Seasonal temperature - temperature in summer and winter - depends on the amount of heat received different regions earth from the sun. For the temperature of an area to be constant, there must be a balance between the amount of heat it receives and the temperature it emits into space. If they receive more heat than they emit, then it becomes warmer. If it's the other way around, it's colder. Why is the amount of energy received this area from the Sun, changes throughout the year.
Two theories of cold winter and warm summer
Two popular theories are used to explain temperature differences in different seasons of the year.
One of them sees the reason that it is cold in winter and warm in summer due to differences in the distances from the Earth to the Sun as it moves along an elliptical orbit. The Earth is located at a distance from the Sun (147.1 million kilometers at a minimum distance - at perihelion and 152.1 million kilometers at a maximum distance - at aphelion).
Another theory considers the reason for the seasons on Earth to be that the Earth's axis is tilted relative to the plane of its orbit.
Farther from the Sun - no colder
If the first theory were completely true, then both hemispheres of the earth - northern and southern - would have the same seasons. However, in general this is not observed.
Moreover, the Earth reaches perihelion - the minimum distance from the Sun just when it is winter in the northern hemisphere - in early January, when it is cold, and the maximum distance - aphelion - in the summer, in July, when it is warm!
The reason for winter and summer is the tilt of the Earth's rotation axis
The second theory sees the reason for the changes in seasons - from winter to summer and back - in the fact that the Earth's rotation axis is inclined by 23.5 degrees relative to the ecliptic - the plane of the Earth's orbit around the Sun. This axis always passes through the Earth's north and south celestial poles. North Pole aimed approximately at North Star.
Figure 1 – Northern Hemisphere of the Earth in winter
(click to enlarge - all pictures)
Figure 2 – Northern Hemisphere of the Earth in summer
When the northern hemisphere of the Earth is tilted away from the Sun, the rays of the Sun strike the northern hemisphere as if “in passing”. And in southern hemisphere the rays fall almost head-on. Then winter begins in the northern hemisphere, and summer, on the contrary, in the southern hemisphere.
Summer and winter - different angles of incidence
When the northern hemisphere of the Earth is tilted towards the Sun, then, on the contrary, the rays of the Sun fall on the northern hemisphere “head-on”, and on the southern hemisphere “casually”. Then summer begins in the northern hemisphere, and winter, on the contrary, begins in the southern hemisphere.
In summer sun rays fall on the earth's surface almost perpendicularly, thereby concentrating energy. This concentrated energy is capable of heating the surface faster than winter time when the sun's rays strike the earth's surface at a more grazing angle. Therefore, it is colder in winter than in summer. The same energy hits different areas earth's surface: less in summer, more in winter (Figures 3 and 4). In other words: in summer the density of solar energy that hits the earth's surface is greater than in winter.
Figure 3 – Low solar energy density in winter
Figure 4 – High density solar energy in summer
In addition, in summer the Sun stays above the horizon longer and therefore has more time to warm everything up. high temperature than in winter (Figures 5 and 6).
Figure 5 – Sunlight For northern hemisphere in winter
Figure 6 – Sunlight for the northern hemisphere in summer
Winter and summer on other planets
Rotation axes of most other planets solar system are also inclined with respect to their orbital planes. Therefore they also have seasonal changes its temperature.
Mercury, Jupiter and Venus have a very small axial tilt - no more than 3 degrees. For these planets, a much larger role in seasonal temperature changes can be played - in contrast to the Earth - by changes in their distance from the Sun. However, only Mercury has big difference between perihelion and aphelion - the maximum and minimum distance to the Sun. Mercury's extremely liquid atmosphere makes it impossible for any solar energy to be stored on the surface. The orbits of Jupiter and Venus are almost circular, and their atmospheres are very dense. Therefore, seasonal changes in their temperatures are almost zero.
Winter and summer on Mars
Mars, as well as Saturn and Neptune, have tilts of their rotation axes similar to those of Earth. However, Saturn and Neptune have zero temperature changes due to their very dense atmospheres as well as their nearly circular orbits.
