Ptolemy formulated a(n) ____________________ model of the solar system to predict positions of the ...

Ptolemy formulated a(n) ____________________ model of the solar system to predict positions of the Sun, Moon, and planets.
 
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Question 2

A star on the ____________________ generates enough energy from nuclear fusion to halt gravitational collapse.
 
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Question 3

A(n) ____________________ is a circle whose center moves in a circular orbit around the Earth.
 
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Question 4

A(n) ____________________ is a cool, dense cloud of dust and gas that is undergoing gravitational contraction.
 
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