The earth rotates around an inclined axis from west to east. Half of the globe is illuminated by the sun, it is day there at that time, the other half is in the shadow, there it is night. Due to the rotation of the Earth, the cycle of day and night occurs. The Earth makes one revolution around its axis in 24 hours - a day.
Due to rotation, moving currents (rivers, winds) are deflected in the northern hemisphere to the right, and in the southern hemisphere to the left.
Rotation of the Earth around the Sun
The Earth rotates around the sun in a circular orbit, completing a full revolution in 1 year. The earth's axis is not vertical, it is inclined at an angle of 66.5° to the orbit, this angle remains constant during the entire rotation. The main consequence of this rotation is the change of seasons.
Consider the rotation of the Earth around the Sun.
- December 22- day winter solstice. The southern tropic is closest to the sun (the sun is at its zenith) at this moment - therefore, it is summer in the southern hemisphere, and winter in the northern hemisphere. Nights in the southern hemisphere are short; on December 22, in the southern polar circle, the day lasts 24 hours, night does not come. In the northern hemisphere, everything is the other way around; in the Arctic Circle, the night lasts 24 hours.
- 22nd of June- day of the summer solstice. The northern tropic is closest to the sun; it is summer in the northern hemisphere and winter in the southern hemisphere. In the southern polar circle, night lasts 24 hours, but in the northern circle there is no night at all.
- March 21, September 23- days of the spring and autumn equinoxes The equator is closest to the sun; day is equal to night in both hemispheres.
All planets move in the Universe. These movements are caused by various physical influences on cosmic bodies and are of a complex nature. The earth also undergoes many movements that can be analyzed and broken down into different components.
In contact with
These movements can be classified on scales:
- Universe;
- Galaxies;
- solar system;
- a common center of mass with the Moon;
- Earth.
The galaxy in which the solar system is located is called the Milky Way. Scientists suggest that this galaxy revolves around the center of the Universe along with other galaxies. The solar system with all objects, including the Earth, rotates around the center of the Milky Way, and it completes this journey in one galactic year, which is approximately 230 million years.
When moving to an even smaller scale, it will be discovered that our planet is making its way around the Sun. In addition, the Earth and the Moon rotate around their common center of mass, which is not in the center of the globe, but close to its surface. Because of this, our planet's orbit follows a slightly spiral path when viewed from the outside rather than from Earth. All these types of movements are invisible or barely noticeable to earthlings.
Rotational speed
You could say that the rotation of a body has two speeds, depending on which measurement system to use:
- linear;
- angular.
If we measure the speed of rotation as the distance that a point travels in a certain time, then the further the point is located from the imaginary axis of rotation, the higher its speed will be. And the closer the point is to the axis, the lower its speed. This speed is called linear. At axis points, the speed is zero.
But if the speed of rotation is measured in degrees, then any point on the surface of the body or inside it will move at the same speed, regardless of whether it is located far from the axis or close. The speed of rotation, measured in degrees, is called angular.
Can be measured Earth rotation speed by observing the movement of two objects on the surface located on the same meridian, but at different latitudes. Let's say object A is at the equator, and object B is in northern latitude. As a result, it will be discovered that object A relative to the axis of the planet has traveled a greater distance per unit time than object B. This means that object A has moved faster than object B.
But if you measure the angular velocity in degrees using the same objects or marks, then their angular velocity will be the same, since they will rotate relative to the axis of the planet at the same angle over a certain period of time. To explore many natural phenomena, such as, for example, the Coriolis force, it is necessary to use linear method rotation speed measurements.
The Earth's surface will have a maximum linear rotation speed near the equator, and this speed is 465 m/s or 1674 km/h. The closer a point on the surface of the globe is to either pole, the lower the speed will be. At the poles, the linear speed of rotation is zero, since these points are on an imaginary axis.
Change of time of day
The most noticeable circumstance for the inhabitants of the Earth and the main geographical consequence of the axial rotation of our planet is the change of times of day, and for earthlings living at a certain distance from the equator - also the seasons.
