Geography earth in outer space. Geophysical fields of the Earth (gravitational, magnetic, electrical, thermal); their origin Gravitational and magnetic fields of the earth

Lecture plan

1.1.Forma and basic parameters of the Earth.

1.2. Gravitational field of the Earth.

1.3. Thermal field of the Earth.

1.4. Earth's magnetic field.

Geology as a science that studies, first of all, our planet and its upper stone shell, does not disregard the surrounding silty world - the Universe. This is due to the fact that in the structure of the Earth there are certain similarities and differences with the planets; some geological processes are directly related to cosmic phenomena.

Earth - a typical planet of the solar system - is characterized by the presence of well-developed inner and outer shells.

1.1. The shape and basic parameters of the Earth

Under the figure, or shape of the Earth, understand the shape of its solid body, formed by the surface of the continents and the bottom of the seas and oceans. The shape of the planet is determined by its rotation, the ratio of the forces of attraction and centrifugal force, the density of matter and its distribution in the body

Geodetic measurements have shown that the simplified firmness of the Earth approaches the ELLIPSOID OF ROTATION (SPHEROID). Polar radius Rn 6356.8 km, equatorial - 6378.2 km, the difference between the radii is 21.4 km.

Detailed measurements have shown that the Earth has a more complex shape. This figure, peculiar only to the Earth, was called GEOID. At any point of the geoid, the gravity vector is perpendicular to its surface, which can be obtained by extending the surface of the World Ocean under the continents. It is the surface of the geoid that is taken as the base when counting heights in topography, geodesy, mine surveying.

The geoid and spheroid do not coincide, and the discrepancy between the position of their surfaces reaches 160 km (100 m in the USSR). According to the most accurate recent data, it has been established that the Earth has a pear-shaped (i.e., heart-shaped) triaxial ellipsoid.

The mass of the Earth is 5.977 10 21 tons, the volume is 1.083 billion km 3, the area is 510 million km 2. The average density of the Earth is 5.52 g/cm 3 . It has been established that the outer, stone part of the earth's crust has an average density of 2.8 g/cm 3 . Thus, for the total density to be 5.52, the interior of the Earth must be denser than the exterior. The increase in density with depth can be explained by differences in composition and by the enormous force with which the outer parts of the Earth press on the inner ones. It is assumed that the inner core has a density of about 13 g/cm 3 , which apparently corresponds to the state of metallic iron at this pressure.

1.2. Earth's gravitational field

The physical fields created by the planet as a whole and by individual isolated bodies are determined by the combination of properties inherent in each physical object. Of great importance is the study of geophysical fields in the study of the physical properties of rocks in samples and massifs. The study of the properties and interpretation of the obtained data should be based on the knowledge of the general and local patterns of the structure of the physical fields of the Earth.

The huge mass of the Earth is the reason for the existence of forces

attraction that affect the howl of the body and objects located on it surfaces. The space within which the forces of attraction of the Earth are manifested is called the field of gravity or the gravitational field (lat. "gravitas" - gravity). It reflects the nature of the distribution of masses in the bowels and is closely related to the figure of the Earth. Each point on the earth's surface has its own magnitude of gravity; in the center of the Earth, gravity is zero.

The force of gravity is numerically equal to the resultant force of attraction and centrifugal force P acting per unit mass of matter

In the CGS system, the magnitude of gravity is expressed in galls (cm / s). In practice, one thousandth of a gala-milligal is often used. Gravity depends on the altitude position of the terrain, since this changes the distance to

the center of the earth. Therefore, gravity measurements are usually reduced to one

level, such as the geoid or ellipsoid level. The value of gravity on the Earth's surface increases from the equator to the poles from 978.049 to 963.235 gal. The average value of gravity on the surface of the geoid is 981 gal.

the magnitude of gravity depends not only on the altitude position, but also on the geographical latitude of the area. It is also influenced by the uneven distribution of masses in the bowels of the Earth. For this reason, there are local deviations in the values ​​of gravity from its theoretically calculated values. Such deviations are called gravitational anomalies.

