MOON: CHARACTERISTICS, STRUCTURE, ORBIT, PHASES, MOVEMENTS - ARTE - 2025

The Moon is the natural satellite of the Earth, linked to it by gravitational attraction. Because it is the closest star to our planet, it is the most familiar to everyone and the first that humanity has visited. It is a rocky body with a radius of 1738 kilometers, close to half the Earth's radius, while its mass is barely 1/81 of the mass of the Earth.

As for its average density, it is 3.3 times that of water, while the average density of the Earth is 5.5. And of course there is its gravity, which is 0.17 times the value of Earth's.

Figure 1. Full moon. Source: Pixabay.

In a scale model with the Earth the size of a basketball, the Moon would be a tennis ball, and the two balls would be about 10 meters apart.

The actual Earth-Moon distance of about 385 thousand kilometers more or less. It takes 1.3 seconds for the light that the Moon reflects from the Sun to reach the Earth.

Another significant characteristic is that the Moon has no atmosphere of its own, there are hardly any traces of some gaseous elements such as hydrogen, helium, neon, argon and others in minute quantities.

And an even more striking detail is that the Moon always shows the same face to the Earth. It is because its period of rotation around its axis is equal to that of its orbit around the Earth: approximately 27 days.

If there were any difference between the two periods, the far side of the Moon would be visible from Earth at some point, but this is not the case and it is due to the effect called tidal coupling. This effect will be discussed in more detail later.

Structure of the Moon

Figure 2. Cross section of the Moon showing its structure of layers and the approximate radius of each of them. Source: Wikimedia Commons. Bryan Derksen at English Wikipedia The internal structure of the Moon is known thanks to the seismographs installed by the Apollo missions. Seismographs are devices that record the movements of the Earth and that on the Moon are capable of recording moonquakes, waves produced by the impact of meteorites.

From these records, the Moon is known to have the following layered structure:

-Cardk, approximately 80 km thick, thinner on the face facing Earth and thicker on the opposite face, due to tidal forces.

-Manto, with an estimated radius of around 1,300 km, composed mainly of iron and magnesium oxides.

-Nucle, small, approximately 587 km in radius, which in turn is made up of an inner solid core, an outer and liquid core plus a semi-melted surrounding layer.

-The Moon lacks tectonic activity, unlike the Earth, since it has lost almost all its internal heat, by cooling very quickly.

Lunar surface

Figure 3. Image of the lunar surface on the far side. Source: NASA via Wikimedia Commons.

The lunar surface is covered in sticky, abrasive dust called regolith. The dark areas that are distinguished are called seas, from the Latin "mare", although they do not contain water, but solidified lava.

These seas are believed to have been caused by the impact of large asteroids about 4 billion years ago, and that they were later filled with lava that flowed from within. The Mare Imbrium is the largest, with 1200 km wide.

The clearest areas that can be seen surrounding the seas are mountainous regions with mountain ranges that have been named after those of the Earth, for example the Alps and the Carpathians.

Distinctive is the presence of numerous craters of all sizes, possibly caused by impacts from small asteroids and meteorites. They are named after famous people, for example the Copernicus crater.

Another theory about the origin of lunar craters considers that they have volcanic origin, although the theory of the origin by meteors has a greater support on the part of the astronomers.

Deep cracks also exist on the surface of the Moon, the origin of which is not yet entirely clear, although it is believed that they come from ancient lava flows. An example is the Hyginus rift, with two branches in the center of which is a crater with the same name.

The images taken by the spacecraft on the side that we cannot see show a surface similar to that on the visible side, although with fewer seas.

Orbit

Thanks to the gravitational attraction exerted by the Earth, the Moon follows an elliptical orbit of little eccentricity from east to west around our planet, according to Kepler's laws.

That is why the Earth-Moon distance indicated at the beginning of 385 thousand kilometers, is an average distance, although due to its small eccentricity, the orbit is almost circular. That is, sometimes the Moon is closer (perigee) and other times it is further away (apogee).

Furthermore, it is not a fixed orbit, as there are other disturbances, such as the gravitational attraction of the Sun and the other planets, which continually modify it.

The plane that follows the lunar orbit does not exactly coincide with that that follows the orbit of the Earth, but is inclined about 5º approximately. During a revolution, the Moon is located about 5º above and below the plane of Earth's orbit. Both orbits intersect at points called lunar nodes.

