Why does the sun illuminate and heat the earth differently? Global influence of the sun on the earth

§ 52. Apparent annual motion of the Sun and its explanation

Observing the daily movement of the Sun throughout the year, one can easily notice a number of features in its movement that differ from the daily movement of stars. The most typical of them are the following.

1. The place of sunrise and sunset, and therefore its azimuth, changes from day to day. Starting from March 21 (when the Sun rises at the point of the east and sets at the point of the west) to September 23, the sun rises in the north-east quarter, and sunset - in the north-west. At the beginning of this time, the sunrise and sunset points move north and then in the opposite direction. On September 23, just like on March 21, the Sun rises at the east point and sets at the west point. Starting from September 23 to March 21, a similar phenomenon will repeat in the southeast and southwest quarters. The movement of sunrise and sunset points has a one-year period.

The stars always rise and set at the same points on the horizon.

2. The meridional altitude of the Sun changes every day. For example, in Odessa (average = 46°.5 N) on June 22 it will be greatest and equal to 67°, then it will begin to decrease and on December 22 it will reach lowest value 20°. After December 22, the meridional altitude of the Sun will begin to increase. This is also a one-year phenomenon. The meridional altitude of stars is always constant. 3. The duration of time between the culminations of any star and the Sun is constantly changing, while the duration of time between two culminations of the same stars remains constant. So, at midnight we see those constellations culminating that are currently located on the opposite side of the sphere from the Sun. Then some constellations give way to others, and over the course of a year at midnight all the constellations will culminate in turn.

4. The length of the day (or night) is not constant throughout the year. This is especially noticeable if you compare the length of summer and winter days in high latitudes, for example in Leningrad. This happens because the time the Sun is above the horizon varies throughout the year. The stars are always above the horizon for the same amount of time.

Thus, the Sun, in addition to the daily movement performed jointly with the stars, also has a visible movement around the sphere with an annual period. This movement is called visible the annual movement of the Sun across the celestial sphere.

Most visual representation We will obtain information about this movement of the Sun if we determine its equatorial coordinates daily - right ascension a and declination b. Then, using the found values ​​of the coordinates, we plot the points on the auxiliary celestial sphere and connect them with a smooth curve. As a result, we get a large circle on the sphere, which will indicate the path of the visible annual movement Sun. The circle on the celestial sphere along which the Sun moves is called the ecliptic. The plane of the ecliptic is inclined to the plane of the equator at a constant angle g = =23°27", which is called the angle of inclination ecliptic to equator(Fig. 82).

Rice. 82.

The apparent annual movement of the Sun along the ecliptic occurs in the direction opposite to the rotation of the celestial sphere, that is, from west to east. The ecliptic intersects the celestial equator at two points, which are called the equinox points. The point at which the Sun passes from the southern hemisphere to the northern, and therefore changes the name of the declination from southern to northern (i.e. from bS to bN), is called the point spring equinox and is designated by the Y icon. This icon denotes the constellation Aries, in which this point was once located. Therefore, it is sometimes called the Aries point. Currently, point T is located in the constellation Pisces.

The opposite point at which the Sun passes from the northern hemisphere to the southern and changes the name of its declination from b N to b S is called point of the autumnal equinox. It is designated by the symbol of the constellation Libra O, in which it was once located. Currently, the autumn equinox point is in the constellation Virgo.

Point L is called summer point, and point L" - a point winter solstice.

Let's follow the apparent movement of the Sun along the ecliptic throughout the year.

The Sun arrives at the vernal equinox on March 21st. The right ascension a and declination b of the Sun are zero. On everything globe The sun rises at point O st and sets at point W, and day is equal to night. Starting March 21, the Sun moves along the ecliptic towards the summer solstice point. The right ascension and declination of the Sun are continuously increasing. It is astronomical spring in the northern hemisphere, and autumn in the southern hemisphere.