There are positive and negative gravitational anomalies. Positive ones are observed when dense masses (iron ores) lie in the bowels of the earth's crust; negative ones are caused by deposits of light masses (gypsum, potassium salt). Gravitational anomalies are detected using gravimeters and pendulum instruments. According to the results of measurements, gravimetric maps are compiled, on which, using isolines, gravity anomalies are shown in milligals.

Changes in gravity can be caused by some phenomena known from astronomy, such as slowing down or speeding up the rotation of the Earth around its axis, changes in the shape and density of the Earth.

1.3. Thermal field of the Earth

The thermal field of the Earth is formed due to external and internal sources. The main source of external energy is solar radiation. The radiant energy of the Sun received by the earth's surface per year is 5.44 * 10J. About 55 % it is absorbed by the atmosphere, vegetation, soil. The rest of the energy is reflected into space.

The sources of internal heat of the Earth are the following: radioactive decay of elements; energy of gravitational differentiation of matter; residual heat, etc.

The resulting solar heat directly heats the rocks and penetrates only to a shallow depth. The surface temperature of the layers varies during the day, season and year. With the depth of the amplitude, temperature fluctuations decrease: first, the influence of daily fluctuations in air temperature disappears, then seasonal and, finally, annual ones. At some depth, the temperature of the rocks remains constant for years - a zone of constant temperature. Above it are layers of long-term, seasonal and daily fluctuations.

The depth of the belt of constant temperatures varies with the latitude of the area and with changes in the thermophysical properties of rocks. In equatorial regions the zone of constant temperature will reach 1-2 m, in middle latitudes 20-30 m (in Moscow - 20 m).

The constant temperature of this zone is approximately equal to the average annual temperature of the surface layer of this area (for Moscow + 4.2 ° C, for Paris + I8). If the average annual temperature of the area is below 0, then precipitation and groundwater turn into ice. This is the main condition for the formation of "permafrost".

Starting from the zone of constant temperatures, there is a constant increase in the temperature of the rocks with depth, which is characterized by a geothermal step and a geothermal gradient. GEOTHERMAL STAGE - numerically equal to the number of meters to which you need to go deep so that the temperature of the rocks rises by 1 and has the dimension m / deg. GEOTHERMAL GRADIENT - the value is inverse and numerically equal to the number of degrees by which the temperature of rocks rises when deepening by 100 m (m / deg).

The geothermal step is taken on average to be 33 m/deg, but its value at different points varies widely from 2 to 250 m/deg. Often the value of the geothermal step deviates significantly at different depths of the same point. It depends: on different thermal conductivity and conditions of occurrence of rocks, groundwater, remoteness from the seas and oceans, terrain, geochemical conditions.

The highest rock temperature in underground mine workings is C and was observed in the copper mines of Magna (USA) at a depth of 1200 m. For the development of mineral deposits occurring at great depths and in the area of ​​permafrost, it is necessary to regulate the thermal regime of deep mines and mines.

1.4. Earth's magnetic field

There are magnetic fields around the globe and inside it. According to space research, it extends beyond the planet for a distance exceeding ten times the radius of the Earth, forming a magnetosphere. A complex asymmetric external shape of the magnetosphere, continuously changing in shape and strength, has been established. From the side of the Earth, illuminated by the Sun, the magnetosphere is significantly compressed, and from the opposite side, it is elongated with the formation of a magnetic plume.

The asymmetry of the magnetosphere is due to the influence of the solar wind (cosmic radiation).

According to the data of 1960, the boundary of magnetism is located at an altitude of 93,000 km. The magnitude of the Earth's magnetic field decreases approximately up to a height of 43 thousand km in proportion to the cube of the distance. In near-Earth space, beyond the limits of terrestrial magnetism, there is a magnetic field of interplanetary space. The nature of the Earth's magnetic field has not yet been fully elucidated. It is known that the impact on it of the processes occurring in the high layers of the atmosphere is small and does not exceed 6 %. On this basis, it is believed that the magnetic field is associated with processes occurring in the deep interior of the Earth. The magnetic field affects the orientation of ferromagnetic minerals (magnetite, ilmenite, hematite) in rocks. Ultrabasic and basic igneous (basalts, gabbro) and red-colored sands react most strongly to the magnetic field. Sedimentary genesis.

