Transcript Earth & Space

Earths & Seasons
• ISN page 124
http://www.youtube.com/watch?v=6lgtCJAWfrk
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
North Pole
• 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?
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 Earth move around the sun?
http://www.collinseducation.com/resources/ict%20activity/earth_FULL.swf
Sun Over Equator
(March 21)
Sun Over
Tropic of
Cancer
(June 21)
Sun Over Tropic
of Capricorn
(December 21)
Sun Over Equator
(September 21)
Sunlight Reaching Earth at…
• Equinox
Sunlight Reaching Earth at…
• Solstice
Again, why are there seasons?
How Seasons Occur
• http://www.collinseducation.com/resources/ict%20activi
ty/earth_FULL.swf
Why do we have days?
• We have day and night because the Earth
rotates on its axis.
Day versus Night
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.
Sunlight
Sunlight
Daytime
Nighttime
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!)