On June 22, approximately 3 months later, the Sun comes to the summer solstice point L. The direct ascension of the Sun is a = 90°, a declination b = 23°27"N. In the northern hemisphere, astronomical summer begins (the longest days and shortest nights), and in the south - winter (longest nights and shortest days)... With the further movement of the Sun, its northern declination begins to decrease, and its right ascension continues to increase.

About three more months later, on September 23, the Sun comes to the point of the autumnal equinox Q. The direct ascension of the Sun is a=180°, declination b=0°. Since b = 0 ° (same as March 21), then for all points earth's surface The sun rises at point O st and sets at point W. Day will be equal to night. The name of the declination of the Sun changes from northern 8n to southern - bS. In the northern hemisphere, astronomical autumn begins, and in the southern hemisphere, spring begins. With further movement of the Sun along the ecliptic to the winter solstice point U, declination 6 and right ascension aO increase.

On December 22, the Sun comes to the winter solstice point L". Right ascension a=270° and declination b=23°27"S. Astronomical winter begins in the northern hemisphere, and summer begins in the southern hemisphere.

After December 22, the Sun moves to point T. The name of its declination remains southern, but decreases, and its right ascension increases. About 3 months later, on March 21, the Sun, having completed full turn along the ecliptic, returns to the point of Aries.

Changes in the right ascension and declination of the Sun do not remain constant throughout the year. For approximate calculations, the daily change in the right ascension of the Sun is taken equal to 1°. The change in declination per day is taken to be 0°.4 for one month before the equinox and one month after, and the change is 0°.1 for one month before the solstices and one month after the solstices; the rest of the time, the change in solar declination is taken to be 0°.3.

The peculiarity of changes in the right ascension of the Sun plays an important role when choosing the basic units for measuring time.

The vernal equinox point moves along the ecliptic towards the annual movement of the Sun. Its annual movement is 50", 27 or rounded 50",3 (for 1950). Consequently, the Sun does not reach its original place relative to the fixed stars by an amount of 50",3. For the Sun to travel the indicated path, it will take 20 mm 24 s. For this reason, spring

It occurs before the Sun completes its visible annual motion, a full circle of 360° relative to the fixed stars. The shift in the moment of the onset of spring was discovered by Hipparchus in the 2nd century. BC e. from observations of stars that he made on the island of Rhodes. He called this phenomenon the anticipation of the equinoxes, or precession.

The phenomenon of moving the vernal equinox point caused the need to introduce the concepts of tropical and sidereal years. A tropical year is the period of time during which the Sun makes a full revolution across the celestial sphere relative to the vernal equinox point T. “The duration of the tropical year is 365.2422 days. The tropical year is consistent with natural phenomena and precisely contains the full cycle of the seasons of the year: spring, summer, autumn and winter.

A sidereal year is the period of time during which the Sun makes a complete revolution across the celestial sphere relative to the stars. The length of a sidereal year is 365.2561 days. Sidereal year longer than tropical.

In its apparent annual movement across the celestial sphere, the Sun passes among various stars located along the ecliptic. Also in ancient times these stars were divided into 12 constellations, most of which were given the names of animals. The strip of sky along the ecliptic formed by these constellations was called the Zodiac (circle of animals), and the constellations were called zodiacal.

According to the seasons of the year, the Sun passes through the following constellations:

From the joint movement of the annual Sun along the ecliptic and the daily movement due to the rotation of the celestial sphere, the general movement of the Sun along a spiral line is created. The extreme parallels of this line are located on both sides of the equator at distances of = 23°.5.

On June 22, when the Sun describes the extreme diurnal parallel in the northern celestial hemisphere, it is in the constellation Gemini. In the distant past, the Sun was in the constellation Cancer. On December 22, the Sun is in the constellation Sagittarius, and in the past it was in the constellation Capricorn. Therefore, the northernmost celestial parallel was called the Tropic of Cancer, and the southern one was called the Tropic of Capricorn. The corresponding terrestrial parallels with latitudes cp = bemach = 23°27" in the northern hemisphere were called the Tropic of Cancer, or the northern tropic, and in the southern hemisphere - the Tropic of Capricorn, or the southern tropic.

