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Moon

Moon

The Moon is the only natural satellite of the Earth.

The Moon is the only natural satellite of the Earth. The closest satellite of the planet to the Sun, since the planets closest to the Sun do not have them. The second brightest object in the earth's sky after the Sun and the fifth largest natural satellite of a planet in the solar system. The average distance between the centers of the Earth and the Moon is 384,467 km

The apparent stellar magnitude of the full moon in the earth's sky is −12.71m[5]. The illumination created by the full Moon near the surface of the Earth in clear weather is 0.25-1 lux.

The Moon appeared about 4.51 billion years ago [6], a little later than the Earth. The most popular hypothesis is that the Moon was formed from fragments left after the “Giant Collision” between the Earth and Theia, a planet similar in size to Mars.

The moon is the only extraterrestrial astronomical object visited by man.

Name

The Russian word "Moon" goes back to Praslav. *luna < Proto-IE *louksnā́ “bright” (zh. R. of the adjective *louksnós), lat. also goes back to the same Indo-European form. lūna "moon"[7].

The Greeks called the Earth's satellite Selena (ancient Greek Σελήνη), the ancient Egyptians - Yah (Iyah) [8], the Babylonians - Sin [9], the Japanese - Tsukiyomi

Moon as a celestial body

Orbit

Since ancient times, people have tried to describe and explain the movement of the moon. Over time, more and more accurate theories appeared.

Brown's theory is the basis of modern calculations. Created at the turn of the 19th-20th centuries, it described the movement of the moon with the accuracy of the measuring instruments of that time. At the same time, more than 1400 terms (coefficients and arguments for trigonometric functions) were used in the calculation.

Modern science can calculate the motion of the moon and verify these calculations with even greater accuracy. Using laser ranging methods, the distance to the Moon is measured with an error of several centimeters [11]. Not only measurements, but also theoretical predictions of the position of the Moon have such accuracy; for such calculations, expressions with tens of thousands of terms are used, and there is no limit to their number if even higher accuracy is required.

In the first approximation, we can assume that the Moon moves in an elliptical orbit with an eccentricity of 0.0549 and a geocentric orbit semi-major axis of 384,399 km (while the semi-major axis in the system relative to the center of mass of the Earth-Moon system is 379,730 km. The actual motion The moon is quite complex, and many factors must be taken into account in its calculation, for example, the oblateness of the Earth and the strong influence of the Sun, which attracts the Moon 2.2 times stronger than the Earth [comm 2].More precisely, the movement of the Moon around the Earth can be represented as a combination several movements

* circulation around the Earth in an elliptical orbit with

a period of 27.32166 days is the so-called sidereal month (that is, the movement is measured relative to the stars);

* rotation of the plane of the lunar orbit: its nodes (points of intersection of the orbit with the ecliptic) are shifted to the west, making a full revolution in 18.6 years. This movement is precessional;

* rotation of the major axis of the lunar orbit (lines of apsides) with a period of 8.8 years (occurs in the opposite direction than the movement of the nodes indicated above, that is, the longitude of the perigee increases);

* periodic change in the inclination of the lunar orbit with respect to the ecliptic from 4°59' to 5°19';

* periodic change in the size of the lunar orbit: perigee - from 356.41 to 369.96 thousand km, apogee - from 404.18 to 406.74 thousand km;

* gradual removal of the Moon from the Earth due to tidal acceleration (by 38 mm per year)[13][14], thus, its orbit is a slowly unwinding spiral[15].

General structure

The moon consists of a crust, a mantle (asthenosphere), the properties of which are different and form four layers, in addition, the transition zone between the mantle and the core, as well as the core itself, which has an external liquid and internal solid[16] parts[17]. Atmosphere and hydrosphere are practically absent. The surface of the moon is covered with regolith, a mixture of fine dust and rocky debris formed as a result of meteorite collisions with the lunar surface. The shock-explosive processes accompanying the meteorite bombardment contribute to the loosening and mixing of the soil, simultaneously sintering and compacting the soil particles. The thickness of the regolith layer ranges from fractions of a meter to tens of meters

Geological layers of the Moon according to GRAIL[17]

Inner solid core 0–230 km

Outer liquid core 230–325 km

Transition zone 325–534 km

Mantle 534-1697 km

Kora 1697—1737 km

The shift of the center of mass towards the center of the CM/CF figure is approximately 1.68–1.93 km, while the thickness of the lunar crust is on average 8–12 km less in the hemisphere facing the Earth. In relation to the center of mass, the opposite side is on average 3.2 km farther than the front side. The average thickness of the crust is 8–12 km greater on the opposite side than on the front side. The equatorial crust is on average 9.5 km thicker than at the poles

surface conditions

Color images of the Moon at different heights above the horizon, obtained by the onboard digital camera of the Columbia spacecraft on January 26, 2003[20][21]

The atmosphere of the Moon is extremely rarefied. When the surface is not illuminated by the Sun, the content of gases above it does not exceed 2⋅105 particles/cm3 (for the Earth, this figure is 2.7⋅1019 particles/cm3), and after sunrise it increases by two orders of magnitude due to degassing of the soil. The rarefied atmosphere leads to a high temperature difference on the surface of the Moon (from -173 °C at night to +127 °C at the subsolar point) [22], depending on the illumination; at the same time, the temperature of the rocks lying at a depth of 1 m is constant and equal to −35 °C. Due to the virtual absence of an atmosphere, the sky on the Moon is always black and with stars, even when the Sun is above the horizon. However, the stars are not visible in daytime photographs, since their display would require such an exposure, in which objects illuminated by the Sun would be overexposed.

