# How old is the moon

### What is the moon

It is the brightest celestial body in the night sky: the moon. It shines so brightly on full moon nights that some people find it difficult to sleep. It appears as big as the sun and the stars look like tiny points of light next to it.

But the impression is deceptive: In reality, the moon (diameter: 3474 km) is only about a quarter the size of the earth (12742 km) - and the sun (1.39 million km) is even four hundred times larger. The moon only appears the same size to us because it is so close to us - the sun (distance to the earth about 150 million km) is about four hundred times further away than the moon. (384,400 km, an airplane needs 18 days for this distance!)

The bright light is also deceptive: unlike the sun, the moon does not shine by itself, but is illuminated by the sun. Some of this light is then reflected back from the surface of the moon and hits the earth. Just because the moon is so close to us, enough light arrives on earth to light up the night - at least if the moon doesn't just seem to have disappeared without a trace ...

21.7.1969

Tense waiting in the control center. A series of warnings from the on-board computer almost led to the mission being aborted, now this: The planned landing site is littered with small craters and rocks. The commander Neil Armstrong grabs the control stick and tries to land the lunar module by hand. But the fuel is running out ...

Finally the redeeming radio message comes: “The eagle has landed.” For the first time, a spaceship with people on board touched down on the moon. A few hours of rest and preparation, then Armstrong opens the hatch and climbs down the ladder. With the words "a small step for a person, but a gigantic leap for mankind" he is the first person to set foot on the moon. Shortly afterwards his colleague Buzz Aldrin follows.

The stay is only short: in two and a half hours on the lunar surface, the astronauts set up an American flag, collect a few kilograms of lunar rock and set up various scientific experiments on the lunar surface.

After another pause, they ignite the engine and fly back into a lunar orbit. Michael Collins is waiting there in the Columbia space capsule, which is supposed to bring them back to Earth.

### A tape measure to the moon

Among the devices Aldrin and Armstrong placed on the moon was a special mirror. It is constructed in such a way that it reflects every ray of light back to its starting point. With a well-aimed laser beam, scientists can now take aim at this mirror - and stop the time until the reflected laser beam arrives at them again. If the watch is accurate enough, you can measure the distance to the moon to within a few millimeters. They made a surprising discovery: the moon moves about 3.8 centimeters away from the earth every year!

### Why do planets have moons?

Earth has one, Mars has two, Jupiter and Saturn even over sixty each! Only two planets in the solar system have to do without moons: Mercury and Venus, all other planets have at least one moon. But why do most planets have moons? And what is a moon anyway?

For us, the moon is first and foremost the bright circle that stands in the sky at night. It looks small, but in reality it is a large rock ball 3475 km in diameter that circles the earth. And it is exactly the same with the other planets: They are also orbited by smaller or larger celestial bodies on regular orbits. Astronomers also call these celestial bodies “moons”.

To get to a moon, a planet usually has two options: Either the moon is created together with its planet, or the planet is created first and later captures a smaller celestial body.

These smaller celestial bodies are asteroids that fly ownerless through the solar system. When they get near a much larger planet, they are drawn to its gravity. This forces the asteroid into an orbit around the planet - the planet has got a moon. This “catching” of a moon works better, the heavier the planet is. This is why the large and heavy planets Jupiter and Saturn also have most of the moons in the solar system.

Other moons formed from debris left over when their planets formed: In the beginning, the solar system was nothing but a large disk of dust, gas, and ice. In the middle, the matter agglomerated particularly strongly - here the sun was created, surrounded by the remaining disk of dust, ice and gas. In this disk the same thing was repeated on a small scale: again compact lumps formed - the planets - and the remaining dust collected in a disk. And if there was enough matter in this disk, smaller lumps were formed there: moons. (Only when the gravitational pull of the planet was very strong were the lumps immediately torn apart. This was the case, for example, close to Saturn, which is still surrounded by rings of dust to this day.)

Both moons that emerged from the dust debris and the captured moons are much smaller than their planets.

The earth is the big exception: its moon is much larger than it should be compared to the earth. Therefore it can neither have originated from leftover dust nor simply been captured. Instead, the earth owes its moon to a cosmic catastrophe that almost destroyed the planet:

Shortly after the earth was formed, it collided with a celestial body that was about half the size of itself. The force of this impact cannot be imagined: The explosion was so strong that most of the young earth melted again - and the other celestial body as well. Part of the molten mass was thrown away and gathered in an orbit to form a second ball. Over time, these two spheres cooled and solidified again. Today the larger sphere orbits the sun as the earth - and the smaller one orbits the earth as the moon.

