libration
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The slow, apparent rocking or wobbling of the Moon as seen from Earth, caused by slight variations in the Moon's orbital speed and the geometry of its rotation. Although the Moon keeps the same face pointed toward us (tidally locked), libration allows us to peek slightly around the edges over time — revealing about 59 percent of the lunar surface rather than just 50. It is a small generosity of orbital mechanics, a glimpse of what is almost hidden.
Etymology
Latin libratio, a balancing, a swinging, from libra, a balance or scales. The Moon swings on its axis like a balance tipping.
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