About 3.5 billion years ago, during large-scale lava outpourings, the lunar atmosphere was denser. Calculations show that volatile substances released from the lava (CO, S, H2O) could form an atmosphere with a pressure of 0.01 Earth pressure. The time of its dissipation is estimated at 70 million years [23].

"On the moon. The earth is rising.”[24] Postage stamp of the USSR, 1967

The Earth's disk hangs almost motionless in the sky of the Moon. The reasons for the small monthly fluctuations of the Earth in height above the lunar horizon and in azimuth (about 7 °) are the same as for librations. The angular size of the Earth when viewed from the Moon is 3.7 times larger[25] than the lunar size when viewed from the Earth, and the area of ​​the celestial sphere covered by the Earth is 13.5 times larger[26] than that covered by the Moon. The degree of illumination of the Earth, visible from the Moon, is inverse to the lunar phases visible on Earth: during the full moon, the unlit part of the Earth is visible from the Moon, and vice versa. Earth's reflected light illumination should theoretically be about 41 times stronger[27] than moonlight illumination on Earth, but in practice only 15 times more[28]; Earth's largest apparent magnitude on the Moon is approximately −16m[29].

The lunar surface is characterized by low reflectivity and reflects only 5-18% of sunlight; color differences on the moon are extremely small. Its surface has a brownish-gray or blackish-brown color (data from 1970)[30].

The best colorimetric images of the lunar surface for 2017 were obtained by the wide-angle, multispectral WAC camera of the LRO spacecraft using filters in three color channels: 689 nm red, 415 nm green, and 321 nm blue[ (map description). On color separation images, the central part of the Sea of ​​Clarity, the eastern part of the Sea of ​​Rains, the Sea of ​​Cold and the Aristarkh plateau have a brownish tint. The Sea of ​​Tranquility, the peripheral part of the Sea of ​​Clarity, the northern part of the Sea of ​​Plenty, the western part of the Sea of ​​Rains, the western and southern parts of the Ocean of Storms have a blue tint. All these color features of individual regions of the Moon were confirmed later The eye almost does not distinguish the color features of individual surface details. The use of ordinary color photography also does not give the desired effect - the lunar surface looks monotonous.

A decrease in the surface albedo in the short-wavelength part of the spectrum leads to the fact that visually the Moon appears slightly yellowish

Gravity field

Gravity

Main article: Moon's gravity

The force of gravity near the surface of the moon is 16.5% of the earth's (6 times weaker).

Gravitational potential

Coefficients of sectoral and tesseral harmonics[36]

C3.1 = 0.000030803810 S3.1 = 0.000004259329

C3.2 = 0.000004879807 S3.2 = 0.000001695516

C3.3 = 0.000001770176 S3.3 = -0.000000270970

C4.1 = −0.000007177801 S4.1 = 0.000002947434

C4.2 = -0.000001439518 S4.2 = -0.000002884372

C4.3 = -0.000000085479 S4.3 = -0.000000718967

C4.4 = -0.000000154904 S4.4 = 0.000000053404

The gravitational potential of the Moon is traditionally written as the sum of three terms[37]:

Ebb and flow on earth

Main articles: Ebb and flow and Tidal forces

The gravitational influence of the Moon causes some interesting effects on the Earth. The most famous of them is the tides of the sea. On opposite sides of the Earth, two bulges are formed (in the first approximation) - on the side facing the Moon, and on the opposite side. In the oceans, this effect is much more pronounced than in the solid crust (the bulge of the water is greater). The amplitude of the tides (the difference between the levels of high and low tide) in the open spaces of the ocean is small and amounts to 30-40 cm. However, near the coast, due to the incursion on a solid bottom, the tidal wave increases the height in the same way as ordinary wind waves of the surf. Given the direction of the Moon's revolution around the Earth, it is possible to form a picture of the tidal wave following the ocean. Strong tides are more susceptible to the eastern coasts of the continents. The maximum amplitude of a tidal wave on Earth is observed in the Bay of Fundy in Canada and is 18 meters.

Although the Sun's gravitational force is almost 200 times greater on the globe than the Moon's, the tidal forces generated by the Moon are almost twice those generated by the Sun. This is due to the fact that tidal forces depend not only on the magnitude of the gravitational field, but also on the degree of its inhomogeneity. As the distance from the field source increases, the inhomogeneity decreases faster than the magnitude of the field itself. Since the Sun is almost 400 times farther from the Earth than the Moon, the tidal forces caused by the Sun's attraction are weaker[38].

A magnetic field

It is believed that the source of the magnetic field of the planets is tectonic activity. For example, for the Earth, the field is created by the movement of molten metal in the core, for Mars - the consequences of past activity.

"Luna-1" in 1959 established the absence of a uniform magnetic field on the Moon[39]:24. The results of research by scientists at the Massachusetts Institute of Technology confirm the hypothesis that it had a liquid core. This fits into the most popular hypothesis of the origin of the Moon - the collision of the Earth about 4.5 billion years ago with a cosmic body the size of Mars “knocked out” a huge piece of molten matter from the Earth, which later turned into the Moon. Experimentally, it was possible to prove that at an early stage of its existence, the Moon had a magnetic field similar to that of the Earth[40].

The GRAIL program for studying the gravitational field and the internal structure of the Moon, as well as reconstructing its thermal history, established that the Moon has an internal solid and external metal parts of the core (consisting of iron and siderophile elements). The very weak magnetic field of the Moon is formed due to residual magnetism in lunar rocks, as well as tidal forces acting on the core [16]

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