The sun is a sphere and the moon is a sphere. No one doubts, because everyone is really watching from a distance. However, people live on the earth and can't look at the earth like the sun and the moon before sailing in space. Compared with human vision, the earth is so vast. People standing on the ground can only see a small part of their vision, that is, the horizon-a flat land with a radius of 4.6 kilometers surrounded by the horizon, so they make various intuitive speculations about the shape of the earth. In ancient China, there was a saying that "the sky is round like a cover and the place is like chess", that is, the earth is regarded as flat and the sky is regarded as a round cover covered on the ground. Ancient Russians imagined that the earth was a plate on the backs of three whales, and they were floating on the ocean. For another example, the ancient Indians thought that the earth was a raised circular shield, which was carried by three elephants. These three elephants are standing on the back of a turtle, which is floating in the vast ocean. These are the most primitive human understanding of the earth. As early as more than 500 BC, Pythagoras believed that the sphere was the most perfect shape from the philosophical point of view, and thus put forward the conjecture that the earth was spherical. In 300 BC, Aristotle saw the phenomenon that the shadow cast by the earth on the moon was arc during the eclipse, and put forward scientific evidence that the earth was spherical. As early as the Warring States Period, Chinese philosopher Hui Shi put forward the view that the earth is spherical. But this view was rarely accepted at that time. It was not until A.D. 1522 that Magellan and his partners completed a circle around the earth that people established the concept that the earth was a sphere. /kloc-before the middle of the 0/7th century, people regarded the earth as a regular sphere, and this view was further revised and improved through scientific practice. 1672, astronomer Ricci brought a clock from Paris (49 N) to Guyana (5° N) in South America, and found that the clock was two minutes and 28 seconds slow every day, and returned to normal after being brought back to Paris. Similar observations in other places in the future will have similar results. This shows that the swinging speed of the pendulum slows down or the vibration period of the pendulum becomes longer when moving from the polar region to the equator. After physical speculation, the earth is not a right sphere, but a rotating sphere with slightly flat poles and protruding equator. The so-called rotating ellipsoid is formed by rotating a coil around the earth axis. All warp circles are equal ellipses, while the equator and all latitude circles are perfect circles. In order to process the results of geodesy, a rotating ellipsoid close to the size and shape of the earth is used and its relationship with the origin of the earth is determined, which is called reference ellipsoid. 19th century, after precise gravity survey and geodetic survey, it was further found that the equator is not a perfect circle, but an ellipse with different diameters. In this way, there are three unequal axes from the center of the earth to the surface of the earth, so the shape of the earth is represented by a triaxial ellipsoid in the measurement. Since then, people have found that the northern and southern hemispheres of the earth are asymmetrical. The South Pole is closer to the center of the earth than the North Pole, with the North Pole protruding 18.9 meters and the South Pole sunken 25.8 meters. 45 north latitude is also a low pressure, and 45 south latitude is also a high pressure. Compared with the reference ellipsoid, the earth looks a bit like a pear, so the name "pear-shaped earth" appeared in surveying. In short, the shape of the earth is very irregular and cannot be expressed by simple geometric shapes. More precisely, the earth has a unique earth shape. Seen from space, the earth looks like neither a pear nor an orange or an egg, but a ball. People photographed the real face of the earth at an altitude of 36,000 kilometers by using the actual observation of spacecraft and synchronous satellites. It can be seen that on this asteroid, the vast ocean is blue, the land is brown, the vegetation on the ground is green, and the white clouds are constantly changing. As can be seen from the above, people's understanding of the shape of the earth is gradually improved with the development of science and technology. Regular ellipsoid, rotating ellipsoid, triaxial ellipsoid and globe are all approximations of the real shape of the earth. On the other hand, people also need to simplify the shape of the earth to varying degrees in production struggle and scientific practice. For example, when making a globe or drawing a global map, we must regard the earth as a regular sphere; When surveying and mapping large-scale topographic maps, it is necessary to treat the earth as a regular reference ellipsoid; When launching an artificial celestial body and calculating its orbit, it is necessary to calculate the oblateness of the equator and the deviation from the reference ellipsoid more accurately. Therefore, the shape of the earth can not be expressed by a certain geometric shape, strictly speaking, it should be called a globe.