Day and night are produced by the rotation of the earth, and seasons are produced by the angle of 23°27′ between the rotation axis of the earth and the rotation axis of the earth’s orbit around the sun. The earth rotates once a day from west to east around the “earth axis” that passes through its own south pole and north pole. Each revolution is a 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 in an elliptical orbit with a small eccentricity. The normal of the earth’s rotation axis and the orbital plane is always 23.5°. When the earth revolves, the direction of the rotation axis remains the same, always pointing to the north pole of the earth. Therefore, when the earth is in different positions of the orbit, the direction of sunlight projected on the earth is also different, thus forming the four seasons on the earth. At noon every day, the height of the sun is always the highest. In low-condensation areas in the tropics (that is, the area between 23°27′ north and south latitudes of the equator), the sun has two vertical incidences in a year. In higher latitudes, the sun is always close to the equator. In the Arctic and Antarctic regions, the sun stays below the horizon for a long time in winter, while it stays above the horizon for a long time in summer.

Since the earth revolves around the sun in an elliptical orbit, the distance between the sun and the earth is not constant, and the distance between the sun and the earth is not the same every day of the year. As we all know, the radiation intensity at a certain point is inversely proportional to the square of the distance from the radiation source, which means that the solar radiation intensity above the earth’s atmosphere will vary with the distance between the sun and the earth. However, due to the large distance between the sun and the earth (the average distance is 1.5×10⁸ km), the intensity of solar radiation outside the earth’s atmosphere is almost constant. Therefore, people use the solar constant to describe the intensity of solar radiation above the earth’s atmosphere. It refers to the solar radiation energy received on the unit surface area of the upper boundary of the earth’s atmosphere perpendicular to the solar radiation at the average distance between the sun and the earth. In recent years, the standard value of the solar constant measured by various advanced methods is 1353 W/m², and the change in solar radiation intensity caused by the change in the distance between the sun and the earth does not exceed 3.4% in a year.

The radiation that the sun hits on the ground plane, or insolation, is composed of two parts: direct insolation and diffuse insolation. When solar radiation passes through the atmosphere and reaches the ground, the absorption, reflection and scattering of solar radiation by air molecules, water vapor and dust in the atmosphere not only weaken the radiation intensity, but also change the direction of radiation and the spectral distribution of radiation. Therefore, the solar radiation that actually reaches the ground usually consists of two parts: direct and diffuse. Direct radiation refers to radiation that comes directly from the sun and its radiation direction does not change; diffuse is solar radiation whose direction has changed after being reflected and scattered by the atmosphere. It consists of three parts: scattering around the sun (bright light from the sky around the surface of the sun), scattering from the horizon (bright or dark light from the sky around the horizon) and other scattered radiation from the sky. In addition, non-horizontal surfaces also receive reflected radiation from the ground. The sum of direct insolation, diffuse insolation and reflected insolation is the total insolation or global insolation. Can rely on lenses or reflectors to focus directly to the sun. If the concentration rate is high, high energy density can be obtained, but diffuse solar radiation is lost. If the concentrating rate is low, it is also possible to condense part of the diffuse solar radiation around the sun. The range of diffuse insolation varies greatly. When the sky is clear and cloudless, the diffuse insolation is 10% of the total insolation. But when the sky is densely covered with clouds and the sun cannot be seen, the total insolation is equal to the diffuse insolation. Therefore, the energy collected by concentrating collectors is usually much less than the energy collected by non-concentrating collectors. The reflected insolation is generally very weak, but when the ground is covered with snow and ice, the reflected insolation on the vertical surface can reach 40% of the total insolation.

The solar radiation reaching the ground is mainly affected by the thickness of the atmosphere. The thicker the atmosphere, the more severe the absorption, reflection and scattering of solar radiation, and the less solar radiation reaching the ground. In addition, the condition of the atmosphere and the quality of the atmosphere also affect the solar radiation reaching the ground. Obviously, the path length of solar radiation through the atmosphere is related to the direction of solar radiation.

In recent years, due to the thinning of the ozone layer, especially in the Antarctic and Arctic regions, more and more ultraviolet radiation reaches the ground. Part of the infrared radiation emitted by humans is absorbed by carbon dioxide, water vapor and other gases, while most of the longer-wavelength infrared radiation from the surface of the earth at night is transmitted to outer space. The accumulation of these greenhouse gases in the upper atmosphere may increase the absorption capacity of the atmosphere, leading to global warming and cloudy weather. Although ozone reduction has little impact on solar collectors, the greenhouse effect may increase scattered radiation and may seriously affect the function of solar collectors.