The joint movement of the Sun, which occurs along the ecliptic with the simultaneous rotation of the celestial sphere, has a number of features: the length of the daily parallel above and below the horizon changes (and therefore the duration of day and night), the meridional heights of the Sun, the points of sunrise and sunset, etc. etc. All these phenomena depend on the relationship between the geographic latitude of a place and the declination of the Sun. Therefore, for an observer located at different latitudes, they will be different.

Let's consider these phenomena at some latitudes:

1. The observer is at the equator, cp = 0°. The axis of the world lies in the plane of the true horizon. The celestial equator coincides with the first vertical. The diurnal parallels of the Sun are parallel to the first vertical, therefore the Sun in its daily movement never crosses the first vertical. The sun rises and sets daily. Day is always equal to night. The Sun is at its zenith twice a year - on March 21 and September 23.

Rice. 83.

2. The observer is at latitude φ
3. The observer is at latitude 23°27"
4. The observer is at latitude φ > 66°33"N or S (Fig. 83). The belt is polar. Parallels φ = 66°33"N or S are called polar circles. In the polar zone, polar days and nights can be observed, that is, when the Sun is above the horizon for more than a day or below the horizon for more than a day. The longer the polar days and nights, the greater the latitude. The sun rises and sets only on those days when its declination is less than 90°-φ.

5. The observer is at the pole φ=90°N or S. The axis of the world coincides with the plumb line and, therefore, the equator with the plane of the true horizon. The observer's meridian position will be uncertain, so parts of the world are missing. During the day, the Sun moves parallel to the horizon.

On the days of the equinoxes, polar sunrises or sunsets occur. On the days of the solstices, the height of the Sun reaches highest values. The altitude of the Sun is always equal to its declination. The polar day and polar night last for 6 months.

Thus, due to various astronomical phenomena caused by the combined daily and annual movement of the Sun at different latitudes (passage through the zenith, polar day and night phenomena) and the climatic features caused by these phenomena, the earth's surface is divided into tropical, temperate and polar zones.

Tropical zone is the part of the earth's surface (between latitudes φ=23°27"N and 23°27"S) in which the Sun rises and sets every day and is at its zenith twice during the year. The tropical zone occupies 40% of the entire earth's surface.

Temperate zone called the part of the earth's surface in which the Sun rises and sets every day, but is never at its zenith. There are two temperate zones. In the northern hemisphere, between latitudes φ = 23°27"N and φ = 66°33"N, and in the southern hemisphere, between latitudes φ=23°27"S and φ = 66°33"S. Temperate zones occupy 50% of the earth's surface.

Polar belt called the part of the earth's surface in which polar days and nights are observed. There are two polar zones. The northern polar belt extends from latitude φ = 66°33"N to the north pole, and the southern one - from φ = 66°33"S to the south pole. They occupy 10% of the earth's surface.

For the first time, the correct explanation of the visible annual movement of the Sun across the celestial sphere was given by Nicolaus Copernicus (1473-1543). He showed that the annual movement of the Sun across the celestial sphere is not its actual movement, but only an apparent one, reflecting the annual movement of the Earth around the Sun. The Copernican world system was called heliocentric. According to this system in the center solar system There is the Sun, around which the planets move, including our Earth.

The Earth simultaneously participates in two movements: it rotates around its axis and moves in an ellipse around the Sun. The rotation of the Earth around its axis causes the cycle of day and night. Its movement around the Sun causes the change of seasons. The combined rotation of the Earth around its axis and the movement around the Sun causes the visible movement of the Sun across the celestial sphere.

