Why does the moon stay up in the sky? Because it’s always falling.

If you go out at night on the 31st of January, remember to look up at the moon.

Wednesday’s full-moon is going to be a special one. To start with, it’s the second full moon of the month. Months don’t get more then one full moon very often, because they’re almost the same length as one full cycle of the moon — from full to new and back again. When it does happen, the second moon is known as a “blue moon”, even though it’s not blue.

This month’s moon is definitely not going to be blue. In fact, it’s going to be red.

That’s because this full-moon is also on the day of a lunar-eclipse. The day when the Moon goes behind the Earth, to the side opposite the Sun. The Earth’s shadow will then fall on the Moon. But it won’t be black. The Earth doesn’t block all the light coming from the Sun: some of it bounces round in the atmosphere and manages to get through to the other side. That’s what will make the Moon glow a dull red.

And that’s not all. This week’s full-moon is also going to be one of the biggest full-moons you can see. While the moon travels round the Earth, it doesn’t always travel at the same distance. It comes closer sometimes and further sometimes. And this month’s full-moon is coming up at a time when the Moon is closest. So this could end up being one of the biggest lunar-eclipses you’ll ever see!

If the Moon travels further and closer, how does it manage to stay up in the sky? Why does it never fall?

Actually, that’s precisely what keeps the Moon in the sky: the fact that it’s always falling.

Normally, gravity makes things fall. Or rather, it attracts two things together, a bit like magnets. When you jump and the Earth pulls you back down, then you pull the Earth up, too. But, because the Earth is so much bigger than you, the second pull is very slight and amounts to almost nothing at all.

The Moon is bigger than you. Much bigger. It’s almost one-fourth as big as the Earth itself. So the pull of the Moon does make a noticeable difference. Have you ever wondered why the ocean has tides? Why the water-level at the beach becomes higher and lower through the day? That’s the work of the Moon’s gravity: it’s pulling the water up!

The Moon also helps the Earth move smoothly. It helps the Earth keep on track as it moves around the Sun.

If it wasn’t for the Moon, the Earth’s journey round the Sun would have become much more wobbly over time. The weather would have become more erratic, and the seasons more extreme. It’s the gentle tug of the Moon, going round and round the Earth and adjusting it from every angle, that keeps the journey smooth.

Astronauts can feel the Moon’s gravity too. They can feel it more directly: by actually landing there.

If you walk on the Moon, you’ll feel much lighter than you did on Earth. You’ll be able to walk around easily, even if you’re wearing heavy space-suits to stay alive. You can also jump much higher than at home — but don’t do it too much, because you might damage your space-suit!

When astronauts return back to the more powerful earth-gravity, they feel very weak and heavy. A bit like how you would feel if you came out of a swimming-pool after being in there for a long time.

Too much time in low-gravity is not healthy. Your body gets used to it, and your bones and muscles become weaker. Your heart won’t pump blood as hard as it used to. When astronauts are returning to Earth after spending a long time in a space-station, they have to first do a lot of exercise to make sure they’re fit enough. Stay up there too long, and you’ll never be able to come back.

How will other animals and plants react, if they’re put on the Moon? Will they be able to stay there and live out a full life? That’s one of the things the Chang’e 4 mission is going to find out.

The Chang’e space programme is named after the Goddess of the Moon. The fourth mission, to be launched in June this year, includes a small aluminium case. The case be carrying potatoes, Arabidopsis seeds, and silkworm eggs. The eggs will hatch into silkworms that breathe out carbon-dioxide, which the plants can use while they’re producing oxygen for the silkworms. That will create a small ecosystem right on the Moon.

Scientists can find out how plants and insects live on the low moon-gravity. Later, they might be able to make a bigger ecosystem, one that humans can live in as well. Of course, a human-base will need more preparation

That’s another objective of the Chang’e 4 mission. It includes a rover to explore the South Pole-Aitken Basin, which has been found to contain lots of water-ice. This part of the Moon is always dark and shadowy: that’s probably why the water lasted so long without evaporating and escaping into space.

Because of the water, many people have said this would be a good place to establish a permanent base on the Moon. The Chang’e rover will run experiments to find out more about the place. The experiments will cover many things, but they could also be the first step to setting up a permanent lunar base.

A lunar base can act as a stepping-stone for missions to other planets. There are already many space-agencies talking of going to Mars, although some details are not quite clear.

Of course, they haven’t decided what to do if they find somebody already living there.

Before going to the Moon, Neil Armstrong and the other Apollo 11 astronauts had to go through a lot of training. Part of that training was to spend time in a desert, whose vast landscape was a lot like the Moon’s.

It was there (so the story goes) that the astronauts came across and old Native American man. He asked what they were doing, and they explained they were preparing to go to the Moon.

On hearing this, the man was very interested. He asked the astronauts if they could do him a favour. “In my tribe”, he said, “we believe that there are holy spirits living on the moon. I was wondering if you could pass on an important message from by tribe to them.”

When the astronauts asked for the message, the man spoke some words in his native tongue. He made the astronauts memorise it, and took great pains to make sure they pronounced the words correctly.

“But what does the message mean?” the astronauts asked.
“That is between my tribe and the moon-spirits,” the old man said. “I cannot reveal the secret to anyone else.”

Of course, that made the astronauts very curious. When they got back from the training, they hunted everywhere for someone to translate the message for them.

Finally, they found one person who could speak the old man’s tongue. They repeated the message to him and asked him what it meant. On hearing the message, the translator began to laugh and laugh.

Finally, he managed to control himself, and tell the astronauts what was so funny. The secret message, he explained, actually meant: “Don’t believe a single word these people are telling you. They have come to take away your lands.”

As it turned out, there weren’t any moon-spirits living up there. (Or, if there were, they were wise enough to stay hidden). Humans probably wouldn’t live there long, either. They’ll either go back after spending a few months, or continue on to other planets like Mars.

A lunar base would make a good “stepping stone”. That’s because it’s much easier to launch a spacecraft from the Moon than from the Earth. Why? Because the gravity’s much lower there.

If you throw a stone, it flies forward for a while but eventually falls down.

If you get a machine that shoots it out fast enough, it could continue over the horizon before hitting the ground.

All these stones are trying to fly straight, but gravity makes their paths bend. It makes them bend so much that they finally hit the ground and stop.

If you make them move fast enough, they’ll still be falling down — but they’ll be travelling straight even faster. So the path they travel will only bend a little bit. And if that bending is little enough, they won’t fall fast enough to actually reach the ground. They’ll fly out, up and away.

On the Moon, gravity is much lower than on Earth. So you don’t need to travel so fast to go out, up and away. That means rockets don’t have to be so powerful, and they don’t have to use so much fuel. Spacecraft and equipment could be assembled on the Moon, made ready, and then launched off into the right direction, with minimal fuel.

Fly out or fall in — that’s not all you can do. What if you travel at an in-between speed? If you go too slow to escape the Moon’s gravity, but too fast to be actually pulled back in?

Why, then your path will bend round just enough to form a perfect circle. You’ll travel right round the Moon, back to where you started…and keep going.

You’ll be falling down all right. But you’ll also be travelling straight so quickly, the Moon’s surface will curve away as you reach it. And so it will continue, over and over again.

In fact, that’s exactly what the Moon itself does, to the planet Earth! And what the Earth does, going around the Sun. It’s what anything does, when it’s “in orbit” around another thing.

And that’s the reason why the Moon stays up in the sky.
Because it’s always falling.

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