The poles of the Earth's magnetic field do not coincide with the geographic poles.

The main characteristics of the magnetic field are as follows:

MAGNETIC DECLATION - the angle between the magnetic needle axis of the magnetic meridians and the geographic meridian.

MAGNETIC TILT - the angle of the magnetic needle to the horizon.

The STRENGTH of the Earth's magnetic field is expressed by a vector quantity - MAGNETIC VOLTAGE. The unit of measurement of magnetic intensity is one hundred-thousandth of an oersted, called gamma ().

Deviations of the elements of the Earth's magnetic field are called magnetic anomalies. They are caused either by the occurrence of large magnetic masses (iron ores) or by disturbances in the homogeneity of the geological structure.

The largest magnetic anomaly in the world, caused by the occurrence of large magnetic masses, is the KMA.

The study of the Earth's magnetic field is widely used to search for mineral deposits, including oil and gas.

Geographic Consequences of the Shape and Size of the Earth

Even the ancient Greek scientist Aristotle suggested that the Earth, like all other planets, has the shape of a ball, but the shape of the Earth can be more accurately called the geoid.

Earth is a small planet in the solar system. In size, it surpasses only Mercury, Mars and Pluto. The average radius of the Earth is 6371 km, while the equatorial radius of the Earth is greater than the polar one, i.e. The earth is "flattened" at the poles, which is caused by the rotation of the earth around its axis. The polar radius of the Earth is 6357 km, and the equatorial radius is 6378 km. The circumference of the Earth is approximately 40 thousand km. And the surface area of ​​our planet is approximately 510 million km2.

The Earth revolves around the Sun and makes a complete revolution in 365 days 6 hours and 9 minutes. The "extra" hours and minutes form an extra day - February 29, so there is a leap year (a year that is a multiple of 4).

The earth also rotates on its axis, resulting in a daily cycle of day and night. The earth's axis is an imaginary straight line passing through the center of the earth. The axis crosses the Earth's surface at two points: the North and South Poles.

The Earth's axis is tilted at 23.5°, which causes the seasons to change on our planet. When the area around the North Pole is facing the Sun, it is summer in the Northern Hemisphere and winter in the Southern Hemisphere. When the area around the South Pole is facing the Sun, the opposite is true. On June 22, the Sun is at its zenith over the Northern Tropic - this is the longest day of the year in the Northern Hemisphere, December 22 -¦ over the Southern Tropic - this is the shortest day in the Northern Hemisphere, but the longest in the Southern. March 21 and September 23 - the days of the spring and autumn equinoxes - the days when the day is equal to the night, and the Sun is at its zenith above the equator.

The sphericity of the Earth leads to uneven heating of the earth's surface. The equatorial regions of the Earth (hot heat zone), located between the tropics, receive the maximum amount of solar heat, while the polar (cold heat zones) receive the minimum, which leads to negative temperatures in the polar latitudes.

The Earth's magnetic field is the area around our planet where magnetic forces act. The question of the origin of the magnetic field has not yet been finally resolved. However, most researchers agree that the presence of the Earth's magnetic field is at least partly due to its core. The Earth's core consists of a solid inner and liquid outer parts. The rotation of the Earth creates constant currents in the liquid core. As the reader may remember from physics lessons, the movement of electric charges results in the appearance of a magnetic field around them.

One of the most common theories explaining the nature of the field, the theory of the dynamo effect, assumes that convective or turbulent movements of a conducting fluid in the core contribute to self-excitation and maintaining the field in a stationary state.

The earth can be considered as a magnetic dipole. Its south pole is located at the geographic North Pole, and the north, respectively, at the South. In fact, the geographical and magnetic poles of the Earth do not coincide not only in "direction". The axis of the magnetic field is tilted with respect to the axis of rotation of the Earth by 11.6 degrees. Due to the fact that the difference is not very significant, we can use a compass. Its arrow points exactly to the south magnetic pole of the Earth and almost exactly to the geographic north. If the compass had been invented 720,000 years ago, it would have pointed to both the geographic and magnetic north poles. But more on that below.