To explain the apparent annual movement of the Sun across the celestial sphere, we will use Fig. 84. The Sun S is located in the center, around which the Earth moves counterclockwise. The earth's axis remains unchanged in space and makes an angle with the ecliptic plane equal to 66°33". Therefore, the equator plane is inclined to the ecliptic plane at an angle e=23°27". Next comes the celestial sphere with the ecliptic and the signs of the Zodiac constellations marked on it in their modern location.

The Earth enters position I on March 21. When viewed from the Earth, the Sun is projected onto the celestial sphere at point T, currently located in the constellation Pisces. The declination of the Sun is 0°. An observer located at the Earth's equator sees the Sun at its zenith at noon. All earthly parallels are half illuminated, so at all points on the earth's surface day is equal to night. Astronomical spring begins in the northern hemisphere, and autumn begins in the southern hemisphere.

Rice. 84.

The Earth enters position II on June 22. Declination of the Sun b=23°,5N. When viewed from Earth, the Sun is projected into the constellation Gemini. For an observer located at latitude φ=23°.5N, (The sun passes through the zenith at noon. Most of the daily parallels are illuminated in the northern hemisphere and a smaller part in the southern hemisphere. The northern polar zone is illuminated and the southern one is not illuminated. In the northern, the polar day lasts, and in the southern hemisphere it is polar night.In the northern hemisphere of the Earth, the rays of the Sun fall almost vertically, and in the southern hemisphere - at an angle, so astronomical summer begins in the northern hemisphere, and winter in the southern hemisphere.

To position III Earth comes September 23rd. The declination of the Sun is bo = 0 ° and it is projected at the point of Libra, which is now located in the constellation Virgo. An observer located at the equator sees the Sun at its zenith at noon. All earthly parallels are half illuminated by the Sun, so at all points on the Earth day is equal to night. In the northern hemisphere, astronomical autumn begins, and in the southern hemisphere, spring begins.

On December 22, the Earth comes to position IV. The Sun is projected into the constellation Sagittarius. Declination of the Sun 6=23°.5S. The southern hemisphere receives more daylight than the northern hemisphere, so daylight in the southern hemisphere longer than the night, and in the north - vice versa. The sun's rays fall almost vertically into the southern hemisphere, and at an angle into the northern hemisphere. Therefore, astronomical summer begins in the southern hemisphere, and winter in the northern hemisphere. The sun illuminates the southern polar zone and does not illuminate the northern one. The southern polar zone experiences polar day, while the northern zone experiences night.

Corresponding explanations can be given for other intermediate positions of the Earth.

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Just recently I went to buy myself a winter wardrobe, because very soon The sun will stop heating that part of the Earth where I live. But why does this happen? Every year, more than half of humanity experiences serious climate change, it is either hot or cold, then moderate weather reigns in their region. For many, spring is their favorite time of year, because it is not too hot yet, but there are no sub-zero temperatures either. This state of affairs is due to the fact that Planet Earth does not stand still for 365 days, but rotates around its axis and the luminary in a certain orbit.

Why does the sun shine differently throughout the year?

The sun's rays hitting the Earth will never be the same, since the planet stands in outer space not exactly, and at an angle of 23 degrees. This is the first reason for this circumstance, because depending on the position of the Earth at a particular moment, photons cannot reach the South and North Poles in the same way, but at the equator the weather always remains warm, but here the second very important factor.

It lies in the fact that the planet’s orbit is not circular, but oval, therefore, at different periods of passage along it, the Earth appears either closer to the luminary or moves away from it. In addition to all this, our planet rotates around its axis within 24 hours, and therefore each hemisphere turns away from the sun's rays and night falls there.

Why does the earth rotate

Despite the fact that the Universe is huge, everything in it is arranged very subtly and harmoniously. All space objects interact with each other through gravitational attraction, which determines constant rotation of the Earth around the Sun, since the mass of the light is greater, accordingly, the strength gravity is many times stronger.