Day and night change because that parallel rays of light from the Sun fall on only one side of the planet at a time. The opposite side of the Earth is in shadow. This means that on the side facing the sun there will be day, and on the opposite side it will be night. If the globe were constantly turned with only one side towards the Sun, then on the illuminated side there would be a temperature of about +100 ° C, all the water would have to evaporate, and on the dark side the surface of the planet would be under a layer of ice. Conditions on both sides of the Earth would then be unsuitable for life.
Due to the rhythmic change of day and night, seasons, and, therefore, light and temperature conditions , on Earth, all living things obey certain biorhythms. At the same time, not only all plants and animals, but also inanimate nature are subject to rhythmic changes.
The Earth rotates on its axis counterclockwise when viewed from the North Star, namely from the north. And if the observation point is from the equator, when the North Pole is at the top, then the planet rotates from left to right or from west to east.
In connection with the rotation of the Earth around its axis, the concept of a day is used. But the days are different:
- starry;
- sunny;
- average solar.
Sidereal days are used for astronomical research and observations. A solar day is the period of rotation of the Earth around its axis relative to the Sun. They may differ in duration, so to measure time in Everyday life the average solar day is used, which lasts 24 average sun hours and longer than the sidereal day by 4 minutes.
Time Zones
With the development of communications between different parts of the world, time zones were invented for convenience and safety. Most of all, such unification was in demand in order to eliminate confusion and accidents on the railway.
Accurate time measurement Time zones began to be used in the 19th century. The first person to come up with this idea was the English doctor William Hyde Wollaston. The earth's surface was conventionally divided into 24 sectors perpendicular to the equator, each of which is 15 degrees, and together they determine the daily cycle. Each zone is assigned its own time (with a difference of one hour from the neighboring one). Moreover, the further west the belt is located, the more time lags behind.
If the boundaries of the time zone do not coincide with state or administrative outlines, they are adjusted to the locality for convenience. Therefore, the boundaries of time zones are not always straight. Their countdown begins from zero, located on the Greenwich meridian. This zone indicates universal time.
Change of seasons
The Earth's axis relative to the orbital plane along which the planet moves around the Sun is not perpendicular, but at an angle. Because of this, an uneven amount of heat from the Sun reaches the surface of the planet in different parts of it.
When the Earth is in orbit on one side of the Sun, it is tilted on its axis so that it faces the North Pole, but when it moves in orbit to the opposite side of the Sun, the planet will be tilted with the South Pole. This means that, in the first case, summer will be in the Northern Hemisphere, and winter in the Southern Hemisphere. In the second case, it will be winter in the Northern Hemisphere, and summer in the Southern Hemisphere. In intermediate positions of the Earth in orbit, its hemispheres will have autumn and spring.
If the Earth's axis were perpendicular to the plane of its orbit, then there would be no seasons, since the Northern and Southern Hemispheres would always receive the same portion of light and heat during the day.
Deflection of falling bodies
All objects located on the surface of the Earth move with it at the same linear speed, caused by the rotation of the planet around its axis. The farther from the axis an object moving with the planet is, the higher its speed will be. The higher an object is above the surface, the greater the linear speed it moves with the Earth around its axis.
Objects thrown from a great height initially move with the Earth and fall to the ground, slightly shifted to the east. This happens due to inertia, which is retained by an object thrown from a height. He maintains the speed he had at his best. This speed is always higher than on the Earth's surface. During the fall, this velocity, directed eastward, is perpendicular to the fall velocity.
As a result, the object does not fall vertically, but slightly to the east. This effect will not occur at the poles, due to the lack of linear speed of movement. An airplane or other aircraft is not suitable for carrying out such an experiment, since they are not rigidly connected to the surface of the earth and do not move synchronously with it. A tower or tall building is better suited for this.
Foucault pendulum
This experiment is the simplest and most visual test of the axial rotation of the Earth.
According to the law of physics, the plane of the trajectory of a swinging pendulum is always in the same position in relation to the World space. But, if you follow the pendulum throughout the day, it will become obvious that the directions of its swings are constantly changing. This occurs due to the rotation of the planet around its own axis.
