What are valence electrons and where are they located

Freya built a model of the earth, sun, and moon. freya can cause her model to move in three different ways. she can turn the crank to model how the earth revolves around the sun, she can spin the globe to model how the earth rotates on its axis, and she can push the moon with her fingers to model how the moon revolves around the earth. how could freya best use her model to demonstrate the cause of the day/night cycle on earth?