The following is a representation of the Earth revolving around the Sun and the Moon around the Earth:

Synchronous rotation

The Moon always shows the same face to the Earth, therefore there is a dark side that cannot be seen from here. The explanation is that the Earth and the Moon form a system under mutual gravitational action, but the Earth has a greater mass.

In this case, the smaller body couples its movement to that of the larger body, that is, it equates its period of rotation with that of translation.

Figure 4. Synchronous rotation of the Moon and Earth. Source: Wikimedia Commons. Fernando de Gorocica The Earth-Moon system has come to this because of tidal forces, just as it was said at the beginning. And at the same time it happens because the gravitational attraction is not uniformly "distributed", since the Earth and the Moon have appreciable dimensions.

In other words, the parts of each of them closest to the other attract more strongly than the extremes further away, and this difference can be large enough to cause a bulge on the planet.

This is how the Moon is responsible for the Earth's tides, because the oceans "rise" in response to the gravitational pull of the satellite. But the lunar crust also deformed, giving rise to frictional forces that caused its period of rotation to gradually decrease.

This phenomenon is frequent between a planet and its moons, for example Pluto and its satellite Charon are in synchronous rotation with each other.

The dark side of the moon

Long ago, when the Moon had just formed, it was spinning faster around its axis and was closer to Earth than it is now. So at some point in early Earth history, it must have looked like a huge silver disk lighting up the night sky.

This hemisphere of the Moon is always the same, seen from Earth, as has been explained. However, one half of the Moon always receives sunlight (and it is very hot there, approximately 134 ºC) and the other half does not, unless an eclipse occurs. But these halves do not correspond to the faces we see from here.

The lunar hemisphere that receives sunlight is the one that looks directly at it, while the other is dark and very cold, about -153 ºC. The thin lunar atmosphere is responsible for this large variation in temperature.

These hemispheres change as the moon continues its translational movement around the Earth, so that in reality the entire Moon receives light from the Sun at some point.

Moon phases

Figure 5. Illustration of the phases of the moon. Source: Wikimedia Commons. Orion 8.

Viewed from Earth, the Moon undergoes changes in its illuminated part over a period of about a month. They are the so-called lunar phases: new moon, first quarter, full moon and last quarter, which are repeated continuously in that same order.

Actually, the time it takes for the Moon to go through all its phases is just under a month. This period is called the lunation or synodic month and lasts 29 days and 12 hours.

The phases of the moon depend on the relative position between the Moon, the Earth and the Sun. Let's see:

New Moon

In a new moon or new moon it is hardly possible to distinguish the Moon, because being interposed between the Earth and the Sun, the visible side from here is not illuminated.

Crescent quarter

Then, over the period of approximately 7.4 days, which is approximately the duration of each phase, the illuminated area gradually increases until it reaches the first quarter, where half of the lunar disk is illuminated. It can be observed from noon to midnight.

full moon

The illuminated area continues to increase after the first quarter until reaching the full moon or full moon, when the Moon is behind the Earth, and the Sun illuminates it completely from the front (figure 1). The full Moon can be seen from the time the Sun sets until sunrise, reaching its maximum height at midnight.

Last quarter

Finally, the size of the Moon decreases little by little, going to a last quarter, when again half of the disk is illuminated. It can be seen leaving around midnight, until it reaches its maximum height at sunrise. Then it continues to decrease to start a new cycle.

It should be noted that from the northern hemisphere the movement of light goes from right to left and in the southern hemisphere it is the opposite.

So we can know for example if the moon is waxing or waning. If it is in crescent, the right side of the Moon is the one that is illuminated in the northern hemisphere and the left side if it is in the southern hemisphere.

Movements of the Moon: rotation and translation

The Moon makes a complete orbit or revolution around the Earth in 27.32 days, called the sidereal month (not to be confused with the synodic month of 29 days and 12 hours). It does so at a rate of 1 km / s.

The difference between the sidereal and the synodic month is due to the fact that while the Moon is drawing its orbit, the Earth advances 27º in its own translational movement around the Sun. When this happens, the relative positions Sun-Earth-Moon become the same.

Our satellite also performs a rotation on its own axis in the same period of time, because of the synchronous rotation.

Hovers

The Moon executes more movements apart from rotation on its axis and translation, which are considered the main movements. In addition to them it has the hovers.

Librations are oscillatory movements of the Moon that allow us to observe 59% of its surface, instead of the 50% expected due to the fact that it always offers the same face to the Earth. They have been known since the time of Galileo.

Composition

The Moon is rocky and has a very thin atmosphere. The presence of liquid water is ruled out in the lunar hemispheres exposed alternately to the Sun, due to the high temperatures that are reached there.

