At the beginning of July, the earth passed the perihelion, 5 million kilometers away from the sun. How did it greet the dog days?
We know that the earth goes round the sun all the time. It takes a year for the earth to go around the sun. So how long does the earth go around the sun in a year? If we regard the earth's orbit around the sun as a circle, then the sun is at the center of the circle, and the distance between the earth and the sun is the radius. The average distance between the sun and the earth is about 6.5438+49.6 million kilometers. From this, it can be concluded that the orbit circumference of the earth around the sun is about 940 million kilometers.
In a year, the earth walked 940 million kilometers around the sun, with an average speed of 29.78 kilometers per second, that is, 107200 kilometers per hour, that is, about 2.57 million kilometers per day. In one day, the earth took us across the solar system with a diameter of more than 200 times. Isn't this incredible?
Interestingly, the earth's orbit around the sun is not a perfect circle, but an elliptical orbit close to a circle. This means that the distance between the earth and the sun is sometimes farther and sometimes closer. The nearest place on earth to the sun is called perihelion. On the contrary, the farthest place on the earth from the sun is called apohelion. Every year at the beginning of 65438+ 10, the earth will reach perihelion and aphelion in early July.
It's early July, and the earth has reached the pole. This is the time when the earth is farthest from the sun in a year. How far is the earth from the sun at this time? The answer is 1.52 1 100 million kilometers. When we reached the perihelion at the beginning of 65438 10, the distance between the sun and the earth was 1.45438+0 billion kilometers. The difference between the two is 5 million kilometers.
Some friends will be curious: "When the earth is at perihelion and apohelion, is the sun in the sky different?" That's true. Our eyes always follow the principle of being near and far and small. The farther away the same object is, the smaller it becomes. So is the sun in the sky. According to the calculation, the apparent diameter of the sun at apohelion in early July is 3% smaller than that at perihelion in early 10.
Of course, this 3% gap is indistinguishable from our naked eyes.
Speaking of this, some friends will have such doubts: "No! At the beginning of July, the earth reached the perihelion farthest from the sun, and the heat received by the earth should be less, so it should be very cold! How can it be a hot summer? And it's going to be dog days soon. "
In fact, when the earth is at perihelion, it receives less solar radiation energy than when it is at perihelion. However, the difference between perihelion and apohelion is 5 million kilometers, which has little effect on the heat received by the earth. 5 million kilometers is a long distance for us. But it is insignificant for the earth orbit, and the eccentricity of the earth orbit is only 0.0 17.
If we draw an ellipse with the semi-major axis of 1 m, the difference between the semi-major axis and the semi-minor axis of the earth's orbit is only 0. 14 mm, so we can completely regard the earth's orbit as a circle.
In this way, when the earth is at perihelion, it has little to do with our hot summer. The real reason for the seasonal variation of alternating cold and summer on the earth is the existence of the ecliptic angle of the earth. The ecliptic angle is the angle between the orbital plane of the earth revolving around the sun and the equatorial plane of the earth. The declination angle is 23 26'.
It is precisely because of the intersection of yellow and red that when the earth revolves around the sun, the direct point of the sun will move back and forth between the Tropic of Cancer. Every summer solstice on June 22nd, the direct point of the sun moves to the Tropic of Cancer. At this time, the sun shines directly into the northern hemisphere, and the solar heat obtained by the northern hemisphere reaches the maximum. Therefore, the northern hemisphere ushered in a hot summer. On the contrary, at this time, the southern hemisphere is in a state of oblique sun, which is the coldest time of the year.
The influence of the direct point of the sun on the temperature in the northern and southern hemispheres is far greater than that of the apohelion and perihelion. Knowing this, it is not surprising that the dog days in the northern hemisphere are hot when the earth is at perihelion.