Sphere
A sphere is a locus of points in the space that are at distance
r from a fixed point S.
The point S is called a center of the sphere, the positive number
r is called a radius of the sphere.
A plane intersection of a sphere is a circle.
A center of a plane intersection of a sphere is an orthogonal projection of the
center of the sphere to the intersecting plane.
If a plane contains the center of a sphere, this plane intersection is a
circle called a great circle.
A radius of a great circle equals the radius of the sphere.
All points of the sphere are regular points.
A tangent plane of the sphere at the point T is a plane
through T perpendicular to the line ST,
i.e. the line ST is a normal line of the sphere at
T.
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Intersections of horizontal planes and the sphere are called the lines
of latitude, the longest among them is called an equator.
Great circles that contain the endpoints of the vertical diameter of
the sphere (poles) are called the lines of longitude or
meridians.


lines of latitude 
meridianslines of longitude 
Constructive assignments
An intersection of a sphere and a line.
A tangent plane of a sphere.
An intersection of a sphere with a projecting plane.
Created by Sonja Gorjanc, translated by Helena Halas and Iva Kodrnja  3DGeomTeh  Developing project of the University of Zagreb