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Why is the edge of Earth’s umbra blurry during a lunar eclipse?
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Eclipse
Total solar eclipse as seen from Hopkinsville, Kentucky, August 21, 2017.
During a lunar eclipse, the edge of Earth’s umbra—the darkest, central part of the shadow—appears blurry or diffuse rather than sharply defined. This effect is caused by Earth’s atmosphere, which refracts sunlight into the shadow cone.12
Because of this atmospheric interference, the transition between the umbra and the penumbra is not distinct, making it difficult for observers to accurately determine the precise times of contact between the Moon and the umbra.12