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Weird Science: Tidal Locking—Why the Man in the Moon Can Always See You

NGSS Crosscutting Concepts:

NGSS Disciplinary Core Ideas:

<p><strong>SF Fig. 6.7.</strong> In this figure, a face represents the near side of the moon. The face is always turned towards Earth. This figure is not drawn to scale.</p><br />

Some people see a face in the lunar mare, which is commonly referred to as “the man in the moon.” Due to the gravitational effect of the earth on the moon, the moon and the earth are tidally locked. This means that it takes the same amount of time for the moon to rotate once about its axis as it takes for the moon to make one orbit around the earth. The moon’s rotation and orbital period are both just under four weeks. This synchronous (or same) rotation causes one side of the moon (the side with “the man in the moon”) to always face Earth (SF Fig. 6.7). This is called the near side of the moon. Most of the major moons in our solar system rotate synchronously with their planets.

<p><strong>SF Fig. 6.8.</strong> (<strong>A</strong>) The side of the moon that faces Earth—the “near side”</p><br />
<p><strong>SF Fig. 6.8.</strong>&nbsp;(<strong>B</strong>) The side of the moon that does not face Earth—“the far side.”</p><br />


Because people can only see one side of the moon from Earth (the “near side”), no one knew what the other side of the moon (often called the “far side”) looked like until 1959, when photographs were transmitted from the Soviet spacecraft Luna 3 (SF Fig. 6.8).

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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.