Mars has very large seasonal temperature changes because it has a very “liquid” atmosphere and a highly eccentric orbit. The Southern Hemisphere is closest to the Sun during its summer and furthest away during its winter. For the same reasons, the northern hemisphere of Mars has milder seasonal variations than its southern hemisphere. Because planets move slowest in their orbits when they are farthest from the Sun, the southern hemisphere experiences short, hot summers and long, cold winters.
Seasons of Uranus
Uranus' seasons are the most interesting because it revolves around the Sun, so to speak, on its side - Uranus' axis is tilted to the plane of its orbit by 98 degrees. Half of the “Uranian” year, one of its hemispheres is constantly under sunlight, and the other hemisphere is always in the shadow. In the other half of the year, these hemispheres change places. Thick atmosphere Uranus distributes solar energy from one hemisphere to the other is very efficient, so that seasonal temperature changes are almost invisible there.
Eternal winter on Pluto
Pluto's axis is also tilted at a large angle - 122.5 degrees, its orbit is the most elliptical of all the planets. In addition, it has a very weak atmosphere. Pluto is always so far from the Sun that it is constantly “frozen” - at a temperature of about minus 220 degrees. It’s already where it’s really cold – both in summer and winter.
The learned men of the 17th century must have been very surprised and experienced a lot of displeasure when they read Johannes Kepler’s book “The New Astronomy”. Of course, the German mathematician called for no less than the abandonment of the circular orbits in which the planets move, and replacing them with ellipses! Astronomers had not yet fully comprehended the revolutionary ideas of Copernicus, who placed the Sun at the center of the world and thus reduced the Earth to the status of an ordinary planet, when the second blow was dealt to the two-thousand-year-old system of the world of Ptolemy.
Ellipses! It's almost sacrilege! A circle is a perfect figure, and how else can bodies move in heavenly world, if not in circular orbits! But Kepler's theory explained the motion of planets better than any theory that used circular orbits. Based on it, it was possible to do more accurate predictions, where in the sky this or that planet will be in a year, ten, a hundred years. The theory worked!
Earth, like other planets, also moves around the Sun not in a circular, but in an elliptical, elongated orbit. This means that our planet spends one part of its journey approaches the sun, and the other part - deleted. The point where the Earth is closest to the star is called perihelion , and the point of the orbit farthest from the Sun is called aphelion . As a consequence, the size of the Sun in our sky should change.
The difference in the size of the Sun when the Earth is at perihelion and at aphelion. Photo: Raffaele Esposito
Since the Earth moves in an ellipse, its motion unevenly. Due to the fact that the force of gravity decreases with increasing distance between gravitating bodies, near aphelion the Earth should move slower than at perihelion. Of course, this is reflected in the movement of the Sun across the sky: the luminary moves against the background of the stars, sometimes faster, sometimes slower (this does not mean the daily movement of the Sun from east to west, but the second, annual movement against the background of the constellations!). They must have different durations and seasons, because the season in which the Earth is closer to the Sun, our planet “skips” faster than others.
All of the above are obvious consequences of Kepler's three laws, but in ordinary life they usually pass by our attention. And this is not surprising - the earth’s orbit is almost a circle, its elongation is small. Without making special observations, it is almost impossible to notice the effects of the Earth’s motion along an ellipse.
This long preface was made in order to say the main thing: today, January 4, 2015, the Earth is at perihelion to the Sun - at the point of its orbit closest to the star. Exact date events - January 4 at 06:36 universal time, or at 09:36 Moscow time.
It is today that the Sun is closer to the Earth than on any other day of the year, which means that today the Earth will receive from the Sun greatest number light and warmth in 2015!
When the Earth is closest to the Sun, it is winter in the northern hemisphere. This remarkable photo of a solar halo was taken on January 2, 2015 in Alaska. © Tracey Mendenhall Porreca
Strange? Not at all! Let us remember that the seasons change not because the Earth is either closer or further from the Sun, but because the axis of rotation of our planet is inclined to the plane earth's orbit. As a result, the Sun illuminates mainly the northern hemisphere of the Earth for half the year, and the southern hemisphere for the other half of the year. Therefore, it is now a real summer in the southern hemisphere of the Earth!
