A figure composed of all points, whose distance to a fixed point in space is equal to a fixed length, is called a sphere. Sphere is a three-dimensional figure with a continuous surface, and the geometry surrounded by sphere is called sphere. The center of the sphere is equal to the sphere everywhere.
Composition of a sphere: The surface of a sphere is a curved surface, which is called a sphere.
Like a circle, a ball has a center, which is called the center of the ball.
Stars, especially "Earth".
Cut a ball with a plane, the cross section is round. The cross section of the ball has the following characteristics:
The straight line connecting the center of the sphere and the center of the section is perpendicular to the section.
The distance d from the center of the sphere to the cross section has the following relations with the radius r of the center of the sphere and the radius r of the cross section: r 2 = r 2-d 2.
A circle whose sphere is cut by a plane passing through the center of the sphere is called a big circle, and a circle cut by a section not passing through the center of the sphere is called a small circle.
On the sphere, the length of the shortest connecting line between two points is the length of the bad arc between these two points through the great circle of these two points. We call this arc length the spherical distance between two points.