Nature thought through everything thoroughly, because if photons always heated only, for example, one side of the planet, or its inclination angle was higher, there would be no balance. Some celestial bodies suffer from such an imbalance, on one side there may be extraordinary heat, while the other at that moment freezes and becomes covered with ice. So, to summarize, it should be noted three reasons uneven lighting throughout the year, namely:

1. The tilt of the earth's axis.
2. Oval orbit.
3. Daily rotation.

As it turned out, everything is quite simple and clear.

On a hot summer day, when the weather is clear outside and we are exhausted from the high temperature, we often hear the phrase “the sun is at its zenith.” In our understanding, we are talking about the fact that the heavenly body is at the highest point and warms as much as possible, one might even say, scorches the earth. Let's try to plunge a little into astronomy and understand in more detail this expression and how correct our understanding of this statement is.

Earthly parallels

More from school curriculum we know that on our planet there are so-called parallels, which are invisible (imaginary) lines. Their existence is due elementary laws geometry and physics, and knowledge of where these parallels come from is necessary in order to understand the entire geography course. It is customary to distinguish three most important lines - the equator, the Arctic Circle and the tropics.

Equator

The equator is usually called the invisible (conditional) line dividing our Earth into two identical hemispheres - the Southern and Northern. It has long been known that the Earth does not stand on three pillars, as was believed in ancient times, but has a spherical shape and, in addition to moving around the Sun, rotates around its axis. So it turns out that on Earth, which has a length of about 40 thousand km, this is the equator. In principle, from a mathematical point of view, everything is clear here, but does this matter for geography? And here, upon closer examination, it turns out that the part of the planet that is located between the tropics receives the most solar heat and light. This is due to the fact that this area The Earth is always turned towards the Sun, so the rays here fall almost vertically. It follows from this that the highest air temperatures are observed in the near-equatorial areas of the planet, and air masses saturated with moisture create strong evaporation. The sun is at its zenith at the equator twice a year, that is, it shines absolutely vertically downwards. For example, such a phenomenon never occurs in Russia.

Tropics

There are Southern and Northern tropics on the globe. It is noteworthy that the sun is at its zenith here only once a year - on the day of the solstice. When the so-called winter solstice occurs - December 22, the Southern Hemisphere turns maximum towards the Sun, and on June 22 - vice versa.

Sometimes Southern is named after the zodiac constellation that is in the path of the Sun these days. For example, the Southern one is conventionally called the Tropic of Capricorn, and the Northern one is called the Tropic of Cancer (December and June, respectively).

Arctic Circles

The Arctic Circle is considered to be a parallel, above which a phenomenon such as polar night or day is observed. The location of the latitude at which the polar circles are located also has a completely mathematical explanation: it is 90° minus the tilt of the planet’s axis. For the Earth, this value of the polar circles is 66.5°. Unfortunately, residents of temperate latitudes cannot observe these phenomena. But the sun at its zenith at the parallel corresponding to the Arctic Circle is an absolutely natural event.

Well-Known Facts

The Earth does not stand still and, in addition to moving around the Sun, rotates around its axis every day. Throughout the year, we observe how the length of the day and the air temperature outside the window change, and the most attentive ones can note the change in the position of the stars in the sky. Passes in 364 full path around the Sun.

Day and night

When it is dark here, that is, it means that the Sun illuminates the other hemisphere at a given period of time. A completely logical question arises: why the day is not equal to the length of the night. The fact is that the trajectory plane is not at a right angle relative to the earth's axis. Indeed, in this case we would not have seasons in which the ratio of the length of day and night changes.

On the 20th of March it tilts towards the Sun. Then at approximately noon on the equator line one can absolutely say that the sun is at its zenith. These are followed by days when a similar phenomenon is observed at more northern points. Already on June 22, the sun is at its zenith on the Tropic of Cancer; on this day it is considered mid-summer and has its maximum longitude. For us, the most familiar definition is the phenomenon of solstice.

