Why is it so cold in winter? Isn't the earth and the sun getting closer and closer?

All celestial bodies in the universe will follow some basic rules, including the earth where we live. Whether the earth rotates or revolves around the sun, there is a fixed directional orbit. Moreover, there are two special points in the orbit of the earth ellipse, namely apohelion and perihelion. It's cold winter at this time, so many people have such doubts: Is perihelion useless? Otherwise, why is the distance between the sun and the earth getting closer and closer, and it is still so cold in winter?

What is the positional relationship between the earth and the sun?

As we all know, the sun is the core celestial body in the solar system, including planets, asteroids and comets, all of which revolve around the sun at different distances. From Mercury, the closest to the sun, to Oort Cloud, the elliptical orbit of Mercury is about 47 million kilometers, and the celestial bodies in Oort Cloud are located at about 15 trillion kilometers.

So, how far is the earth from the sun? We can compare the Oort Cloud with the Earth. The earth's orbit around the sun is about 654.38+ million times closer than that of Oort Cloud, with an average orbit of about 654.38+049597870 km. The familiar unit of length, light years, is actually the average distance between the sun and the earth.

When the earth revolves around the sun, it will follow an invisible track and pass through every point on the route. It is in this regular movement that the earth forms a closed curve on the same plane, that is, the earth approaches the elliptical orbit of a perfect circle, and the sun is at a focus of this ellipse.

I believe everyone knows what the key elements of an ellipse are. We can use a to represent the semi-major axis of an ellipse, b to represent the semi-minor axis, and c to represent the semi-focal length. Moreover, a is also the distance from both ends of the short axis to the focus. Then, we can find that the relationship between semi-focal length and semi-major axis and semi-minor axis will be: C2 = A2-B2; Eccentricity of an ellipse is the ratio of semi-major axis to semi-focal length, that is, e (eccentricity) = c/a.

According to this formula, we can calculate that the eccentricity of the earth's orbit is about 0.0 16722, which is why everyone says that the earth's orbit is approximately a perfect circle, but its elliptical orbit has perihelion and apohelion. In fact, the position of the earth's revolution around the sun is not the center of the sun, but the common center of mass of the two. Perhaps many people don't understand that the sun and the earth are actually moving around this special point.

Just because the mass of the earth is only one-third of that of the sun, and the distance from this center of mass to the center of the sun is only 450 kilometers. The radius of the sun is as high as 700,000 kilometers, and the distance between the earth and the sun is 65.438+0.5 billion kilometers. Therefore, even if the revolution of the earth is regarded as the earth running around the center of the sun, there will be no big error with the actual situation.

Why is the earth still cold at perihelion?

Because the earth's orbit around the sun is not a perfect circle, it will move to the nearest or farthest point from the sun at a specific time, which is called perihelion and apohelion. At the beginning of 1 month, when the distance between the earth and the sun is about 65.438+0.46 billion kilometers, the actual distance between the earth and the sun at perihelion will be slightly shorter than 654.38+0 AU. By then, in early July, the distance between the sun and the earth will be about 65.438+0.52 billion kilometers. At this time, the distance between the earth and the sun at apohelion will be slightly longer than 654.38+0 AU.

Like all planets in the solar system, the earth has its own elliptical orbit around the sun. First of all, we can simply and directly compare the distance value of this special point. That is, the perihelion is 65.438+0.46 billion kilometers and the apohelion is 65.438+0.52 billion kilometers, and the difference between them is only 0.06 billion kilometers. Such a distance difference will not greatly change the solar heat received by the earth.

Secondly, the eccentricity of the earth's orbit is small, and only those planets with high eccentricity around the sun may affect the temperature of the planets when they run to the perihelion. Why is it very hot in the northern hemisphere during the summer solstice? Not only is there an angle of 23.5 degrees between the equatorial plane and the orbital plane of the earth, which leads to the sun's direct exposure to the Tropic of Cancer, but the northern hemisphere of the earth is also leaning towards the sun. Moreover, at this time, the earth moves faster at perihelion.

The temperature on the earth has not warmed up because the earth passed the perihelion, nor has it cooled down because the earth passed the perihelion. The four seasons on the earth are distinct, not because of the distance between the sun and the earth, but because of the inclination of the earth axis. When the earth is at perihelion, it is summer in the southern hemisphere and winter in the northern hemisphere. When the earth is at apohelion, it is winter in the southern hemisphere and summer in the northern hemisphere.

Perihelion affects the winter length of the two hemispheres of the earth.

As mentioned above, although our earth is actually getting closer and closer to the sun in winter, it has no effect on the weather. However, because the earth will move to perihelion every year 1 month, the winter in the northern hemisphere will be 5 days longer than that in the southern hemisphere.

The reason is actually very simple. Why does our earth change all year round? The reason is that the axis of the earth's rotation around the sun is tilted, and the tilted planetary axis makes the other hemisphere that deviates from the star receive less sunlight. The two hemispheres switch places about once every six months.

So there are obvious seasonal differences in different hemispheres. When the hemisphere pointing to the sun is in summer, the other hemisphere slightly deviating from the sun is experiencing cold winter. From this point of view, it actually shows that perihelion is not useless, just because your hemisphere is in winter.

When the sun is at its lowest position in the sky, that is, the shortest day in a year, it represents the beginning of winter (winter solstice in the northern hemisphere). If the speed of the earth in its orbit is constant, then the time between the vernal equinox and the vernal equinox should be the same. However, this is not the case.

According to Kepler's law of motion, orbiting objects will sweep the same size area at the same time, that is, when the distance between the planet and the star becomes closer, its moving speed will become faster, and when the distance is far, its moving speed will be slower.

In short, the earth will move faster in 1 month than in July. It is this movement law that causes the time from June (winter solstice) to March (vernal equinox) in the northern hemisphere to be actually shorter than that from June (winter solstice) to September (vernal equinox) in the southern hemisphere. During this period, the difference between the two hemispheres is about 5 days.

To sum up briefly:

1. Because the earth's orbit is elliptical and its eccentricity is very small, there are two special positions in the orbit: perihelion and apohelion.

When the distance between the earth and the sun is getting closer and closer, even reaching perihelion, people in winter will not obviously feel the temperature rise. Because the sun shines directly on the tropic of cancer, the earth moves faster than usual, and there is little difference between perihelion and apohelion.

3. Although the perihelion has no influence on the earth's temperature, the winter in the northern hemisphere will be about 5 days longer than that in the southern hemisphere because the earth will move faster in May 438+10 than in July.