However, as we wrote last year, six months is a rough estimate. Let's take a closer look.
The boundaries of the astronomical seasons are the moments of the equinoxes and solstices. (These are not random dates, but singular points in the Earth’s orbit, marking the key “stages” of the process of illumination of the Earth by the Sun/) For example, astronomical summer lasts from the moment summer solstice, which in different years occurs on June 20, 21, or 22, until the autumnal equinox occurs on September 22 or 23. Thus, the duration of summer is 93.6 days. Autumn lasts from the autumnal equinox until the winter solstice, which occurs on December 21 or 22. Let's count the number of days on the calendar between these dates and make sure that autumn is 4 days shorter! - its duration is 89.8 days! Winter is even shorter - only 89 days. Finally, the duration of spring is 92.8 days. Here visual evidence the fact that the Earth moves in an ellipse and is closer to the Sun in winter than in summer!
However, the difference in distances to the Sun in summer and winter is small - only about 5 million km. Today it is equal to 147 million 096 thousand 204 kilometers. At aphelion it will exceed 152 million kilometers. The distance varies by approximately 3%. The size of the Sun in our sky changes just as much - completely invisible to the naked eye!
We all know very well that in different times The Sun behaves differently every year. In summer it rises early, goes high in the sky and sets late. In winter, on the contrary, the Sun appears above the horizon late and, having made a low and shortcut across the sky, sets early. In summer the days are long and the nights are short; In winter the days are short and the nights are long. In spring and autumn, day and night differ little in duration. How can all this be explained? After all, we know that the change of day and night, that is, the rising and setting of the Sun, occurs because the Earth rotates around its axis. Why doesn’t it spin the same way all year round? Or maybe the length of day and night depends on some other reason?
To find out this, let's take a closer look at how the Sun behaves at different times of the year and what connection there is between the behavior of the Sun and weather changes.
In both summer and winter, the Sun rises in the eastern part of the horizon, sets in the western part, and at noon is in the south highest above the horizon. But in summer the Sun rises between the east and the north, that is, in the northeast, and sets between the west and the north, that is, in the northwest. Due to this, its visible path across the sky is long, and a lot of time must pass before the Sun can reach the south; During this time the Sun will have time to rise high. In winter, the Sun rises between east and south, that is, in the southeast, and sets between west and south, that is, in the southwest. Its path across the sky is shorter than in summer. The Sun reaches the south in a relatively short time and does not have time to rise to a significant height (Fig. 5).
Rice. 5. Visible path Sun above the horizon at different times of the year
Let's take Moscow, for example. In summer in Moscow, at the end of June, the Sun is above the horizon for approximately 17 and a half hours, and in winter, at the end of December, only 6 and a half. At noon, when the Sun is in the south, in summer it is more than 5 times higher above the horizon than in winter.
It is not difficult to understand that it is precisely due to this difference in the behavior of the Sun in winter and summer that it is cold in winter and warm in summer. After all, in summer the Sun illuminates the surface of the Earth much longer than in winter. And the sun's rays not only give light to the Earth, but also warm it.
But also higher value has a difference in the height of the Sun's path above the horizon. When the Sun is low in the sky, its rays have to pass through a thick layer of air envelope, which not only weakens the light of the Sun, but also delays the heat of its rays. In addition, in this case the sun's rays fall on the earth's surface not directly, but obliquely, as if sliding along it. As a result of all this, when the sun is low, the sun's rays warm the soil very little.
It’s completely different when the Sun is high above the horizon. Then the sun's rays pass through a relatively thin layer air and fall to the earth's surface almost vertically. Thanks to this, they greatly warm the soil.
