The rising Earth

Topsy-turvy world – the Earth rises above the lunar horizon. On Christmas Eve 1968, the American astronaut William Anders took this famous photo.

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 again during one of these orbits, they saw the Earth’s sphere emerge from behind the lunar horizon. They were deeply impressed by this sight and took several photos – although Anders jokingly remarked that this was not even foreseen in the mission plan.

However, it is not possible to experience a real “earth rise” – as we see the moon rising while standing on the earth – on the lunar surface. This is because the Moon always faces the Earth on the same side. When you are on this side, you see the Earth all the time – and always at 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 7 October 1959, the Russian probe “Luna 3” took the first photo of the far side of the moon. However, the world still had to wait eleven days for this historic photo: only when the probe flew back towards Earth was the radio connection good enough to send the picture.

At first glance, the image does not look very spectacular. The resolution is poor, and since the sun shines almost vertically on the lunar surface, no shadows of mountains and craters can be seen.

But there was one 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 Earth, you only ever see the same side of the moon.

The first man on the moon

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

Finally the relieving radio message comes: “The Eagle has landed.” For the first time, a spaceship with people on board has touched down on the moon. A few hours of rest and preparation, then Armstrong opens the escape hatch and climbs down the ladder. With the words “a small step for a man, but a giant leap for mankind”, he sets foot on the moon as the first human being. Shortly afterwards, his colleague Buzz Aldrin follows.

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

After another rest period, they ignite the engine and fly back into lunar orbit. There, Michael Collins is waiting in the Columbia space capsule to take them back to Earth.

A tape measure to the moon

Among the devices Aldrin and Armstrong set up on the moon was a special mirror. It is constructed in such a way that it throws every beam of light back to its point of origin. With a well-aimed laser beam, scientists can now aim at this mirror – and stop time until the mirrored laser beam arrives back at them. If the clock is accurate enough, they can measure the distance to the moon to within a few millimetres. In doing so, they have made a surprising discovery: the moon moves away from the Earth by about 3.8 centimetres every year!

Why do we only see one side of the moon?

Have you 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.

This seems quite normal at first – until you consider that the moon moves around the earth in the course of a month. So we should see it from all sides during this time. But the moon always looks the same – why?

You could also ask differently: the moon is a celestial body like the earth – and we know that the earth rotates on its own axis. This is another reason why the moon should show itself to us from all sides.

Both thoughts are correct: The moon travels around the earth and rotates in the process. The special thing about it: Both movements have exactly the same speed. During a month, the moon goes around the earth once and also rotates around its own axis exactly once. As a result, the same side always faces the Earth, as if the moon were bound. Scientists speak of “bound rotation”.

This is no coincidence and was not always the case. When the moon was newly formed, it rotated much faster. But the Earth’s gravitational pull slowed the Moon down until its rotational speed was aligned with its orbital speed.

Conversely, the Moon’s gravitational pull also slows down the Earth’s rotation, so that the days always get a little longer. But since the Earth is much heavier than the Moon, this “brake” does not have such a strong effect. It will take about 200 million years until the Earth’s rotation has slowed down so much that a day lasts 25 hours. Whether there will still be people then and how they will spend this extra hour cannot be predicted, unfortunately.

What is the moon?

It is the brightest celestial body in the night sky: the moon. On full moon nights, it shines so brightly that some people have trouble sleeping. 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 as big as the earth (12742 km) – and the sun (1.39 million km) is even four hundred times bigger. The moon only appears to be the same size because it is so close to us – the sun (distance to earth about 150 million km) is also about four hundred times further away than the moon. (384400 km, an aeroplane needs 18 days to cover this distance).

The bright light is also deceptive: unlike the sun, the moon does not shine by itself, but is illuminated by the sun. Part of this light is then reflected back from the moon’s surface and hits the earth. It is only because the moon is so close to us that enough light arrives on Earth to light up the night for us – at least when the moon does not seem to have disappeared without a trace …

Why do planets have moons?

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

For us, the moon is first of all the bright circle that is in the sky at night. It may look small, but in reality it is a large sphere of rock 3475 km in diameter orbiting the Earth. And it is exactly the same with the other planets: They are also orbited by smaller or larger celestial bodies in regular orbits. Astronomers also call these celestial bodies “moons”.

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

These smaller celestial bodies are asteroids that fly through the solar system without a master. When they come close to a much larger planet, they are attracted by its gravity. This forces the asteroid into orbit around the planet – the planet has been given a moon. This “capture” of a moon works better the heavier the planet is. That is why the large and heavy planets Jupiter and Saturn have the most moons in the solar system.

Other moons have formed from dust remnants left over when their planets were formed: In the beginning, the solar system was nothing but a large disc of dust, gas and ice. In the middle, the matter concentrated particularly strongly – this is where the sun was formed, surrounded by the rest of the disc of dust, ice and gas. In this disc, the same thing happened again on a small scale: Again, compact clumps formed – the planets – and the remaining dust gathered in a disc. And if there was enough matter in this disc, even smaller lumps were formed there: moons. (Only if the gravitational pull of the planet was very strong, the lumps were broken up again immediately. This was the case, for example, close around Saturn, which is still surrounded by dust rings today).

Both moons that were formed from the dust remnants and the captured moons are much smaller than their planets.

The Earth is the great exception: its moon is much larger than it should be compared to the Earth. Therefore, it could neither have formed from dust remnants nor been captured just like that. 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 about half the size of itself. The force of this impact cannot be imagined: The explosion was so powerful that most of the young Earth melted again – and so did the other celestial body. Part of the molten mass was hurled away and gathered in orbit to form a second sphere. Over time, these two spheres cooled and became solid again. Today, the larger sphere orbits the sun as the Earth – and the smaller one orbits the Earth as the moon.