libration

/laɪˈbreɪʃən/
noun
  1. A slight, regular back-and-forth movement or oscillation, especially of a celestial body like the Moon as seen from Earth.
    • Astronomers study lunar libration to map the edges of the Moon's far side.
    • Because of libration, the Moon appears to rock gently from side to side in the sky.
    • The Moon's libration allows us to see slightly more than half of its surface over time.
  2. A balancing or swaying motion, like a scale tipping back and forth.
    • The dancer's libration between stillness and movement captivated the audience.
    • In physics class, we observed the libration of a hanging weight as it swung.
    • The pendulum's libration slowed as it came to rest at the center.
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