Rice. 6. Path of sunlight at low and high high positions Sun
Look at fig. 6. The left side of the picture shows how a beam of sunlight falls on the Earth when the Sun is low in the sky. The right side of the picture shows a beam of rays from the Sun falling on the Earth when it is high in the sky. The same beam of rays in the first case (when the Sun is low) illuminates significantly large area on the earth's surface and a thicker layer of air passes through than in the second case. This makes it clear why winter sun It barely warms, but the summer one, on the contrary, warms very much.
Thus, we see that winter cold is explained by the fact that in winter the Sun is not above the horizon for long and its rays almost do not warm the surface of the Earth. In summer, on the contrary, the Sun stays above the horizon for a long time, and its rays greatly warm the Earth. That's why it gets warm in the summer.
The sun is main source heat and the only star our solar system, which, like a magnet, attracts all the planets, satellites, asteroids, comets and other “inhabitants” of space.
The distance from the Sun to the Earth is more than 149 million kilometers. It is this distance of our planet from the Sun that is usually called the astronomical unit.
Despite its significant distance, this star has a huge impact on our planet. Depending on the position of the Sun on Earth, day gives way to night, summer comes to replace winter, and magnetic storms and the most amazing things are formed auroras. And most importantly, without the participation of the Sun, the process of photosynthesis, the main source of oxygen, would not be possible on Earth.
Position of the Sun at different times of the year
Our planet moves around a celestial source of light and heat in a closed orbit. This path can be schematically represented as an elongated ellipse. The Sun itself is not located in the center of the ellipse, but somewhat to the side.
The Earth alternately approaches and moves away from the Sun, completing a full orbit in 365 days. Our planet is closest to the sun in January. At this time, the distance is reduced to 147 million km. The point in the Earth's orbit closest to the Sun is called "perihelion".
The closer the Earth is to the Sun, the more the South Pole is illuminated, and summer begins in the countries of the southern hemisphere.
Closer to July, our planet moves away as far as possible from main star Solar system. During this period, the distance is more than 152 million km. The point of the earth's orbit farthest from the Sun is called aphelion. The further the globe is from the Sun, the more light and heat is received by the countries of the northern hemisphere. Then summer comes here, and, for example, in Australia and Young America winter reigns.
How the Sun illuminates the Earth at different times of the year
Illumination of the Earth by the Sun in different times years directly depends on the distance of our planet at a given period of time and on which “side” the Earth is turned towards the Sun at that moment.
The most important factor influencing the change of seasons is the earth's axis. Our planet, revolving around the Sun, manages at the same time to rotate around its own imaginary axis. This axis is located at an angle of 23.5 degrees to heavenly body and always turns out to be directed towards the North Star. Full turn around earth's axis takes 24 hours. Axial rotation also ensures the change of day and night.
By the way, if this deviation did not exist, then the seasons would not replace each other, but would remain constant. That is, somewhere constant summer would reign, in other areas there would be constant spring, a third of the earth would be forever watered by autumn rains.
The earth's equator is always under the direct rays of the Sun. The sun's rays falling vertically bring more light and heat, they are not scattered in the atmosphere. Therefore, residents of countries located on the equator never know the cold.
Poles globe alternately find themselves in the rays of the Sun. Therefore, at the poles, day lasts half the year, and night lasts half the year. When the North Pole is illuminated, spring begins in the northern hemisphere, giving way to summer.
Over the next six months the picture changes. The South Pole turns out to be facing the Sun. Now summer begins in the southern hemisphere, and winter reigns in the countries of the northern hemisphere.
Twice a year our planet finds itself in a position where the sun's rays equally illuminate its surface from the Far North to South Pole. These days are called equinox days. Spring is celebrated on March 21, autumn on September 23.
Two more days of the year are called solstice. At this time, the Sun is either as high as possible above the horizon, or as low as possible.
In the northern hemisphere, December 21 or 22 marks the longest night of the year—the winter solstice. And on June 20 or 21, on the contrary, the day is the longest and the night is the shortest - this is the day of the summer solstice. In the southern hemisphere, the opposite happens. There are long days in December and long nights in June.