This pendulum was first used in his experiment by the French scientist Jean Foucault, after whom the instrument was named.
Compression of the Earth from the poles
During rotation, centrifugal force occurs, which is no exception in the case of planets. Thus, under the influence of centrifugal force, acting perpendicular to the axis especially strongly in the equator region, our planet over a long time acquired the shape of an ellipsoid (a ball flattened at the poles).
The influence of the Moon's gravity
The Earth's natural satellite influences not only the earth's surface, but also the layers lying underneath it. This occurs under the influence of gravity or gravity. The gravity of the Moon is most visible on the surface of the World Ocean. Earth's water is attracted by the satellite and forms a wave that follows the Moon. The satellite moves around the Earth in the opposite direction to the rotation of our planet along its axis. And, since the rotation of the globe around its axis is faster than the movement of the satellite around the Earth, the tidal wave does not move from east to west how the moon moves, and from west to east.
This opposition of movements contributes to a gradual slowdown in the rotation of both celestial bodies. The Moon is always located on the same side in relation to the Earth. Scientists claim that in the distant future the same will happen to our planet, that is, both celestial bodies will be directed towards each other by one of their sides and will continue to rotate around their common center of mass.
Coriolis force
A body performing rectilinear motion in a rotating medium is deflected to the side relative to this medium. Such a rotating medium is called a non-inertial coordinate system. The Earth is such a system. If the medium rotates clockwise, then a body moving in this system will deviate to the left, relative to the medium. When a non-inertial system rotates counterclockwise, the body deviates to the right.
Using an example, it will look like this: if a cannonball located at the North Pole is fired in the direction of the equator, then for an observer on Earth, the cannonball will gradually begin to deviate to the right. This happens because the planet moves, rotating around its axis, and while the core is flying, it has time to turn. If the observer is not on the Earth, that is, does not move with it, then the motion of the nucleus will be rectilinear.
In the Southern Hemisphere, such a deviation of moving bodies will occur to the left, since, when viewed from the South Pole, the planet rotates around its axis clockwise.
This effect is called the Coriolis force. It is named after the French scientist who discovered the phenomenon. It is noteworthy that this principle operates in any direction of the body along the earth's surface. If you shoot a cannonball from a cannon standing on the equator towards North Pole, then the projectile for an observer on Earth will deviate to the right, just as with reverse direction, that is, when shooting from the North Pole to the equator.
When shooting from the equator to the South Pole, the projectile will deflect to the left, as when shooting from the South Pole to the equator. This effect is observed due to the inertia of the core directed towards the rotation of the planet. At the beginning of its movement, the projectile was at the equator (at the point on earth with the most high speed, arising due to axial rotation). As the core moves toward the pole, it flies over points on the earth's surface that move slower than the equator, and, therefore, the lateral movement of the core, which is maintained due to inertia. Thus, the core gradually “overtakes” the earth’s surface in the lateral direction and is deflected to the side.
The Coriolis force always acts perpendicular to the motion of an object. This force acts not only on bodies moving in the direction of the meridians, but also in any other directions, regardless of which direction the movement occurs.
It is not entirely correct to call the Coriolis force a force, since it, in fact, by itself does not pull anyone anywhere. This effect is strictly relative and exists only in a non-inertial frame.
But the consequences of this effect are quite noticeable. For example, due to the Coriolis force, cyclones form on the planet. Air from high pressure zones tends to move to low pressure areas and the Coriolis force deflects the air masses relative to the moving surface to the right or left, depending on the hemisphere. Therefore, cyclones spin counterclockwise in the Northern Hemisphere, and clockwise in the Southern Hemisphere.
The Coriolis force acts on rivers and their channels. In the Northern Hemisphere, usually the right banks of rivers are steeper and washed away by water, which is pulled to the right by the rotating planet; in the Southern Hemisphere, on the contrary, they are on the left.
