The sun is like a big stove, and the closer we get to it, the warmer it gets. Mercury, for example, is the closest of the eight planets to the Sun, and the side illuminated by the sun's rays has a temperature of 427 degrees Celsius.
Earth is on average 150 million kilometers from the Sun, and in terms of distance from the Sun, it ranks third among the eight planets, and the second closest to the Sun is Venus. Although Venus is not as close to the Sun as Mercury, due to the high content of greenhouse gases in the internal atmosphere, the average temperature on the surface of Venus is as high as 450 ° C, which is higher than the temperature on the surface of Mercury.
At present, the coldest place on the Earth's surface is at the South Pole, where the temperature can reach minus 90 ° C; the temperature of the hottest place on the Earth's surface can reach more than 50 ° C. However, on average, the average temperature on the Earth's surface is now around 15 ° C.
Bounded by the equator, the Earth can be divided into the northern and southern hemispheres. Our country is in the northern hemisphere, and more than half of the world's population lives in the northern hemisphere. When the Northern Hemisphere is in winter, the southern hemisphere is the opposite, and it is a hot summer. So for people in the southern hemisphere, the title of this article is problematic, but almost all the people who see this article are people from the northern hemisphere.
In general, the temperature is generally cold in winter and hot in summer. But you know what? When the Northern Hemisphere is in winter, the Earth is just close to the Sun.
The Earth's orbit around the Sun is an ellipse. December 21 or 22 or 23 in the Gregorian calendar is the winter solstice of the twenty-four solar terms, while the summer solstice is june 20 or 21 or 22 in the Gregorian calendar. Currently, about 14 days after the winter or summer solstice, the Earth passes through perihelion or aphelion. That is to say, at the beginning of January of the Gregorian calendar, the earth is closest to the sun; at the beginning of July, the earth is farthest from the sun. The data shows that the Earth's distance from the Sun at perihelion and aphelion is about 5 million kilometers.
Supposedly, the closer you get to the sun, the hotter it gets, so why is the northern hemisphere so cold when it's close to the sun?
The closer you get to the sun, the higher the temperature, and there's nothing wrong with that. However, the eccentricity of the Earth's orbit is only 0.0167, and the 5 million kilometers is only one-thirtieth of 150 million kilometers, and the impact of this distance change on the Earth's temperature is not large. At perihelion, the Earth receives less than 6 percent more solar radiation than at the aphelion.
The greater impact on the Earth's temperature is the direct and oblique sunlight, as well as the length of time the sun's rays are illuminated. Both are related to the tilt of the Earth's axis.
The plane of the Earth's orbit is also called the ecliptic plane; the Earth's axis is a hypothetical axis of the Earth's rotation, connecting the north and south poles through the center of the earth. The angle between the Earth's axis and the ecliptic plane is 66 degrees and 34 angle minutes, and the angle of the yellow axis of the Earth axis is 23 degrees and 26 angle minutes (this angle is also called the yellow-red angle). Geographically, the division of the Tropic of Cancer and the Arctic circle is related to these two angles.
It is precisely because of the tilt of the Earth's axis that sunlight produces direct or oblique sunlight during the Earth's rotation.
During the Earth's rotation, the North Pole of the Earth's axis always points in the same direction in the universe - the direction of the North Star. When the Earth is relatively close to the Sun, at that time, due to the tilt of the Earth's axis, most of the sunlight is obliquely emitted in the northern hemisphere. Due to oblique radiation, sunlight has to pass through a thicker atmosphere to reach the ground, and the solar radiation received per unit area becomes weaker, and the temperature in the northern hemisphere naturally becomes lower.
At this time, the southern hemisphere is the opposite, most of the region can be directly sunburned, in the summer. However, this does not mean that the summer in the southern hemisphere is hotter than the summer in the northern hemisphere, because the southern hemisphere is mostly oceanic, while the land is mostly distributed in the northern hemisphere, and the summer in the southern hemisphere is not very hot due to the influence of the ocean.
Affected by the tilt of the Earth's axis, the days are long and the nights are short in summer and short and long in winter; and the higher the latitude of the northern and southern hemispheres, the greater the change in the length of day and night in a year. The north and south poles occur half the time of each year, polar day and polar night. The daytime becomes shorter, that is, the length of the sunshine becomes shorter.
Whether it is the oblique sunlight or the length of the sun, this can be vividly felt on the ground.
Looking at the sun in the northern hemisphere, far from the equator, the noonday sun always hangs in the southern sky, and the north-facing south is derived from this. When the sun is oblique in winter, the height of the sun in the southern sky also decreases, and the more north the sun shines, the more severe the oblique radiation of the sun, and the lower the height of the sun in the sky at noon. The sun rises in the east and sets in the west is related to the rotation of the earth, and the rotation speed of the earth is constant. When the sun is low, the path (a large arc) traveled in the sky each day becomes shorter, and the length of the sunshine naturally becomes shorter.
It should be noted that the earth's axis is not static, the earth's axis is actually moving, can be divided into precession, chapter movement and polar shift, but this change is very slow and weak, and cannot be felt in a short time.
The impact of the precession is the greatest. Precession, also known as precession, refers to the conical motion of the Earth's axis around the yellow axis, with a period of about 25,800 years. This change accumulates over time and will have an impact on the calendar and solar terms.
Influenced by the gravitational pull of jupiter and other celestial bodies, the Earth's orbit is not constant, and drift occurs, with a period of about 21,000 years. 800 years ago, the northern hemisphere's winter solstice and perihelion coincided, and about 12,000 years later, the northern hemisphere's winter solstice will coincide with the aphelion point, when the northern hemisphere winter will be colder than it is now.
For local surface temperatures, there are more influencing factors, including sea and land distribution, topography, ocean currents, atmospheric circulation and other factors.
For example, Northern Europe, which is also in a high latitude region, is not as cold as Siberia and Alaska, mainly because the Nordic sea is surrounded by the sea and has the influence of warm currents from the North Atlantic, so that the local temperature is not as cold as other regions at the same latitude. In those coastal areas, the daily temperature difference and the annual temperature difference are smaller than inland areas, and the phenomenon of "wearing leather jackets and wearing yarn at noon and eating watermelon around the stove" will not occur like some inland areas.
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