to divide among 50 people what would each person get?
 If you had the same $100.00 to divide among 100 people what would each person get?
 If you had the same $100.00 to divide among 200 people, what would each person get?

Sun’s Angle = money

Think of the money as the sun’s power
 The sun always shines with the same amount of power, just like the $100.00.
 If you had to spread the power over a larger area, there would be less power
(heating) to go around on that area on
Earth.
 Just like there is less money for each person if the number of people grows.

Changes in the Sun’s Angle

Summer

Sun is more directly overhead

Sun’s energy is concentrated

Winter


The sun is less directly overhead
Sun’s energy is less concentrated

Seasons and Earth’s Tilt

Big Idea:
The seasons are the result of:
1) the tilt of the Earth's axis, and
2) revolution around the sun
 If the tilt of the Earth's axis was 0 ° there would be no difference in how the rays from the sun hit its different regions (there would be no seasons)

Review
Look closely at where the Sun is hitting the Earth during each season: http://Search.Lycos.com/setup.asp?r=5&src=clear2&query=weather+savvy

Solstice
Solstice: Either of the two times in the year, the summer solstice and the winter solstice , when the sun reaches its highest or lowest point in the sky at noon, marked by the longest and shortest days.

Make a quick T chart in your notes
Winter Solstice Summer Solstice

• The northern hemisphere is pointed furthest away from the sun
• The sun is directly over the Tropic of Capricorn
• The shortest day of the year in the northern hemisphere

• The northern hemisphere is pointed furthest toward the sun
• The sun is directly over the Tropic of Cancer
• The longest day of the year in the northern hemisphere

Solstices
 On two days each year, the noon sun is overhead at either
23.5
° S or 23.5° N – solstices.
 Winter Solstice
 23.5
° S
 in the northern hemisphere
 December 21st
 shortest day of the year
 Summer Solstice
 23.5
° N
 in the northern hemisphere
 June 21st
 Longest day of the year

Equinox
 Equinox: the date (twice each year) at which the sun crosses the equator, therefore day and night are of equal length
(about September 22 and March 20).

Make a quick T chart in your notes
Spring Equinox Fall Equinox

• Vernal (spring) and autumnal (fall) equinox the sun is directly over the equator
• Days and nights are equal in length

Earth in March and September
Equinox (means - Equal night)
 Halfway between the solstices, neither hemisphere is tilted toward or away from the sun
 The noon sun is directly overhead at the equator

Vernal (Spring) Equinox
 March 21st
 Marks the beginning of spring in the northern hemisphere

Autumnal Equinox
 September 23rd
 Marks the beginning of fall in the northern hemisphere

Equinoxes and Solstices

Videos https://www.youtube.com/watch?v=p6Tz836
Avh4
Seasons Animation