Diurnal libration and VA

Diurnal libration is the daily wobble in the Moon's orientation toward the Earth.
  • Press the animation button to see the moon orbiting the earth.
  • The gap between terminators on the surface of the moon subtends an angle of 1' for an observer at the earth's centre E when the moon's elongation changes by about 3.7° (or, equivalently, when about 7.35 hours have elapsed).
  • Yet, because neither the earth nor the moon remain stationary as time goes by, an observer on the earth's surface will see something different, as shown in this interactive illustration.