The railway rails are also affected by this force. Right-hand rails on single-track roads in the Northern Hemisphere will wear out more as the train pulls to the right. In the Southern Hemisphere, left-hand rails wear out more.
These are the general consequences of the rotation of our planet around its axis, which, in turn, influence a huge number of circumstances and events both on Earth and around it. A similar topic is covered in the geography textbook “Axial Rotation of the Earth,” 5th grade.
Astronomers have found that the Earth simultaneously participates in several types of motion. For example, in a composition it moves around the center Milky Way, and as part of our Galaxy it participates in intergalactic movement. But there are two main types of movement known to mankind since ancient times. One of them is around its axis.
Consequence of the Earth's axial rotation
Our planet rotates uniformly around an imaginary axis. This movement of the Earth is called axial rotation. All objects on the earth's surface rotate with the Earth. Rotation occurs from west to east, that is, counterclockwise when looking at the Earth from the North Pole. Because of this rotation of the planet, sunrise in the morning occurs in the east, and sunset in the evening in the west.
The Earth's axis is inclined at an angle of 66 1/2° to the orbital plane in which the planet moves around the Sun. Moreover, the axis is strictly in outer space: its northern end is constantly directed towards the North Star. The axial rotation of the Earth determines the apparent movement of the stars and the Moon across the sky.
The rotation of the Earth around its axis has a great influence on our planet. It determines the change of day and night and the emergence of a natural unit of time given by nature - the day. This is the period of complete rotation of the planet around its axis. The length of the day depends on the speed of rotation of the planet. According to the existing time system, a day is divided into 24 hours, an hour into 60 minutes, and a minute into 60 seconds.
Due to the axial rotation of the Earth, all bodies moving on its surface deviate from their original direction in the Northern Hemisphere to the right as they move, and in the Southern Hemisphere - to the left. In rivers, the deflection force presses the water to one of the banks. Therefore, rivers in the Northern Hemisphere usually have a steeper right bank, while rivers in the Southern Hemisphere tend to have a steeper left bank. The deviation affects the direction of winds and currents in the World Ocean.
Axial rotation affects the shape of the Earth. Our planet is not a perfect sphere, it is a little compressed. Therefore, the distance from the center of the Earth to the poles (polar radius) is 21 kilometers shorter than the distance from the center of the Earth to the equator (equatorial radius). For the same reason, the meridians are 72 kilometers shorter than the equator.
Axial rotation causes daily changes in the supply of sunlight and heat to the earth's surface and explains the apparent movement of the stars and the Moon across the sky. It also determines the difference in time in different parts globe.
World Time and Time Zones
At the same moment in different parts of the globe, the time of day can be different. But for all points located on the same meridian, the time is the same. It is called local time.
For the convenience of counting time, the surface of the Earth is conventionally divided into 24 (according to the number of hours in a day). The time within each zone is called standard time. Zones are counted from zero time zone. This is a belt in the middle of which the Greenwich (zero) meridian passes. Time on this meridian is called universal time. In two neighboring zones, the standard time differs by exactly 1 hour.
In the middle of the twelfth time zone, approximately along the 180 meridian, runs the international date line. On both sides of it, the hours and minutes coincide, and the calendar dates differ by one day. If a traveler crosses this line from east to west, then the date is moved forward one day, and if from west to east, then it goes back one day.
The earth is always in motion. Although we seem to be standing motionless on the surface of the planet, it continuously rotates around its axis and the Sun. This movement is not felt by us, as it resembles flying in an airplane. We're moving at the same speed as the plane, so we don't feel like we're moving at all.
At what speed does the Earth rotate around its axis?
The Earth rotates once on its axis in almost 24 hours (to be precise, in 23 hours 56 minutes 4.09 seconds or 23.93 hours). Since the Earth's circumference is 40,075 km, any object at the equator rotates at a speed of approximately 1,674 km per hour or approximately 465 meters (0.465 km) per second (40075 km divided by 23.93 hours and we get 1674 km per hour).
At (90 degrees north latitude) and (90 degrees south latitude), the speed is effectively zero because the pole points rotate at a very slow speed.
