why do we see shadow on the moon
Answers
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The shadow is from the Moon itself. Just like Earth, half of the Moon is facing the Sun at any given time.
From Earth, we only see one side of the Moon because it is tidally locked to the Earth (it does not rotate as viewed from Earth).
However the Moon completes a full orbit around the Earth once each month. As it orbits the Earth, the Moon's side facing the Sun changes during this period. When it is between the Earh and Sun, the dark (night) side of Earth can not see the Moon. When it is on the side of Earth furthest from the Sun, the dark side of Earth sees the Moon fully illuminated by the Sun.
Try this experiment:
Find a bright light source in your home. Hold a ball at arms length away from your face. The ball represents the Moon. You head is the Earth and your eyes are your location on Earth. Spin your body around slowly to represent one month of the Moon's orbit around you. Notice how the shadow appears on the side of the ball facing away from the light.