OUR MOON

The Dark Side of the Moon and the Tidal Effect

The side of the Moon that cannot be seen from Earth is also called the "dark side of the Moon". However, this is a misnomer. Because the back side of the Moon also receives sunlight. For example, when the Moon is in a full moon phase, the side facing the Earth receives full sunlight, while when it is in a new moon phase, the back side of the Moon, which is not visible from Earth, receives sunlight.

The Moon is said to orbit the Earth. However, in fact, both celestial bodies orbit around their common center of mass. Since the mass of the Earth is much greater than the Moon's, the common center of mass is closer to the center of the Earth and is located 1,700 km below the surface of the Earth.

Due to the gravitational interaction between them, a tidal effect occurs between the Moon and the Earth. Due to the tidal effect, oceans and even the earth's crust change places a little.The tidal effect is felt more in large water bodies such as the ocean.

The tidal effect created by the Moon on the Earth is 2.2 times greater than the tidal effect created by the Sun.

When the Earth, Moon and Sun are almost in line, that is, when the Moon is in the new or full moon phase, the tidal effect caused by the Moon reaches its maximum level.

When the angle between the lines connecting the Moon and the Earth and the Earth and the Sun is ninety degrees, that is, when the Moon is in the first quarter or last quarter phase, the tidal effect is at its minimum level. Due to the tidal effect, the Earth's rotation around its own axis slows down by an average of 2 milliseconds per century. For this reason, the Earth's day is getting longer. However, this event is happening very slowly. For example, the day, which is 24 hours today, may reach 25 hours in 180 million years.

Since the Earth's rotation is constantly slowing down, one Earth day will eventually be equal to 27.3 days. In this case, one side of the Earth will always be facing the Moon. However, the distance between the Moon and the Earth is not constant either.

The Moon moves away from the Earth by an average of 4 cm per year. Therefore, the time it takes for the Moon to complete a full rotation around the Earth is gradually increasing. As the distance between the Moon and the Earth increases, the Moon's tidal effect on the Earth will decrease, and its slowing effect on the Earth's rotation speed will also decrease. However, long before the Earth's day becomes this long, the Sun will have turned into a red giant.

The Moon does not only affect the Earth through tidal force. The oscillation movement that occurs during the Earth's rotation around its own axis becomes more regular due to the Moon. This situation ensures that the climate on Earth is more stable.

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My Moon's Orbital Features

As the Moon moves in its orbit around the Earth, part or all of its surface periodically brightens or darkens due to the light it receives from the Sun. As a result, it has phases called new moon, crescent, first quarter (half moon), waning moon (before full moon), full moon, waning moon (after full moon), last quarter (half moon) and crescent.

The time it takes for the Moon to complete its orbit around the Earth can be defined in two ways. Let's take a distant star as a reference for an observer looking at the Moon from Earth. When the Moon comes back into line with that star during its orbit around the Earth, it completes one revolution around the Earth. The time it takes for the Moon to complete one full revolution around the Earth is 27.3 days. This is called the "sidereal period".

The time it takes for the Moon to return to the same phase from one phase (for example, crescent) is called the "conjunction period" and lasts 29.5 days. The reason why the sidereal period is shorter than the conjunction period is that the Earth also moves around the Sun. The phases of the Moon change depending on the angle of sunlight reaching the Moon. As a result, the Earth-Moon direction aligns with the distant reference star before the Moon's conjunction period ends, and a sidereal period is completed.

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Why Do We Always See the Same Side of the Moon?

The surface of a satellite belonging to a planet can make a real or apparent oscillation movement depending on the observation time and perspective of the observer looking from the planet. This is called libration in astronomy.

The period of the Moon's rotation around its own axis and the period of its revolution around the Earth are almost equal. This is called rotation-orbit locking or synchronous rotation. For this reason, the Moon always shows us the same face. However, due to the oscillation of the Moon's rotation axis, we see a little more than 50% of the Moon's surface.

There is an angle of 1.5 degrees between the Moon's rotation plane around its own axis and the plane of its orbit around the Earth. For this reason, an observer looking from Earth during one revolution of the Moon around the Earth can also observe some of the Moon's polar regions. This is called latitudinal libration. Since the Moon's orbit around the Earth is elliptical, it moves at different speeds at different points in its orbit. Its rotation around its own axis is more regular. As a result, the Moon oscillates slightly east and west during one full orbit. This is called longitudinal libration. As a result of the combination of these two effects, we see 59% of the Moon's surface, not 50%.

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Is There Water on the Moon?

In the past, studies of samples taken from the Moon's surface showed that the Moon's surface did not contain water. The first evidence of the existence of water on the Moon's surface was obtained by India's Chandrayaan-1 rover, which landed on the Moon's surface in 2008, and the hydroxyl (OH) molecule was found. Later observations and measurements revealed that there was water ice in the inner regions of the craters in the polar regions of the Moon, which do not receive any sunlight. The surface of the Moon is covered with a layer of dust several centimeters thick. Thanks to the micro-scale meteorites hitting the surface of the Moon, it was understood that water molecules under the surface and covered by dust grains spread to the surface. In 2020, water molecules were found not only in the regions close to the poles of the Moon, but also in the regions that receive sunlight.

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The Moon's Internal Structure

The Moon's internal structure consists of the core, mantle layer and surface crust. The Moon is thought to have a solid inner core with a radius of about 250 km rich in iron. Above the inner core is an outer core of liquid iron that is 90 km thick. The core and the liquid iron layer outside it are surrounded by another layer of molten material that is 150 km thick.

After this layer, there is a mantle layer that extends to the crust layer that surrounds the Moon's surface. The Moon's mantle layer is thought to consist of minerals consisting of magnesium, iron, silicon and oxygen atoms.

While the thickness of the Moon's crust is 70 km on the side facing the Earth, it can reach 150 km on the side of the Moon that cannot be seen from the Earth. In other words, the crust layer in some parts of the Moon is thicker than the Earth's crust.

It is thought that the Moon had a strong magnetic field in the past. Today, the Moon's magnetic field strength is thousands of times weaker than the Earth's magnetic field.

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Atmosphere of the Moon

The Moon's very thin atmosphere, called the exosphere, contains argon, neon, hydrogen and helium gases, especially in the regions close to the surface. Meteorites hitting the Moon's surface, high-energy particles carried by solar winds and violet wavelength rays affect the structure of the Moon's atmosphere.

The Moon's day and night last for about two weeks. During the day, atoms and molecules on the Moon's surface are ionized by solar winds, that is, they become electrically charged as a result of electron exchange.

The micro-scale meteorites hitting the Moon's surface move at very high speeds and cause a large amount of heat to be released when they hit the surface. This causes the dust particles on the Moon's surface to vaporize. The released gases mix with the atmosphere.

Because the atmosphere is very thin, there is a significant temperature difference between the parts of the Moon that receive and do not receive sunlight. For example, while the temperature can reach 121°C during the day, it can drop to -133°C at night. In addition, the temperature is much lower in the craters on the Moon's surface, which do not receive any sunlight. The atoms or molecules in these regions can hardly move at all due to the extreme cold and are virtually trapped.