To determine the speed at any other latitude, simply multiply the cosine of the latitude by the planet's rotation speed at the equator (1674 km per hour). The cosine of 45 degrees is 0.7071, so multiply 0.7071 by 1674 km per hour and get 1183.7 km per hour.
The cosine of the required latitude can be easily determined using a calculator or looked at in the cosine table.
Earth rotation speed for other latitudes:
- 10 degrees: 0.9848×1674=1648.6 km per hour;
- 20 degrees: 0.9397×1674=1573.1 km per hour;
- 30 degrees: 0.866×1674=1449.7 km per hour;
- 40 degrees: 0.766×1674=1282.3 km per hour;
- 50 degrees: 0.6428×1674=1076.0 km per hour;
- 60 degrees: 0.5×1674=837.0 km per hour;
- 70 degrees: 0.342×1674=572.5 km per hour;
- 80 degrees: 0.1736×1674=290.6 km per hour.
Cyclic braking
Everything is cyclical, even the speed of rotation of our planet, which geophysicists can measure with millisecond accuracy. The Earth's rotation typically has five-year cycles of deceleration and acceleration, and Last year The slowdown cycle is often associated with a surge in earthquakes around the world.
Since 2018 is the latest in the slowdown cycle, scientists expect an increase in seismic activity this year. Correlation is not causation, but geologists are always looking for tools to try to predict when the next big earthquake will happen.
Oscillations of the earth's axis
The Earth rotates slightly as its axis drifts toward the poles. The drift of the Earth's axis has been observed to accelerate since 2000, moving eastward at a rate of 17 cm per year. Scientists have determined that the axis is still moving east instead of moving back and forth due to the combined effect of the melting of Greenland and , as well as the loss of water in Eurasia.
Axial drift is expected to be particularly sensitive to changes occurring at 45 degrees north and south latitude. This discovery led to scientists finally being able to answer the long-standing question of why the axis drifts in the first place. The axis wobble to the East or West was caused by dry or wet years in Eurasia.
At what speed does the Earth move around the Sun?
In addition to the speed of the Earth's rotation on its axis, our planet also orbits the Sun at a speed of about 108,000 km per hour (or approximately 30 km per second), and completes its orbit around the Sun in 365,256 days.
It was only in the 16th century that people realized that the Sun is the center of our solar system, and that the Earth moves around it, rather than being the fixed center of the Universe.
The Earth rotates around an axis from west to east, that is, counterclockwise when looking at the Earth from the North Star (North Pole). In this case, the angular velocity of rotation, i.e. the angle through which any point on the Earth’s surface rotates, is the same and amounts to 15° per hour. Linear speed depends on latitude: at the equator it is highest - 464 m/s, and the geographic poles are stationary.
The main physical proof of the Earth's rotation around its axis is the experiment with Foucault's swinging pendulum. After the French physicist J. Foucault carried out his famous experiment in the Paris Pantheon in 1851, the rotation of the Earth around its axis became an immutable truth. Physical evidence of the Earth’s axial rotation is also provided by measurements of the arc of the 1° meridian, which is 110.6 km at the equator and 111.7 km at the poles (Fig. 15). These measurements prove the compression of the Earth at the poles, and this is characteristic only of rotating bodies. And finally, the third evidence is the deviation of falling bodies from the plumb line at all latitudes except the poles (Fig. 16). The reason for this deviation is due to their inertia maintaining a higher linear velocity of the point A(at height) compared to point IN(near the earth's surface). When falling, objects are deflected to the east on the Earth because it rotates from west to east. The magnitude of the deviation is maximum at the equator. At the poles, bodies fall vertically, without deviating from the direction of the earth's axis.
The geographic significance of the Earth's axial rotation is extremely large. First of all, it affects the figure of the Earth. The compression of the Earth at the poles is the result of its axial rotation. Previously, when the Earth rotated faster angular velocity, polar compression was more significant. The lengthening of the day and, as a consequence, a decrease in the equatorial radius and an increase in the polar one is accompanied by tectonic deformations of the earth's crust (faults, folds) and a restructuring of the Earth's macrorelief.
