The spacecraft Lunar Reconnaissance Orbiter photographed the moon from behind in 2011. (Photo: NASA / GSFC / Arizona State University)
Reading aloud The "Man in the Moon" always looks down at the earth with the same face. Its downside, on the other hand, is hidden from our view. Stefan T. asked us why the moon does not show us another view - thank you for the question. Bdw editor Ralf Butscher knows the answer.

When the Russian probe Lunik 3 first transmitted images of the back of the moon to Earth on October 7, 1959, the scientists were astonished. Because this side of the earth satellite offers a very different view than its appearance from the earthly perspective. The other side of the moon is characterized by mountainous highlands and many impact craters. Extensive lowlands (Mare) as on its front, the celestial body has not to offer there.

The fact that astronomers had to wait until the first Lunik 3 probe to find out is due to the Moon's "bounded rotation": it turns at the same speed and direction around its own axis as it circles the Earth - once in 27.3 days. The consequence of this common mode of rotation: The moon always turns the same side to us.

When the moon turned away

That was not always the case. After the emergence of the moon about 4.5 billion years ago, he turned around himself much faster than today. The days on the Erdbegleiter were therefore shorter. However, the so-called tidal friction - the constant kneading of the moon's interior by the gravitational force of the earth - has steadily depleted its own spin until it reached a bound rotation and there was no tidal friction.

Conversely, the moon also exerts a braking effect by tidal forces on the earth - but much weaker, since the Earth's mass is more than 80 times as large as the mass of the Moon. But after all, the earthly days are gradually getting longer - every year by 16-millionths of a second. display

Because of another physical effect, the libration, just over half of the lunar surface can be seen from Earth. Since the moon orbits the earth not on a circular, but an elliptical orbit, it seems to sway a little back and forth. As a result, depending on the season, you can look out over one or the other edge of the moon's front. In addition, the axis of rotation of the moon is slightly inclined to the plane of its orbit around the earth. So you see about 60 percent of the lunar surface over the year.

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© science.de - Ralf Butscher
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