### What is our solar system and how did it come about?

The earth is not alone in space: people have been observing the sun, moon and stars in the sky for a long time. They discovered early on that some stars are moving. These wandering stars were observed and their paths followed. But for a long time they did not understand their movements - until about five hundred years ago a man by the name of Nicolaus Copernicus solved the riddle: The earth and the "wandering stars" are actually planets that all orbit the sun at different distances.

Today we know eight planets. To remember their names in the correct order, the first letters of the sentence "M.a Vater eclarifies mir jEden S.monday uurens Nachthimmel. “- or in short: M-V-E-M-J-S-U-N.

M.Erkur is the planet that orbits closest to the sun. Then come Venus, E.rde and M.ars. These four inner planets have a solid surface made of rock and are still relatively close to the sun - only a few hundred million kilometers.

They are circling further out, at a distance of about one to 4.5 billion kilometers from the sun outer planets: Jupiter, S.aturn with his rings, Uranus and all the way outside Neptun. They are made of gas (mostly hydrogen and helium) and are much larger than the inner planets. Jupiter and Saturn are about ten times the size of the earth, that's why they are also called that Gas giants.

And finally there are asteroids, comets, and clouds of dust that also orbit the sun. The gravitational pull of the sun holds all these heavenly bodies together and forces them to fly in a circle like on a long line. Everything together is called that Solar system. The moons are one of them - but they are held in place by the gravitational pull of the planets.

But why does the sun even have planets? This has to do with how the sun came into being: a cloud of gas and dust contracted by its own gravity and became a star. But not all of the material in this cloud was "built into" the star - around one percent was left over. And when the sun began to shine, the radiation pushed the remaining matter back outwards.

The light gases were pushed far outwards, the heavier dust and rocks remained close to the sun. From these clouds of dust and gas, the planets emerged over time. Therefore there are the gas planets outside in the solar system, further inside the rock planets - including our earth - and in the very center the sun. It contains 99% of the mass of the solar system and holds everything together with its gravity.

### 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 further on its orbit, that changes: The rays of the sun now hit it from the left side 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 the sun hit the side facing us again. 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 can we see the moon during the day too?

The tasks are clearly distributed: the sun shines during the day and the moon shines at night. But that's not true at all: The moon can sometimes be seen during the day - what is it doing there?

Day and night have a simple cause: the earth rotates. If our location on earth is pointing towards the sun, it is light, i.e. day. Later, when the earth continues to rotate, our location moves to the side facing away from the sun. We watch the sun go down and it gets dark.

The moon rises and sets too - for exactly the same reason: because the earth rotates. But the moon also moves: in the course of four weeks it circles the earth once. Half of this time, its orbit is on the side of the earth facing away from the sun. From there you can always see it when your location has just turned away from the sun - or in short: when it's night. But two weeks later the moon is on the side facing the sun. Then it is exactly the other way round: You can see it together with the sun during the day when your own location is facing the sun.

So the moon can sometimes be seen during the day and sometimes at night, even if for us it actually belongs to the night. But that's simply because the moon is the brightest light in the sky at night, which makes it so much more noticeable.

### 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 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 impacts 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-colored crustal rock and, in the lowlands, darker lava rock. From the earth it looks like spots, seas - or a face.

### ebb and flow

Anyone who has already vacationed at the North Sea or the Atlantic knows the problem: You go to the beach to swim and the water is much further away than when you last bathed. The water level has sunk: it is ebb tide. If you want to get into the water now, you either have to walk a bit over damp sand and silt or wait a few hours until the tide comes in and the water rises again.

Ebb and flow alternate in a regular rhythm. This change is called the tide. The time interval between ebb and flow is a little more than six hours. There are twelve hours and 25 minutes between one flood and the next. How much the water rises and falls depends on the coast. At the North Sea, the difference between high and low water is about two to three meters. Elsewhere, however, it is much larger: In the Bay of Fundy in Canada, the water level fluctuates by 15 to 21 meters - this is the highest tidal range in the world!

But why is it that the water sloshes back and forth in the oceans? The solution lies in the gravitational pull of the moon. This force causes two huge flood mountains under which the earth rotates. One of the two comes about directly through the gravitational pull of the moon, because it pulls the water towards itself. The second flood mountain is exactly on the opposite side of the earth. This arises because the earth does not rotate perfectly evenly due to the gravitational pull of the moon, but “rocks” a bit. As a result, there is a centrifugal force that pulls the water away from the moon. Both flood mountains are about half a meter high.

Not only the moon, but also the gravitational pull of the sun has an effect on the water. When the sun and moon are on the same line, the tide rises higher than normal due to the mutual attraction: there is a "spring tide". If, on the other hand, the sun and moon are at a 90 degree angle to the earth, then their forces partially cancel each other out. The result is a less high tide, the nipp tide.