An important consequence of the Earth’s axial rotation is the deflection of bodies moving in a horizontal plane (winds, rivers, sea currents, etc.). from their original direction: in the northern hemisphere – right, in the south - left(this is one of the forces of inertia, called the Coriolis acceleration in honor of the French scientist who first explained this phenomenon). According to the law of inertia, every moving body strives to maintain unchanged the direction and speed of its movement in world space (Fig. 17). Deviation is the result of the body participating simultaneously in both translational and rotational movements. At the equator, where the meridians are parallel to each other, their direction in world space does not change during rotation and the deviation is zero. Toward the poles, the deviation increases and becomes greatest at the poles, since there each meridian changes its direction in space by 360° per day. The Coriolis force is calculated by the formula F = m x 2ω x υ x sin φ, where F– Coriolis force, T– mass of a moving body, ω – angular velocity, υ – speed of a moving body, φ – geographical latitude. The manifestation of the Coriolis force in natural processes is very diverse. It is because of it that vortices of different scales arise in the atmosphere, including cyclones and anticyclones, winds and sea currents deviate from the gradient direction, influencing the climate and through it the natural zonality and regionality; The asymmetry of large river valleys is associated with it: in the northern hemisphere, many rivers (Dnieper, Volga, etc.) for this reason have steep right banks, left banks are flat, and in the southern hemisphere it’s the other way around.
Associated with the rotation of the Earth is a natural unit of time measurement - day and it happens the change of night and day. There are sidereal and sunny days. Sidereal day– the time interval between two successive upper culminations of a star through the meridian of the observation point. During a sidereal day, the Earth makes a complete rotation around its axis. They are equal to 23 hours 56 minutes 4 seconds. Sidereal days are used for astronomical observations. True solar days– the period of time between two successive upper culminations of the center of the Sun through the meridian of the observation point. The length of the true solar day varies throughout the year primarily due to uneven movement Earth in an elliptical orbit. Therefore, they are also inconvenient for measuring time. For practical purposes they use average sunny days. Mean solar time is measured by the so-called mean Sun - an imaginary point that moves evenly along the ecliptic and makes a full revolution per year, like the true Sun. The average solar day is 24 hours long. They are longer than sidereal days, since the Earth rotates around its axis in the same direction in which it moves in its orbit around the Sun with an angular velocity of about 1° per day. Because of this, the Sun moves against the background of the stars, and the Earth still needs to “turn” by about 1° for the Sun to “come” to the same meridian. Thus, during a solar day, the Earth rotates approximately 361°. To convert true solar time to mean solar time, a correction is introduced - the so-called equation of time. Its maximum positive value is + 14 minutes on February 11, its maximum negative value is –16 minutes on November 3. The beginning of the average solar day is taken to be the moment of the lowest culmination of the average Sun - midnight. This kind of time counting is called civil time.
In everyday life, it is also inconvenient to use mean solar time, since it is different for each meridian, local time. For example, on two adjacent meridians, drawn with an interval of 1°, the local time differs by 4 minutes. The presence of different local times at different points lying on different meridians led to many inconveniences. Therefore, at the International Astronomical Congress in 1884, zone time was adopted. To do this, the entire surface of the globe was divided into 24 time zones, 15° each. Behind standard time The local time of the middle meridian of each zone is accepted. To convert local time to standard time and back, there is a formula Tn – m = N – λ°, Where Tp– standard time, m- local time, N– number of hours equal to the belt number, λ° – longitude expressed in hourly units. The zero (also known as the 24th) belt is the one through the middle of which the zero (Greenwich) meridian passes. His time is taken as universal time. Knowing universal time, it is easy to calculate standard time using the formula Tn = T0+N, Where T0- universal time. The belts are counted to the east. In two neighboring zones, the standard time differs by exactly 1 hour. For convenience, the boundaries of time zones on land are drawn not strictly along meridians, but along natural boundaries (rivers, mountains) or state and administrative boundaries.