The magnetic field protects the inhabitants of the Earth and artificial satellites from the harmful effects of cosmic particles. Such particles include, for example, ionized (charged) particles of the solar wind. The magnetic field changes the trajectory of their movement, directing the particles along the field lines. The need for a magnetic field for the existence of life narrows the range of potentially habitable planets (if we start from the assumption that hypothetically possible life forms are similar to earthly inhabitants).

Scientists do not exclude that some of the terrestrial planets do not have a metallic core and, accordingly, are devoid of a magnetic field. Until now, it was believed that the planets, consisting of solid rocks, like the Earth, contain three main layers: a solid crust, a viscous mantle, and a solid or molten iron core.

The change of poles is accompanied by a change in the configuration of the magnetic field. During the "transitional period" much more cosmic particles that are dangerous to living organisms penetrate the Earth. One of the hypotheses explaining the extinction of dinosaurs claims that the giant reptiles became extinct precisely during the next change of poles.

In addition to the "traces" of planned activities to change the poles, the researchers noticed dangerous shifts in the Earth's magnetic field. An analysis of data on his condition over several years showed that in recent months dangerous changes began to occur in him. Scientists have not recorded such sharp "movements" of the field for a very long time. The area of ​​concern to researchers is located in the South Atlantic Ocean. The "thickness" of the magnetic field in this region does not exceed a third of the "normal" one. Researchers have long paid attention to this "hole" in the Earth's magnetic field. The data collected over 150 years show that the field here has weakened by ten percent over this period.

Despite the fact that the magnetic field cannot be seen, the inhabitants of the Earth feel it well. Migratory birds, for example, find their way, focusing on it. There are several hypotheses that explain exactly how they feel the field. One of the latter suggests that birds perceive the magnetic field visually.

In addition to birds, sea turtles use the Earth's magnetic field instead of GPS. And, as shown by the analysis of satellite photographs presented as part of the Google Earth project, cows. After studying photographs of 8510 cows in 308 regions of the world, scientists concluded that these animals preferentially orient their bodies from north to south (or south to north). Moreover, the “reference points” for cows are not geographic, but precisely the magnetic poles of the Earth. The mechanism of the cows' perception of the magnetic field and the reasons for such a reaction to it remain unclear.

In addition to these remarkable properties, the magnetic field contributes to the appearance of auroras. They arise as a result of abrupt field changes occurring in remote regions of the field.

Gravity is the resultant of the attraction of the mass of the Earth and the centrifugal force from the rotation of the planet. In equatorial latitudes, it is equal to an average of 978 gal, and in polar latitudes it increases to 983 gal, which is associated both with the figure of the Earth and with a decrease in centrifugal force with latitude.

The importance of gravity for the geographic shell was discussed above in various aspects. Let us generalize this, since the gravitational field of the Earth is extremely important for its nature.

