Earths & Seasons ISN page 124

Earth & Seasons Changes in season Length of day OBJECTIVE: I will be able to explain how the axis (tilt) of the earth causes the changes in seasons.

What is a season? One of the major divisions of the year, generally based on cyclic changes of climate –One of the four natural divisions of the year, in the North and South

Why are there seasons? The Earth has changes in seasons because Earth's orbit is in the shape of an ellipse, so it gets closer or further away from the sun as it orbits the sun. This is a common mistake…

Ultimate Cause of Seasons Earth’s axis is NOT oriented straight up and down from the North and South Pole Instead, Earth is tilted from straight up and down by an angle of ~23.5 degrees of arc –Actually the angle ranges from 22 to 24.5 degrees But why? North Pole South Pole

Why is it 23.5°? No one knows… Astronomers have different theories: –About 5 billion years ago, when the Earth was still very young, it was struck by a Mars-sized planet. This impact could have tipped our planet over. –As the cloud of dust and gas collapsed when the universe was forming, the solar system did not form uniformly, the spinning of the gases and other planets is what made each different, hence the tilting of the planets –Essentially, the numerical value of this axis tilt is an artifact of the way the Earth formed. It didn't have to have any specific value, and in fact the other planets all have different axis tilts owing to the differing details of their formation.

What does the tilt do? It allows the sun’s rays to shine more directly and for longer periods of time on some locations than other places of Earth

How does the move around the sun? How does the Earth move around the sun? Sun Over Equator (March 21) Sun Over Tropic of Capricorn (December 21) Sun Over Tropic of Cancer (June 21) Sun Over Equator (September 21)

Sunlight Reaching Earth at… Equinox

Sunlight Reaching Earth at… Solstice

Again, why are there seasons?

How Seasons Occur ty/earth_FULL.swfhttp:// ty/earth_FULL.swf

Why do we have days? We have day and night because the Earth rotates on its axis.

Day versus Night Sunlight Daytime Nighttime When where you are is pointed toward the Sun, it is day. Then the Earth rotates you away from the Sun, and it is night.

Tilt of Earth and Days The length of a day changes because the earth spins at a tilt The length of a day depends on where you are on the earth –sometimes the North Pole points towards the sun, while the South Pole points away –this gives the North Pole 24 hours of daylight for about 6 months, while the South Pole is plunged into darkness –on the equator, the sun is always nearly overhead, so the days are more constant with approximately 12 hours of daylight and 12 hours of darkness everyday

Earth Models Activity: –Create a model of the earth using a piece of fruit to show how the earth’s tilt creates the seasons

Earth Models 1. Explain how your model shows how the seasons change as the earth spins and rotates on it’s axis 2. Explain how it can be summer in the Northern Hemisphere while it is winter in the Southern Hemisphere. 3. Describe what earth’s climate would be like if the earth didn’t have a 23.5 degree tilt. Would we have seasons? Why or why not?

Independent Practice Use the knowledge you’ve gained, and your notes to complete the worksheet.

Closure Fill out YOUR ‘L’ on YOUR ‘KWL’ chart (ISN page 121!)