Why is it so cold in winter! Aren’t the Earth and the Sun getting closer?

All celestial bodies in the universe follow some basic rules, including the earth we live on. Whether it is the rotation of the earth or its orbit around the sun, it has a fixed orbit. Moreover, there are two special points on the Earth's elliptical orbit, which are aphelion and perihelion. It is the cold winter at this time, so many people have doubts: Does perihelion have no effect? Otherwise, why is the distance between the sun and the earth getting closer and closer, but the winter is still so cold?

What is the positional relationship between the Earth and the Sun?

As we all know, the sun is the core object of the solar system, including celestial bodies such as planets, asteroids and comets, all orbiting the sun at different distances. Mercury's elliptical orbit is approximately 47 million kilometers from its closest location to the Sun to its farthest Oort cloud, while objects in the Oort cloud are located approximately 15 trillion kilometers away.

So, how far apart are the Earth and the Sun? We can compare the Oort Cloud with the Earth. The Earth's orbit around the sun is about 100,000 times closer than the Oort Cloud, with an average orbit of about 149,597,870 kilometers. The familiar unit of length, light year, is actually the average distance between the sun and the earth.

As the Earth orbits the Sun, it follows an invisible orbit and passes every point along its path. It is during this regular movement that the earth forms a closed curve on the same plane, which is an elliptical orbit of the earth close to a perfect circle, and the sun is at a focus of this ellipse.

I believe everyone knows the key elements of an ellipse. We can use a to represent the semi-major axis of the ellipse, b to represent the semi-minor axis, and c to represent the semi-focal length. Moreover, a is also the distance between the two ends of the short axis to the focus. Then, we can find that the relationship between the semi-focal length, the semi-major axis and the semi-minor axis will be: c2=a2-b2; and the eccentricity of the ellipse is the ratio between the semi-major axis and the semi-focal length, that is: e( Eccentricity)=c/a.

Based on this formula, we can calculate that the eccentricity of the Earth’s orbit is approximately 0.016722. This is why everyone says that the Earth’s orbit is approximately a perfect circle, but its elliptical orbit also has perihelion and aphelion. point reason. In fact, the position where the earth revolves around the sun is not the center of the sun, but the common mass center between the two. Perhaps many people don't understand that the sun and the earth actually revolve around this special point.

It’s just that the mass of the earth is only one 330,000th of the sun, and the distance between the center of mass of this planet and 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 150 million kilometers. Therefore, even if the earth’s revolution is regarded as the earth orbiting the center of the sun, it will not be consistent with the actual situation. There is too much error.

Why is the Earth still cold when it is at perihelion?

Since the earth's orbit around the sun is not a perfect circle, it will move to the closest or furthest point to the sun at a specific time, which is the so-called perihelion and aphelion. In early January, when the distance between the Earth and the Sun was approximately 146 million kilometers, the actual distance between the Earth and the Sun at perihelion would be slightly shorter than 1 AU; and when the time came to early July, the distance between the Sun and the Earth The distance between the Earth and the Sun is about 152 million kilometers. At this time, the distance between the Earth and the Sun at the aphelion will be a little longer than 1AU.

Like all planets in the solar system, the Earth also has its own elliptical orbit around the sun. First, we can simply and directly compare the distance values ​​of this particular point. That is, 146 million kilometers at perihelion and 152 million kilometers at aphelion. The difference in distance between the two is only 0.06 billion kilometers. This distance difference will not cause much change in the solar heat received by the earth.

Secondly, the Earth's orbit has a small eccentricity. Only those planets with higher eccentricities orbiting the sun may affect the temperature of the planet when the planet moves to perihelion. Why is it so hot in the Northern Hemisphere during the summer solstice? Not only is the 23.5-degree angle between the Earth's equatorial plane and its orbital plane causing the sun to hit the Tropic of Cancer directly, but the Earth's northern hemisphere is also tilting toward the sun. Moreover, the Earth is also moving faster at perihelion at this time.

The temperature on the Earth does not become warmer as the Earth passes through perihelion, nor does it become colder as the Earth passes through aphelion. The reason why the earth has brightly colored seasons is not caused by the distance between the sun and the earth, but because of the tilt of the earth's 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 aphelion, it is winter in the southern hemisphere and summer in the northern hemisphere.

Perihelion affects the length of winter in both hemispheres of the earth

As mentioned above, although our earth is actually getting closer and closer to the sun in winter, it does not change the weather What impact will it bring.

However, because the Earth moves to perihelion in January every year, winter in the Northern Hemisphere lasts five days longer than in the Southern Hemisphere.

The reason is actually very simple. Why does our earth change throughout the year? The reason is that the axis of the Earth's rotation around the sun is tilted. The tilted planetary axis causes the other hemisphere away from the star to receive less sunlight. The two hemispheres will swap positions in about 6 months.

There are obvious seasonal differences in different hemispheres. When the hemisphere pointing towards the sun is in summer, the other hemisphere slightly away from the sun is experiencing cold winter. When we look at it from this perspective, it actually shows that it’s not that perihelion is useless, it’s just that the hemisphere you’re in is in winter.

When the sun is at its lowest position in the sky, which is the shortest day of the year, it represents the beginning of winter (the winter solstice in the Northern Hemisphere). If the Earth's speed in its orbit is constant, then the Earth should spend the same amount of time between the equinox and the solstice. However, this is not the case.

According to Kepler’s law of motion, orbiting objects will sweep out areas of the same size in the same time. That is to say, when the distance between the planet and the star becomes closer, It moves faster, and adjusts to move slower as it moves further away.

In short, the Earth is moving faster in January than it is in July. It is this movement pattern that causes the time between December (Winter Solstice) and March (Vernal Equinox) in the Northern Hemisphere to actually be shorter than the time between June (Winter Solstice) and September (Vernal Equinox) in the Southern Hemisphere. The difference in time between the two hemispheres is about 5 days.

A brief summary:

1. Since the Earth’s orbit is elliptical and has a small eccentricity, there are two perihelion and aphelion points on the orbit. Special location;

2. When the distance between the earth and the sun gets closer and closer, even reaching perihelion, people in winter will not feel an obvious increase in temperature. Because the sun is directly shining on the Tropic of Cancer at this time, the earth is moving faster than usual, and the difference between perihelion and aphelion is already very small.

3. Although perihelion does not affect the temperature of the earth, because the earth moves faster in January than in July, the northern hemisphere will have more winter than the southern hemisphere. Out about 5 days.