1. Gravitational forces exceeding cohesive forces created the figure of the Earth. In practice, the inverse problem is solved: the potential of gravity is used in the study of the figure of the Earth.
2. The Earth's gravity condensed the internal matter of the Earth and, regardless of its chemical composition, formed a dense core.
3. The core, together with the rotation of the Earth, created the magnetosphere, the role of which is enormous for the biosphere.
4. The magnitude of the earth's gravity is such that it holds the gaseous shell, allowing only light elements - helium and hydrogen - to escape. Partly due to this, there is a gas mismatch between the Earth's atmosphere and the Universe: in the Universe, hydrogen accounts for 93%, and in the Earth's atmosphere it is negligible.
5. Atmospheric cover ensures the existence of the hydrosphere; otherwise, the water would instantly evaporate and escape.
6. The pressure of the deep masses, along with radioactive decay, generates thermal energy - the source of internal (endogenous) processes that rebuild the lithosphere.
7. The force of gravity determines the tendency of the earth's crust to isostatic equilibrium. Isostasy was discovered when studying the distribution of gravity. Mountain ranges create additional mass on the surface and should cause an increase in gravity proportional to the mass of the mountainous country. In the oceans, 4-5 km are composed of water with a density of about 1.0 g/cm 3 , so here the force of gravity should be less than in the mountains. The lowland plains of the continents occupy an intermediate position and should have a gravity of medium value. The measurements showed that, in fact, the force of gravity on the same parallel everywhere - at sea, on low land, in mountainous countries - is generally the same. This means that in the mountains it is less than normal, or, as is commonly believed, a negative gravimetric anomaly is found here, at sea the gravity force is greater than the calculated one, or its anomaly is positive, in the lowlands its actual value is close to theoretical, i.e. there is no anomaly. This distribution of gravity and its anomalies is explained by isostasy.
8. The asthenosphere - a layer softened by heat that allows the movement of the lithosphere - is also a function of gravity, since the melting of matter occurs at a favorable ratio of the amount of heat and the amount of compression - pressure.
9. The spherical figure of the gravitational field determines two main types of landforms on the earth's surface - conical and flat. They correspond to two universal forms of symmetry - conical and bilateral (I. I. Shafransky). Above each small and large area of ​​the earth's surface there is a cone-shaped field of earth's gravity. It is imprinted on all the bodies that grow on Earth. If the body grows up, or, what is the same, down, then it acquires a shape close to conical (mountain peaks, volcanoes, sinkholes, sandy landforms, trees, etc.). If the body grows horizontally, then gravity makes it leaf-like (deltas, accumulative plains, leveling surfaces, etc.). The transition of conical forms to flat forms slopes. The entire relief of the lithosphere is essentially slope.
10. The force of gravity determines gravitational tectogenesis - the formation of the structures of the earth's crust and, in general, the movement of masses of the lithosphere under the action of gravity. Since the development of relief is the movement of matter, the force of gravity in it plays one of the decisive roles.
11. Earth's gravity determines the upper limit of the height of mountain ranges. The uplifting of the folds of the earth's crust cannot be higher than 9 km, because this is prevented by the force of gravity.
12. The combination of the gravitational field and specific bodies on the Earth creates a disproportionality of the earth's space. A few examples will reveal its essence. Cohesion forces act on small bodies, up to mountain ranges, and on large bodies, mountainous countries, the lithosphere as a whole, and even more so on the whole Earth, gravity forces, which is associated with isostasy. Under the conditions of the earth's gravitational field, each type of animal has the most convenient dimensions for it, a change in which would entail a change in shape. If the length, height and width of an animal is reduced or increased by 10 times, then its mass will change by 1000 times, and its surface by 100 times. It is clear that the whole body must be rebuilt in this case. The ratio of volumes, sizes and masses determines the windage of pollen and seeds of plants and the methods of their transfer.
13. The force of gravity, in combination with the size of the bodies, determines the force of the surface tension of water, which is associated with its uplift through the capillaries and, consequently, one of the sides of the water regime of the soil.
14. The direction of gravity downward, towards the center of the Earth, helps animals to maintain an upright position.
15. In the downward flow of water and, consequently, in the work of rivers, the gravitational field plays a secondary role. Of paramount importance is the energy of solar radiation, which causes the evaporation of water and the rise of steam to the continents and mountains.

The shape of the Earth - the geoid - does not have a regular geometric shape. Therefore, where it is permissible, the surface of the geoid is replaced by approximate mathematical models, which in some cases are taken as the earth's spheroid, in others - the globe. The terrestrial spheroid - an ellipsoid of revolution is obtained by rotating the ellipse around its minor axis b, which coincides with the axis of rotation of the Earth, and the center of the ellipsoid is aligned with the center of the Earth. Features of the structure of the figure of the Earth are fully taken into account in the mathematical processing of high-precision geodetic measurements. In view of the smallness of compression in solving many problems, for the figure of the Earth with sufficient accuracy for practical purposes, one can take a sphere equal in volume to the earth's ellipsoid. The dimensions of which are: equatorial radius -6378 km, polar radius -6357 km, average radius 6371, meridian length 40009 km, equator length -40077 km, its diameter 12756 km, surface W - 510 million km2, average altitude 875 m, cf. depth MO 3800m.

