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# The Moon, our neighbour in space (for youth)

As our neighbour and the brightest object in the sky after the Sun, the Moon has inspired and guided humans for thousands of years. It is also at the heart of many stories and legends.

Are you fascinated by our celestial companion? Would you like to find out more about it and understand it better?

Here are the answers to some questions about the Moon and how you see it in the sky.

## How far away is the Moon, and how does it move?

The Moon is approximately 384,000 kilometres (km) from Earth. This distance is greater than we often imagine. In most pictures where we see Earth and the Moon, the distance is not to scale, because the Moon would be too far away and too small.

The Moon is always in motion. It orbits Earth, which means that it revolves around our planet. How does the Moon continue to follow this motion without falling toward Earth? The reason is that it is moving fast enough. If the Moon did not move, it would fall to Earth like an apple being dropped to the floor. And without Earth's gravity pulling the Moon toward us, the Moon would zip away in a straight line. To stay in orbit, it is necessary to have the right balance between the force of gravity that pulls the Moon toward Earth and the speed of the Moon revolving around our planet. Despite moving at a speed of over 3,600 km/h, the Moon takes almost a month (27.3 days to be exact) to orbit Earth! It has a long way to go!

Meanwhile, Earth is also orbiting the Sun. Are you able to visualize all these movements at the same time?

If you wanted to travel the distance between Earth and the Moon on your bicycle at 12 km/h, which is a normal speed for an 8- to 12-year-old, it would take you almost three years and eight months, without stopping!

## What is the far side of the Moon?

When you look at these two images, which one looks more like the Moon as you see it in the sky? The one on the left, correct? The image on the right is also a picture of the Moon, but it is probably less familiar to you. That is the far side of the Moon, the one we never see from Earth.

Before the space age, we had no idea what was on the other side of the Moon, but we now have photographs taken by probes and astronauts on the Apollo missions. Unlike the near side, the far side has very few dark spots, called maria.

Since we always see the same side of the Moon, one might think that it does not rotate on its axis. But it does! The Moon's rotation period is the same as its orbital period: it takes as much time to rotate on its axis as it does to go around Earth (27.3 days). This phenomenon, called synchronous rotation, occurs because of the gravitational pull of Earth on the Moon. Several moons in the solar system have a synchronous rotation and always show the same side to their planet.

If we built a lunar base on the far side of the Moon, we would never see Earth in the sky.

## Is it possible to see the Moon in broad daylight?

Of course it is! We often associate the Moon with night and the Sun with day, but the Moon is in the sky both day and night.

The Moon has always had a strong influence on our rhythms of life and our stories related to the sky. Many cultures have a calendar based on the Moon (think of the length of a month, which coincides with the lunar cycle) as well as legends and myths based on our celestial neighbour. The Moon is also very important to many Indigenous peoples.

## Why does the Moon seem to change shape from day to day?

If you look at the Moon every day, you can see that it does not rise at the same time, but also that it has a different shape. This is called the phases of the Moon. The main phases have names: new moon, crescent moon, quarter moon, gibbous moon and full moon.

To understand the phases of the Moon, you must first understand that the Moon does not emit its own light. You can see it because it reflects the light of the Sun. At any given time, there is a side of the Moon that is lit by the Sun, where it is daytime, and a side that is not lit, where it is nighttime.

Sometimes, from Earth, you can see the entire day side of the Moon: this is a full moon. From your perspective, the Moon appears to be fully illuminated and you can see its entire face. On the other hand, sometimes Earth faces the night side of the Moon: this is the new moon and you cannot see it in the sky.

Between these two extremes, there are different phases:

Even if you only see part of the Moon shining in the sky, you must remember that the rest is still there! The Moon is a sphere and its shape does not really change. A part of it is simply hidden in the darkness.

Be sure to look up at the sky often to see the Moon during its different phases. Better yet, write down your observations and notice the changes from day to day!

If you travel around the world, you will see the Moon positioned in different ways in the sky. For example, a quarter moon appears to be backwards in the Southern Hemisphere compared to the Northern Hemisphere and completely on its side if you are located near the equator!

## What is a solar eclipse?

A solar eclipse occurs when the Moon passes directly in front of the Sun in the sky, like when someone passes in front of you and blocks your view of a lamp. Another way to think of a solar eclipse is that the Sun, the Moon, and Earth are perfectly aligned in space and the Moon's shadow is cast on Earth. This rather rare alignment occurs only about once every six months because the trajectory of the Moon around Earth is not perfectly aligned with that of Earth around the Sun. Therefore, there are usually at least two solar eclipses per year somewhere on Earth, but in a different region each time. That is why they are very rare for a specific location, such as where you live.

A solar eclipse always occurs during the new moon and lasts a few hours. The Moon gradually covers the Sun in the sky during the periods of partial eclipse. If the alignment is perfect, the Moon completely covers the Sun for a few moments to produce a total solar eclipse. It is quite a show!

One day, there will no longer be any total solar eclipses on Earth, because the Moon is moving away from us very slowly (about 3.8 cm per year). It will therefore appear a little smaller in the sky as time goes by. In about 600 million years, it will not be at the right distance to cover the Sun completely when the two are aligned in the sky. We are so fortunate to be able to enjoy these beautiful spectacles of nature now!

## What is a lunar eclipse?

A lunar eclipse occurs when the Moon enters Earth's shadow. This type of eclipse always occurs at the full moon, when the Sun, Earth and the Moon are aligned. As the Moon enters Earth's shadow, part of the Moon "disappears." This is the partial eclipse period. After several minutes, the Moon is completely in Earth's shadow and appears much darker. It does not disappear completely, however, and takes on a beautiful reddish tint. This is the total eclipse!

It is also possible, when the alignment is not perfect, that the eclipse is never total and that only a part of the Moon is hidden at the peak of the eclipse.

As the Moon continues its orbit around Earth, it emerges from the shadow on the other side. There is a second period of partial eclipse until the Moon is completely out of the shadow. In total, the eclipse can take a few hours and is always very impressive to watch!

The second full moon in a month is sometimes called a blue moon, but that does not mean it is blue! It comes from the expression "once in a blue moon," meaning something that rarely happens. It is indeed rare to see two full moons in the same month since the time between two consecutive full moons is 29.5 days. Therefore, the first full moon must fall on the first of the month. The second full moon in the month will be like any other full moon, but it will be called a blue moon. A strange expression, don't you think?

## Are all eclipses dangerous to watch?

Only solar eclipses are dangerous to look at, because you are looking at the Sun. It is always dangerous to look directly at the Sun because its light is so intense that it can permanently damage your eyesight. Fortunately, there are ways to protect your eyes so that you can enjoy the show, such as special eclipse glasses or a solar eclipse projector, which you can build yourself.

During a lunar eclipse, you are looking at the Moon, so it is safe. The Moon is never very bright because it does not emit its own light. So it is safe to look directly at it.

Bottom line: never look directly at the Sun, eclipse or not!