The sun is the closest star to Earth, and it’s a fiery sphere of gas. Its diameter is about 1.4×106km, its mass is about 1.99×1030kg, and its average density is 1.4×103kg/m³. The main components are gaseous hydrogen and helium, of which hydrogen accounts for about 80%, helium accounts for about 19%, and other elements account for about 1%. According to the research and exploration of scientists, the sun can be divided into two parts: atmosphere and interior. The structure of the sun’s atmosphere has three layers, the innermost layer is the photosphere, the middle is the chromosphere, and the outermost is the corona. Figure 1 shows a schematic diagram of the structure of the sun.
The thickness of the photosphere is about 500km, accounting for only 7/10,000 of the radius of the sun, and the temperature is about 5700K. The brilliance of the sun is basically emitted from here. The chromosphere is located outside the photosphere and is a sparse and transparent layer of atmosphere. Its thickness varies from place to place, with an average of about 2000km. The corona is the outermost layer of the sun’s atmosphere, and beyond it lies the vast interstellar space. The corona is large and irregular in shape, and has no clear boundary with the chromosphere. It can extend 5 million to 6 million km.
Studies have shown that the nuclear fusion reaction in which hydrogen is fused into helium continues to be carried out inside the sun. The mainstream theory holds that there are two kinds of reactions inside the sun that can generate a lot of energy, one is the cycle of protons and protons, and the other is the cycle of carbon and nitrogen. The proton-proton cycle process is expressed as the nuclear reaction equation is
Among them, 21D is a deuterium atom; e+ is a positron; v– is a neutrino; hv is a photon. The carbon-nitrogen cycle process is expressed as a nuclear reaction equation, then
During the carbon-nitrogen cycle, the total amount of carbon and nitrogen involved in the reaction remains unchanged. In both thermonuclear reactions, 4 hydrogen nuclei are synthesized into 1 ammonia nucleus (a particle). In the process of synthesis, the mass loss is 0.7%. According to Einstein’s mass-energy equation E=mc², it can be calculated that for every 1kg of hydrogen consumed, the energy released is
E=1kg×0.7%×(3X108m/s)×(3×108m/s)=6.3×1014J.
Relevant studies estimate that the nuclear reaction inside the sun consumes 6x1011kg of hydrogen nuclear fuel every 1min, the actual mass loss is 4.2x 109kg, and the released energy is about 2.65x 1021J. This part of the energy accounts for 99% of the total energy produced by the sun and is exported outward by convection and radiation. According to estimates by relevant experts, the hydrogen contained in the sun is enough to sustain its nuclear fusion reaction for 60 billion years, so solar energy can be said to be inexhaustible.
As we all know, the earth rotates once a day from west to east on its own “axis” passing through its south and north poles. Each revolution is one day and night, so the earth rotates 15° per hour, in addition to its rotation, the earth also revolves around the sun once a year along an elliptical orbit with a small eccentricity, and the axis of rotation and the normal of the orbital plane are always 23.5°. When the earth revolves, the direction of its axis of rotation does not change and always points to the north pole of the earth. Therefore, when the earth is in different positions of its orbit, the direction of sunlight projected on the earth is also different, thus forming the seasonal changes on the earth. Since the earth orbits the sun in an elliptical orbit, the distance between the sun and the earth is not a constant, and the distance between the sun and the earth is different every day of the year. Physical theory states that the intensity of radiation at a point is inversely proportional to the square of the distance from the source, which means that the intensity of solar radiation above Earth’s atmosphere varies with the distance between the sun and the earth. However, due to the large distance between the sun and the earth, the intensity of solar radiation outside the earth’s atmosphere is almost constant. Therefore, people use the so-called “solar constant” to describe the intensity of solar radiation above the earth’s atmosphere. The so-called solar constant refers to the total solar radiation energy received per unit area and unit time on the surface outside the earth’s atmosphere that is perpendicular to the sun’s rays when the average distance between the sun and the earth is expressed by 1, and the unit is W/m². In general, the change in the solar constant 10 can be approximately expressed as
I0=1357×[1+0.034cos(2πn/365)],
In the formula: n is the serial number of the date in the year starting from New Year’s Day. In recent years, the standard value of the solar constant measured by various advanced means is 1367W/m². In a year, the variation of solar radiation intensity caused by the change of the distance between the sun and the earth does not exceed ±3.4%.
Solar light is a kind of electromagnetic wave, which is not fundamentally different from radio waves, only the wavelength and frequency are different. Studies have shown that nuclear fusion in the core of the sun releases gamma rays (wavelengths less than 10-3nm) rays. When gamma rays pass through the cooler regions of the sun, they lose energy and their wavelengths increase, turning into x-rays (wavelength λ is 10-3~10nm), ultraviolet rays (wavelength λ is 10~400nm) and visible light (wavelength λ is 400~700nm). When solar radiation passes through the atmosphere and reaches the ground, due to the absorption, reflection and scattering of solar radiation by air molecules, water vapor and dust in the atmosphere, not only the radiation intensity is weakened, but also the direction of radiation and the spectral distribution of radiation are changed. The actual solar radiation reaching the ground is usually composed of direct and diffuse components. Direct radiation refers to the radiation that comes directly from the sun and its radiation direction does not change; diffuse is the solar radiation whose direction changes after being reflected and scattered by the atmosphere. The data shows that most of the solar radiation energy on the earth’s surface is concentrated in the wavelength range of 0.3~3.0μm, accounting for 99% of the total energy, which belongs to a kind of short-wave radiation. Among them, the visible light band accounts for 50%, the infrared band accounts for 43%, and the ultraviolet band accounts for about 7%.