Page:Spherical Trigonometry (1914).djvu/19

1. Definition. A sphere is a solid bounded by a surface every point of which is equally distant from a fixed point which is called the centre of the sphere. The straight line which joins any point of the surface with the centre is called a radius. A straight line drawn through the centre and terminated both ways by the surface is called a diameter.

2. The section of the surface of a sphere made by any plane is a circle.

Let ${\displaystyle AB}$ be the section of the surface of a sphere made by any plane, ${\displaystyle O}$ the centre of the sphere. Draw ${\displaystyle OC}$ perpendicular