What causes the changes in the earth’s rotation? There are several causes. First, random events such as earthquakes can shuffle the earth’s material and change the earth’s moment of inertia. When the earth’s moment of inertia changes, conservation of angular momentum requires that the rotation rate must change as well. For instance, the 2004 Indian Ocean earthquake that caused the large tsunami shrunk the earth slightly and shortened the earth’s rotation by about 2.7 millionths of a second. Second, annual events such as seasonal growing and melting glaciers and ice caps change the earth’s moment of inertia. Third, there is a long-term periodic trend caused by astronomical bodies.

Finally, there is a long-term secular (non-periodic) slowing in the earth’s rotation caused by the tidal interaction of the earth and moon. As the earth slows its rotation, the moon spirals away from the earth. Therefore, in the past the earth spun more rapidly and the moon was much closer to the earth. Direct computation shows that the earth and moon would have been in contact about 1.3 billion years ago. Even a billion years ago the moon would have been so close to the earth that tides would have been a mile high. No one—including those who believe that the earth is far older than a billion years—thinks that tides were ever that high or that the moon and the earth touched a little more than a billion years ago.

However, since the earth and moon are only thousands of years old as the Bible clearly indicates, the long-term change in the earth-moon system is no problem. Indeed, what we see in the interaction between the earth and moon offers powerful evidence that the earth and moon are young.