| In our country, standard time was introduced on July 1, 1919. Russia is located in ten time zones: from the second to the eleventh. However, in order to more rationally use daylight in the summer in our country, in 1930, by a special government decree, the so-called maternity time, ahead of standard time by 1 hour. So, for example, Moscow is formally located in the second time zone, where standard time is calculated according to the local time of the meridian 30° east. etc. But in fact, time in winter in Moscow is set according to the time of the third time zone, corresponding to local time on the meridian 45° east. d. This “shift” operates throughout Russia, except for the Kaliningrad region, where the time actually corresponds to the second time zone. |
| Rice. 17. Deviation of bodies moving along the meridian in the northern hemisphere - to the right, in the southern hemisphere - to the left |
In a number of countries, time is moved forward one hour only in the summer. In Russia, since 1981, for the period from April to October, summer time by moving the time another hour ahead compared to maternity leave. Thus, in summer time in Moscow actually corresponds to local time on the meridian 60°E. d. The time according to which residents of Moscow and the second time zone in which it is located live is called Moscow. According to Moscow time, our country schedules trains and planes, and marks the time on telegrams.
In the middle of the twelfth zone, approximately along the 180° meridian, in 1884 a international date line. This is a conventional line on the surface of the globe, on both sides of which the hours and minutes coincide, and the calendar dates differ by one day. For example, in New Year at 0 hours 00 minutes to the west of this line, January 1 of the new year begins, and to the east - only December 31 of the old year. When crossing the date boundary from west to east in the count calendar days go back one day, and from east to west one day is skipped in the count of dates.
The change of day and night creates daily rhythm in live and inanimate nature. The circadian rhythm is associated with light and temperature conditions. The daily variation of temperature, day and night breezes, etc. are well known. The daily rhythm of living nature is very clearly manifested. It is known that photosynthesis is possible only during the day, in the presence of sunlight, and that many plants open their flowers at different hours. Animals can be divided into nocturnal and diurnal according to the time of their activity: most of them are awake during the day, but many (owls, bats, moths) are awake in the darkness of the night. Human life also flows in a circadian rhythm.
Rice. 18. Twilight and white nights
The period of smooth transition from daylight to night darkness and back is called at dusk. IN they are based on an optical phenomenon observed in the atmosphere before sunrise and after sunset, when the sun is still (or already) below the horizon, but illuminates the sky from which the light is reflected. The duration of twilight depends on the declination of the Sun (the angular distance of the Sun from the plane of the celestial equator) and geographical latitude observation sites. At the equator, twilight is short and increases with latitude. There are three periods of twilight. Civil twilight are observed when the center of the Sun plunges below the horizon shallowly (at an angle of up to 6°) and for a short time. This is actually White Nights, when the evening dawn meets the morning dawn. In summer they are observed at latitudes of 60° and more. For example, in St. Petersburg (latitude 59°56" N) they last from June 11 to July 2, in Arkhangelsk (64°33" N) - from May 13 to July 30. Navigational twilight observed when the center of the solar disk plunges 6–12° below the horizon. In this case, the horizon line is visible, and from the ship you can determine the angle of the stars above it. And finally, astronomical twilight are observed when the center of the solar disk plunges below the horizon by 12–18°. At the same time, the dawn in the sky still prevents astronomical observations of faint luminaries (Fig. 18).
The rotation of the Earth gives two fixed points – geographic poles(the points of intersection of the imaginary axis of rotation of the Earth with the earth's surface) - and thus allows us to construct a coordinate grid of parallels and meridians. Equator(lat. aequator- leveler) - the line of intersection of the globe with a plane passing through the center of the Earth perpendicular to its axis of rotation. Parallels(Greek parallelos– running side by side) – lines of intersection of the earth’s ellipsoid with planes parallel to the equatorial plane. Meridians(lat. meridlanus- midday) - the line of intersection of the earth's ellipsoid with planes passing through both of its poles. The length of the 1st meridian is on average 111.1 km.
