Tiānpíngdòng shì yuèqiú guǐdào tuǒyuán xìng zàochéng de xiànxiàng.
Libration is a phenomenon caused by the ellipticity of the Moon's orbit.
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天平 (tiānpíng) is a balance scale, and 动 (dòng) is 'movement' - 'balance-scale rocking', exactly the to-and-fro tilt the Moon shows. The Latin libration also comes from libra 'balance scale', so the metaphor matches across languages.
