Define the following terms and answer the related questions.

I.  PRECESSION: _the way in which the Earth wobbles as it revolves ____

	1.  How frequently does the Earth complete a cycle of precession?
	
	__once every 25,000 years________________________________

	2.  Where is the Earth now in the cycle?

	__closest to the sun in Northern Hemisphere winter____________

II. ORBITAL ELLIPITICITY: _Earth’s orbit flexes from more circular shape to more oval shape____
	
	1.  What happens when the Earth’s orbit is most elliptical?

	___distance from Earth to Sun & intensity of sunlight varies greatly__

	2.  How long does it take to complete an elliptical cycle?

	___about 100,000 years_____________________________________

III. NUTATION: _variation in axial tilt_______________________

	1.  What causes the Earth’s seasons?

	___tilt of Earth’s axis_______________________________________

	2. How do the seasons vary with nutation?

	__variation in axial tilt – greater tilt = harsher seasons_____________

	3.  How long is a cycle of nutation?
	
	__about 40,000 years_______________________________________

IV. CLIMATE VARIATION: _precession, orbital ellipticity, and nutation cylces combining to produce changes in climate___

1.  What happens when all three cycles unite to produce cooler summers?

	_winter ice doesn’t melt, glaciers form, sea levels fall – ice age_____

2.  What happens when all three cycles unit e to produce warmer summers?

	_glaciers melt, sea level rises, Southern Hem. has cooler summers___

Why doesn’t the climate in the Southern Hemisphere affect the cycle of Ice 

Ages?  ___there is less land in Southern Hemisphere so less glacial area___

Where are we now in the cycle?  __scientists believe we are coming out of_

__an ice age.___________________________________________________