To maintain the flow of ideas in the passage about eclipses, the sentence that best fits is:
"A lunar eclipse occurs when the Earth moves."
This sentence provides additional information about different types of eclipses, specifically focusing on a lunar eclipse. It connects well with the existing context that mentions people observing and interpreting eclipses for thousands of years. By introducing the idea of a lunar eclipse and how it occurs, it complements the discussion about eclipses without disrupting the flow of ideas in the passage.