### The relationship between the sun and the earth

- Solar energy

The sun is a hot gas planet composed mainly of hydrogen and helium, with a diameter of about 1.392×10^{6}km and a distance of about 1.496×10^{8}km from the earth. The sun is the most energetic, natural and stable natural radiation source. Its center temperature is 1.5×10^{7}k, and the effective surface temperature is 5762K. The polynuclear reaction that converts hydrogen into helium occurs inside, and 6.57×10^{11}kg of hydrogen is polymerized per second to produce 6.53 ×10^{11}kg of helium, the total radiation power can reach 3.8×10^{26}w. This energy is emitted in the form of electromagnetic waves at a speed of 3×10^{8}m/s, although only about one part of 2.2 billion can reach the earth’s atmosphere (about 1.7×10^{17}w ), but there is also an energy equivalent to 5.9×10^{6}t coal. When this part of solar radiation energy travels through the atmosphere, about 19% is absorbed by the atmosphere, and about 30% is reflected back to space by atmospheric dust particles and the ground, and finally about 8.5×1016w reaches the surface of the earth. - Solar spectrum

The sun is a yellow dwarf, and the solar spectrum belongs to the G2V spectral type. The wavelength range of solar radiation covers the entire electromagnetic spectrum from X-rays to radio waves. Outside the atmosphere, the sun’s spectral distribution curve is similar to that of a 5900K blackbody. The wavelength range of solar radiation observed on the ground is approximately 0.295 ~ 2.5um. Solar radiation with wavelengths less than 0.295 um and greater than 2.5um cannot reach the ground because it is strongly absorbed by ozone, water vapor and other atmospheric molecules in the earth’s atmosphere.

Solar radiation is mainly concentrated in the visible light part (0.4~0.76um), and the infrared light (>0.76um) with wavelength greater than visible light and the ultraviolet light (<0.4um) less than visible light are less. Among the total radiant energy, more than 99% have wavelengths between 0.15 and 4um, and they are mainly distributed in the visible and infrared regions. The former accounts for about 50% of the total solar radiation energy, and the latter accounts for about 43%. Solar radiation energy is very small, accounting for only about 7% of the total.

- Geographical coordinate system

The geographic coordinate system is a coordinate system used to determine the position of an object on the earth. In addition to revolving around the sun, the earth also rotates on its own axis. The axis of rotation of the earth (earth axis) coincides with the short axis of the earth ellipsoid and intersects the ground at two points, namely the two poles of the earth: the North Pole and the South Pole. The plane perpendicular to the earth’s axis and passing through the center of the earth is called the equatorial plane. The large circle (line of intersection) between the equatorial plane and the earth’s surface is called the equator. The equatorial plane divides the earth into the southern hemisphere and the northern hemisphere.

The position of any point on the surface of the earth can be determined by latitude and longitude. The lines of latitude and longitude are two sets of orthogonal (intersecting at 90°) curves on the surface of the earth. The coordinates formed by these two sets of orthogonal curves are called the geographic coordinate system.

1) Latitude (Latitude) The circumferential line parallel to the equator passing through any point M on the earth’s surface is called the line of latitude; the angle of intersection between a plumb line pointing to the center of the earth and the equatorial plane at any point on the latitude line is called the geographic latitude of the point ( Abbreviated as latitude), represented by the letter φ. The latitude is calculated from the equator. The latitude on the equator is 0°. The farther the knot is from the equator, the greater the latitude. The latitude of the pole is 90°. The north of the equator is called the north latitude, and the south is called the south latitude.

2) Longitude (Longitude) The circumferential line perpendicular to the equatorial plane that passes through any point M on the surface of the earth and passes the north and south poles, called the line of longitude (also called the meridian); the meridian passing through a certain point is sandwiched by the meridian passing through the Greenwich Observatory The dihedral angle is called the geographic longitude of the point (longitude for short), which is represented by the letter L. According to international regulations, the meridian passing through the Greenwich Observatory in the United Kingdom is the prime meridian (or called the first meridian) as the starting point for calculating the longitude. The longitude of this line is 0°, and 0~180° to the east is called the east longitude, and 0 to 180 to the west. ° is called the west longitude.

- Celestial coordinate system

Due to the study of the relative positional relationship between the sun and the earth, a celestial coordinate system that indicates the position of the sun can be used. The celestial sphere is an imaginary sphere with the center of the earth as the center and the average distance between the sun and the ground as the radius. The sun is moving relative to the earth from east to west on this sphere. Some physical quantities in the celestial coordinate system can correspond to the physical quantities in the geographic coordinate system, but the spheres are different, which can be obtained by extending or zooming in parallel. The commonly used celestial coordinate system is divided into the equatorial coordinate system and the horizon coordinate system.

1) Equator coordinate system The equatorial coordinate system is a celestial coordinate system with the celestial equator QQ’ as the basic circle and the origin of the intersection Q of the celestial equator and the celestial meridian circle. The position S of the sun is determined by the time angle ∂ and the declination angle δ, two coordinates that have a mutually perpendicular relationship.

2) The horizon coordinate system The basic circle in the horizon coordinate system is the horizon circle, and the basic points are the zenith and the bottom. The position S of the sun is determined by the zenith angle ∂, (or the sun’s altitude angle α) and the sun’s azimuth angle γ, which are mutually perpendicular coordinates.