The interesting thing is that after this day everything happens again, only in reverse order, and continues until the moment when on the equator line at noon the sun is again at its zenith - this happens on September 23. At this time, midsummer begins in the southern hemisphere.

From all this it follows that when the sun is at its zenith at the equator, throughout the entire globe the duration of the night is 12 hours, and the day is equal to the same period of time. We are used to calling this phenomenon the day of the autumn or spring equinox.

Despite the fact that we have analyzed the correct explanation of the concept of “the sun at its zenith”, it is still more familiar to us with a formulation that simply means that the sun is as high as possible on a given specific day.

The sun influences the Earth quite strongly. The sun emits light and, since the Earth rotates on its own axis, it becomes day and night. Sunlight brings heat, which, with the rotation of the Earth around the Sun and the tilt of the Earth's axis (23.5°), causes the seasons to change. Most light and heat come from direct sunlight.

sunlight

The sun's rays can illuminate only one half of the earth's surface at any time. Sunlight equally reaches the North and South Poles only twice a year - on September 23 and March 21 - the equinoxes (Figure 1). On these two days, the direct rays of the Sun fall vertically on the Equator.
From September 23 to December 21, the Sun's rays gradually expand their zone of impact on the Earth from the South Pole and retreat from the North Pole. On December 21, the rays reach 23.5° per South Pole(Antarctic zone) and are unable to reach the North Pole at the same 23.5° (Arctic zone). On this day, the area south of the Antarctic Circle (Antarctica) receives constant sunlight, while the area north of the Arctic Circle (Arctic) remains without sunlight. Try to analyze this using a globe. Find the Southern and Northern Arctic Circles on the globe (parallels in the Northern and Southern Hemispheres with latitudes of 66.5°).
On December 22, the rays of the Sun cover the entire zone up to the Antarctic Circle and leave the Arctic Circle at 23.5° (Figure 2). And on June 21, the opposite is true - the rays completely leave the region of the Antarctic Circle and illuminate the region of the Arctic Circle. Now the South Pole is in darkness, and North Pole receives constant sunlight (Figure 3). This explains the six-month day and night at the North and South Poles.
When light falls directly on the Tropic of the North (23.5° north of the Equator), the day in the Northern Hemisphere is at its longest possible than the night (June 21).
When light falls directly on the Tropic of South (23.5° south of the equator), the day in the Northern Hemisphere is shortest at night (December 22).

Is the Sun exactly south at 12 noon?

At noon the Sun reaches its highest position in the south. When it is at this point, true local time is said to be 12 o'clock. At this moment, the shadow from a vertically standing pillar is the shortest. Unfortunately, due to uneven movement In Earth's orbit, the Sun does not move across the sky evenly. So it doesn't end up exactly south every 24 hours.

In order for the calculation of time not to depend on the “whims” of the true Sun, astronomers came up with the “average Sun”, moving uniformly. It exists, of course, only on paper. When the "mean Sun" reaches its highest position in the south, it is considered to be 12 o'clock local mean time. The difference between true and mean local time is called the equation of time. It varies throughout the year ranging from -14.3 to +16.3 minutes.

But there is another problem. For example, when in Hamburg the Sun is at its highest point, in Berlin it has already passed it, but in Bremen it has not yet reached this position. Thus, the local mean time in the three cities would be different. However, this is very inconvenient for the operation of transport and other services. In Central Europe, all people live according to Central European time, which does not correspond to the true position of the Sun in the sky.

But the governments of several countries have agreed that Central European Time will be considered the mean solar time at 15 degrees east longitude. In summer, another hour is added to this time to lengthen the morning hours and shorten the evening hours. This is already the so-called summer time. Therefore, in the summer in areas of Europe that live according to this schedule, the Sun reaches its highest point in the sky at about 13 o'clock. The same thing is happening in Russia.