Tidal Evolution of the Earth-Moon System

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Due to the tides and the gravitational interactions that cause them, the Earth-Moon system is gradually evolving over time: the Moon is moving farther away from the Earth, the rotation of the Earth (and of the Moon as well) is slowing down, and the size of the tidal bulge is slowly decreasing as it lines up with the Moon. The system will eventually reach a stationary state in the far future. This more detailed discussion explains this evolution and its cause in greater depth.

This animation illustrates the evolution of the Earth-Moon system over roughly the last 4.5 billion years. It shows:

  • the Earth's rotation slowing down over time;
  • the Moon's rotation slowing down as well, eventually becoming locked with its revolution around the Earth;
  • the Moon moving farther away from the Earth;
  • the Moon travelling more slowly (in accordance with Kepler's Third Law);
  • the Moon taking longer to revolve around the Earth, as a result;
  • the size of the tidal bulge decreasing;
  • the tidal bulge partially lining up with the Moon (the thin gray line shows the the axis of the tidal bulge to make this process easier to observe).

The text in the upper corners of the animation shows how the length of the day (in hours), the length of the month (in current 24-hour days), the period of the Moon's rotation (in current days) and the distance of the Moon from the Earth (in multiples of the radius of the Earth) all increase over time. The data are also presented as percents of the current values (24-hour day, 27.3-day month and Moon rotation period, and 60.3 Earth radii from the Moon to the Earth)

For simplicity, we are ignoring the influence of the Sun on the system. Please note that this animation is not to scale. More information about its limitations and approximations can be found in the detailed discussion.