However, at the lunar poles there are craters that have not been reached by solar heat for millions of years. Temperatures can drop to a cold -240ºC.

There the probes sent by India and the United States managed to detect water in the form of ice.

Regarding the composition of lunar rocks, they are abundant in oxygen: up to 43%. In addition, 20% silicon, 19% magnesium, 10% iron, 3% calcium, 3% aluminum, 0.42% chromium, 0.18% titanium and 0.12% manganese are estimated. Silver and mercury have also been detected in lunar dust.

But instead there is no free carbon, nitrogen and hydrogen, the elements that make up living matter. And in lunar rocks there is no water, unlike terrestrial rocks, in whose structure it is found.

Training

The most accepted theory among the scientific community is that the Moon had its origin in a collision between the Earth and an object similar to Mars or greater than it, named Theia, during the formation of the solar system.

Apart from giving rise to the Moon, the collision with Theia altered the tilt of the Earth's axis of rotation and destabilized the early atmosphere.

This theory explains why the Moon is less dense than the Earth, since the collision with Theia tore part of its mantle, whose density is similar to the lunar density. However, it does not explain the existence of the semi-molten core of the Moon, which is known to exist thanks to seismic information.

Another alternative theory considers that the Moon formed elsewhere in the solar system and was captured by Earth's gravity at some point.

The basis for these ideas is that lunar rocks, although they contain the same elements as those on Earth and are of the same age, have many differences from a chemical point of view.

Eclipses

lunar eclipse

Figure 6. Eclipse of the Moon. Source> Wikimedia Commons.

The apparent diameters of the Sun, Earth, and Moon are the same as seen from Earth. So when the Earth is between the Sun and the Moon, it is possible to observe a lunar eclipse.

The lunar eclipse can only occur during the full moon and when it falls within the shadow of the Earth, called the umbra. In this way it is darkened, acquiring a reddish or orange hue, depending on the atmospheric conditions of the Earth. It can be seen in the following image:

The Moon can fall totally in the shadow of the Earth or only in part, in the first case the eclipse is total and otherwise it is partial. Partial eclipses can be mistaken for a phase of the moon, until the eclipse ends and the full moon rises again.

Unlike eclipses of the sun, lunar eclipses can be seen from anywhere in the world where it is at night and can also last several hours.

Sun eclipse

Figure 7. Eclipses of the Sun. Source> Wikimedia Commons.

When the discs of the Sun and the Moon coincide, seen from some point on Earth, an eclipse of the Sun takes place. The Moon seems to pass in front of the Sun, for which it is necessary that it is in the new moon, although solar eclipses they do not occur on every new moon.

For the solar eclipse to occur, the alignment between the Sun, Earth and Moon must be total, and this does not happen all the time, but at least twice a year, up to a maximum of five. As for the duration, the time that the Sun remains obscured is variable, in the order of about 8-10 minutes.

Eclipses of the Sun can be total, partial or annular, depending on whether the moon covers the Sun completely or in part. In the case of annular eclipses, the relative diameter of the Moon is not enough to completely cover the Sun, leaving a luminous ring of this visible. The following is a total solar eclipse:

Total eclipses of the Sun are awesome celestial phenomena and provide a wonderful opportunity to study details of the outermost layers of the Sun.

Influence on life on Earth

The Earth and the Moon form a wonderful duet that has influenced life and humanity in particular since the beginning of time:

-Thanks to the Moon there are seasons.

-Each year the Moon moves about 4 cm away from the Earth, which helps to slow down the Earth's rotation and lengthens the days by a few thousandths of a second. This distance is not constant, since it depends a lot on the disposition of the continental and aquatic masses of the Earth, which, as we know, has changed a lot since the formation of both.

-Thanks to this lengthening of the days, the plants have had enough time to carry out photosynthesis.

-If the theory of the impact with Theia is true, the Earth's atmosphere underwent modifications that made it more appropriate for the emergence of life.

-The Moon has served as a guide during the development of humanity, for example farmers, even today, use the lunar phases to cultivate fields.

-Ocean tides are produced thanks to the gravitational interaction between the Earth and the Moon and are extremely important for fishing and the climate, as well as being sources of energy.

Figure 8. Old tide mill in Huelva, Spain. Source: Wikimedia Commons.

-There is a popular belief that the full moon influences people's mood, making them more susceptible from a psychological point of view during this period.

-The Moon has served as inspiration for countless science fiction novels and films, even before the space race began.

References

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