Why is it so hot here in the summer?

The Earth in Space 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 Earth’s rotation on its axis causes day and night It takes 24 hours for Earth to rotate once on its axis It takes 24 hours for Earth to rotate once on its axis Earth rotates in a counter- clockwise direction Earth rotates in a counter- clockwise direction Earth rotates at about 1,600 kilometers per hour Earth rotates at about 1,600 kilometers per hour Day and Night SongRotation Animation

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

Revolution

Now You Try! Stand in place, and make one complete turn (You have just rotated) Stand in place, and make one complete turn (You have just rotated) Take a trip around your chair (You have just revolved 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! 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 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… What is the difference between Earth’s rotation and Earth’s revolution?

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

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 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 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) 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 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 toward the sun – summer When tilted away from the sun - winter 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 Direct Indirect

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 same amount of energy spread over a larger area of the earth therefore it receives less energy (heat) 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)

Summer Sun (Direct) Winter Sun (Indirect)

Seasons 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. 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! 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. 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 $ to divide among 50 people what would each person get? If you had the same $ to divide among 100 people what would each person get? If you had the same $ to divide among 200 people, what would each person get? Sun’s Angle Analogy

Think of the money as the sun’s power The sun always shines with the same amount of power, just like the $ 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. Sun’s Angle = money

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

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 SolsticeSummer Solstice

Winter 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

Summer Solstice 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). 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 EquinoxFall Equinox Spring EquinoxFall Equinox

Spring and 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 Halfway between the solstices, neither hemisphere is tilted toward or away from the sun The noon sun is directly overhead at the equator The noon sun is directly overhead at the equator

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

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

Equinoxes and Solstices

Videos Avh4 Avh4 Seasons Animation