Why is the moon visible to us

Why do we only see one side of the moon?

Already noticed? The moon always looks the same. Sure, because of the phases of the moon we sometimes only see parts of it, but the dark spots, for example, always stay in the same place.

At first it seems completely normal - until you consider that the moon moves around the earth in the course of a month. So we would have to see him from all sides during this time. But the moon always looks the same - why?

One could also ask another way: The moon is a celestial body like the earth - and we know that it rotates on its own axis. This is one of the reasons why the moon should show itself to us from all sides.

Both thoughts are correct: the moon wanders around the earth and it rotates in the process. The special feature: Both movements have exactly the same speed. During a month, the moon walks around the earth once and during that time it also rotates exactly once on its own axis. So it happens that the same side always points to the earth, as if the moon were tied up, so to speak. Scientists speak of “bound rotation”.

This is no coincidence and has not always been the case. When the moon was freshly formed, it spun much faster. But the gravitational pull of the earth slowed the moon down until the speed of rotation had adjusted to the speed of rotation.

Conversely, the gravitational pull of the moon slows down the rotation of the earth, so that the days are getting a little longer. However, since the earth is much heavier than the moon, this “brake” does not work as strongly. It will take about 200 million years for the earth's rotation to slow down to the point where a day lasts 25 hours. Unfortunately, it is not possible to calculate in advance whether there will still be people and how they will spend this additional hour.


Inverted world - the earth rises above the moon horizon. The American astronaut William Anders took this famous photo on Christmas Eve 1968.

Together with Frank Borman and James Lovell, he circled the moon several times on the Apollo 8 mission. When their space capsule came out from behind the moon during one of these orbits, they saw the globe emerge behind the lunar horizon. They were deeply impressed by the sight and took several photos - although Anders jokingly remarked that this was not provided for in the mission plan.

A real "earth rise" - like the way we see the moon rise while standing on earth - cannot be experienced on the surface of the moon. Because the moon always turns the same side to the earth. If you stay on this side, you can see the earth all the time - and always in the same place in the sky. And from the back of the moon, the earth can never be seen.

The second face

The moon shows itself from a completely new side: On October 7, 1959, the Russian probe "Luna 3" took the first photo of the back of the moon. However, the world had to wait eleven days for this historic photo: only when the probe flew back towards earth was the radio link good enough to send the image.

At first glance, the picture doesn't look very spectacular. The resolution is poor, and since the sun shines almost perpendicularly on the surface of the moon, no shadows from mountains and craters can be seen.

But there was a surprise: The moon has far fewer dark spots on the back than on the front. Astronomers are still puzzling over the reason!

Until this photo was taken, mankind had no idea what it looked like there. Because from the earth you only ever see the same side of the moon.

How do the phases of the moon arise?

The moon is funny: it changes shape all the time. Sometimes it's round like a disk, sometimes just a thin sickle - and sometimes we don't see it at all. Why is that?

The moon (like the earth) does not shine by itself. We only see it because it is illuminated by the sun. More precisely, we can only see half of the lunar sphere that faces the sun. The other half receives no light and stays dark.

What we see of this half changes over the course of a month as the moon orbits the earth once. When we see it from the earth with the sun behind us, we look closely at the illuminated side and see the moon fully illuminated, as a full circle. (Therefore: "Full moon“)

If the moon moves on on its orbit, that changes: The rays of the sun now hit it from the left as seen from us. The right edge is not illuminated, so it is not visible. The visible part of the moon continues to decrease on this part of the orbit. ("waning moon“)

Two weeks after the full moon, the moon is facing exactly in the direction of the sun, the side facing us is completely unlit - the moon seems to have disappeared. This point in time is called "new moon“, Because the moon does not disappear permanently, of course, but continues to run and appears again in the sky.

Because little by little, some rays of sun again hit the side facing us. Because the waxing moon is now on the other side of the earth than when you were losing weight, the rays of the sun now come from the right as seen from us. At first we only see a narrow strip on the edge, but it quickly widens. After a week, half of it is illuminated - we are looking exactly from the side at the light-shadow boundary.

And a week later we see the moon again with the sun behind us as a fully illuminated circle in the sky - and the process starts all over again.

Why does the moon have spots?

The man in the moon - known from songs, films and stories. Indeed, there are conspicuous dark spots on the lunar surface, and with a little imagination you can see a face in them. But what are these spots really?

At first, scientists thought the dark spots were seas. But at least since the first visit to the moon in 1969 it has been clear: The moon is dust-dry, the entire surface of the moon consists of fine gray rock powder. And the dark spots are great plains that are simply filled with darker dust. This makes the moon appear speckled light and dark. But how did these plains come about?

The lowlands are almost as old as the moon itself. When the surface of the moon had already solidified into a crust in the early days of the solar system, large asteroids repeatedly hit the moon and tore holes in the fresh crust. There lava ran out of the still hot, liquid interior of the moon and filled the lowlands. Lava rock is darker than the crustal rock, so the plains appear darker.

There are now hardly any large asteroids hitting the moon, but still a lot of smaller ones. Since the moon (unlike the earth) has no atmosphere, they do not burn up but hit the surface. Most of the time, the force of the impact is only enough to crumble some rock and stir up a bit of dust, which quickly sinks back to the ground. Therefore, the surface of the moon today consists of rock dust, mainly light crustal rock and, in the lowlands, darker lava rock. From the earth it looks like spots, seas - or a face.