When the Moon swings between the Earth and the Sun, it's in the sky all day with its dark side facing us. Normally we can't see it. When the alignment is precise, the Moon partially or fully obscures the disk of the Sun, which allows us to see the Moon silhouetted against the Sun. Only when the Sun's disk is fully obscured can the new moon be observed safely with the naked eye, backlit by the Sun's corona during a total eclipse.
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