Final answer:
During the new moon phase, the moon is not visible to observers on Earth because Earth blocks light from reaching the moon.
Step-by-step explanation:
During the phase called the new moon, the moon is not visible to observers on Earth because Earth is between the sun and the moon, and blocks light from reaching the moon. The moon does not generate its own light, but instead reflects sunlight. When the moon is in the new moon phase, the side of the moon facing Earth is not lit by sunlight, so it appears dark and is not visible.
Learn more about New moon visibility