Z-li movement. It is customary to take into account the orbital and diurnal rotation, the movement of the Earth-Moon system, the change in the speed of rotation of the Earth, as well as the oscillation of the axis of rotation. Orbital motion: moves in an elliptical orbit, in one of the focuses of which the Sun is located, the speed is 29.8 km / s, the period is a year. The speed of movement is higher, the smaller the radius - vector (distance from the Earth to the Sun). It changes slightly during the year: at perihelion (early January) it decreases, at aphelion it increases. The earth's axis is inclined with respect to the plane of the orbit at an angle of 66 33. The presence of the tropics and polar circles is associated with the inclination of the earth's axis. The time it takes the earth's axis to describe a full cone is called the precessional rhythm. The daily rotation of the Earth around its axis is counterclockwise, as viewed from the north pole. Consequences: 1) change of day and night; 2) deformation of the figure of the Earth (polar compression - an increase in centrifugal force); 3) the existence of the Coriolis force (the greater the angular velocity of rotation, the greater the Coriolis force); 4) superposition of centrifugal force and gravitational force, giving gravity (centrifugal - from zero at the poles to the maximum value at the equator; the maximum value of gravity at the pole).

Movement of the Earth-Moon system. The moon creates a tidal drag on the daily rotation of our planet. Tidal drag, causing a slowdown in rotation, reduces polar oblateness and the Coriolis force, i.e. affects the circulation of the atmosphere and oceanosphere, on which climate conditions depend. Change in the speed of the Earth's rotation. Irregularity of daily rotation - average monthly deviation. Movement of the Earth's poles. If the axis of rotation does not coincide with the axis of the figure of the Earth, then there must be a movement of the geographical poles around the poles of the figure with a period of 305 sidereal days. Continuous displacement of the axis of rotation inside the body of the Earth - precession, (through a change in centrifugal force) Movement of the poles in space - nutation. As a result of nutation, the air masses are redistributed during the change of seasons. Change in the slope of the World Ocean level, the intensity of ocean currents, the nature of the interaction between the ocean and the atmosphere, changes in atmospheric circulation.

The physical fields of the Earth include gravitational, magnetic, and thermal fields. They cover at least 2 million km. These limits are determined by gravitational and electromagnetic fields. Gravity field is 2spheres: 1. Hill's sphere, the radius of this sphere is about 1.5 million km and determines the distance at which bodies can move, remaining satellites of the Earth.2. A sphere with a radius of 260 thousand km, within which the earth's gravity exceeds the sun's. The gravitational interactions of the Sun, as well as other planets in the Earth's orbit, cause secular disturbances of an oscillatory nature, which significantly affect the state of the biosphere and man. The gravite field determines the force of gravity on a surface. Acceleration of free fall by 3 differs depending on the distribution of rock density, surface roughness for a particular area. Wed for the entire surface 9.8 m/s. A magnetic field extends over a distance of about 10 Earth radii (100-200 thousand km). The strength of the magnetic field on the surface of the Earth is not the same. In the polar regions it reaches 8.103 -9.103 A/m, and at the equator the intensity decreases to 5.103 A/m. As you move away from the Earth, the intensity decreases in proportion to the cube of the distance. thermal field The earth has, like any heated body. The factors causing the heating of the Earth are divided into external (solar energy, tidal friction, cosmic radiation) and internal (heat transfer from the depths of the Earth, thermal waters, volcanism, earthquakes, human economic activity). The main source of the thermal field is the Sun. The temperature on the Earth's surface fluctuates quite widely.

Around the globe, in addition to the hydrosphere* and atmosphere*, there are also concentrated physical fields. This is the name of special forms of matter, different from solid, liquid and gaseous bodies, but united by the property to create conditions for interaction at a distance of a field source and some object (for example, particles of a body, liquid or gas). Naturally, these interactions are transmitted at some finite rate. The earth has two physical fields: gravitational and magnetic.

