Page:Spherical Trigonometry (1914).djvu/19

SPHERICAL TRIGONOMETRY.

CHAPTER I.

GREAT AND SMALL CIRCLES.

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