Why did ancient observers often fail to perceive the Moon's topographical irregularities when it was viewed fully illuminated?

Answer

Shadows were minimized near the center when the Moon was fully illuminated.

Near the center of the full moon, shadows were minimized, making the terrain look relatively uniform, which obscured the features that were clearly visible near the terminator.

Why did ancient observers often fail to perceive the Moon's topographical irregularities when it was viewed fully illuminated?
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