Gravity field The earth obeys the law of universal gravitation. The latter was established by I. Newton in 1747 and expresses the universal property of matter, which consists in the fact that the force of mutual attraction of two material points is proportional to the product of the masses of these points, divided by the square of the distance between them. The proportionality factor is called gravitational constant. Mathematically, this force is expressed by the formula

where G is the gravitational force, f is the gravitational constant, f ~ 6.673*10-11 m3s-2kg-1, M is the mass of the gravitational source, m is the mass of the gravitating point, r is the distance between the gravitational source and the gravitating point.

For the case of the gravitational field of some large celestial body (for example, the Earth), it is convenient to write the formula expressing the law of universal gravitation in the form

where is the gravitational constant of the considered celestial body, = f M. In particular, for the Earth = 3.986*1014 m3s-2, the Moon - 4.890*1012 m3s-2, the Sun - 1.321*1020 m3s-2.

If a gravitating point is considered, which is located on the surface of a celestial body (for example, the Earth), then the last formula is written as

where g is the acceleration of a freely falling material point, g = /r2. If we calculate the value of g, based on the average value of the Earth's radius r = 6371 km, then we find that g = 9.8 m s-2. This well-known physical constant, unfortunately, is very often misnamed: the acceleration of free fall, the acceleration of gravity. But there is no concept of "acceleration of motion" (for example, falling), just as there is no concept of "acceleration of force" (for example, the force of gravity).

It is essential that the acceleration of a freely falling (ie, falling in vacuum) material point depends on the location of its determination on the Earth's surface. In each particular case, this value can only be found experimentally. If it is necessary to calculate it, then you can use the approximate formula

g = 9.7805 (1 - h/r)2 (1 + 0.0053 Sin),

where h is the height of the point above the Earth's surface,

r is the distance between the center of the Earth and the point,

The latitude of the place.

For example, for the latitude of St. Petersburg (= 60o), for a point located on the Earth's surface (h=0), from the last formula we obtain g = 9.819 m s-2.

One of the most amazing properties of the gravitational field is its permeability: it is impossible to isolate oneself from its influence; it acts on any material object and penetrates through any screen. Another property of the gravitational field is that its action, continuously decreasing, extends to practically unlimited distances.

The presence of a gravitational field on the Earth is one of the most necessary conditions for the existence of life on it: it keeps the atmosphere and the World Ocean from scattering in space; it attracts people, animals and all other material objects to the surface of the Earth; it directs the flow of rivers and creates buoyant (Archimedean) forces on the surface of water bodies that keep ships on it, etc.

In addition to the gravitational field, the Earth has another field - magnetic. Compared to the gravitational field, it is quite weak: its average value is only 0.5 oersteds, while the magnitude of the field created by an ordinary school demonstration magnet reaches several tens of oersteds.

The geomagnetic field is similar to a dipole. This is the name of the field of a magnet, in which the poles are very close to each other. If the dipole is in the center of the ball, then the magnetic field on its surface has poles located at diametrically opposite points. Magnetic poles The lands do not coincide with the geographic ones. The magnetic axis of our planet is inclined to the axis of its rotation by 11.5o. The magnetic pole in the Northern Hemisphere is located near the coast of North America (71o N, 96o W), and the pole in the Southern Hemisphere is near the coast of Antarctica (70o S, 150o E). Thus, the Earth's magnetic poles are not located at diametrically opposite points on the globe, and the magnetic axis not only does not coincide with the Earth's axis of rotation, but also does not pass through its center. The magnitude of the geomagnetic field at the poles is approximately twice that at the equator, and the magnitude of the field in the Northern Hemisphere is somewhat greater than in the Southern.

The action of magnetic forces in near-Earth space and at accessible depths inside the Earth is detected: by the moment of forces applied to freely suspended magnetic needles; electromotive force (emf) induced in the rotating turns of the conductor; deflecting action experienced by charged particles of cosmic radiation; the effect of polarization of radio waves, etc.

There are two types of geomagnetic field sources: internal and external. The first are located inside the planet, the second - outside it. The first create a fairly constant magnetic field, which has small secular variations, the second - a much weaker, but variable magnetic field. The field created by internal sources is called the main field, and the field created by external sources is called variable. The nature and origin of these fields are different, but there is a deep relationship between them.