Circle Parts Explorer

Geometry · Axiom Academy

Discover the vocabulary and parts of a circle — then put the names on the picture yourself. A circle is the set of all points in a plane that are an equal distance from one fixed point — the center. Almost every circle problem comes back to that one idea, so it pays to see exactly where the circle comes from before naming its parts. Watch one radius sweep a full turn around the center. Its tip stays the same distance out the whole way around, and the path that tip traces is the circle. The fixed length it keeps is the radius. Same distance, every direction — that constant reach from the center is what makes a circle a circle. Light up each part of the circle Tap a part below and watch it appear on the circle. Each name describes a specific piece — a point, a segment, a piece of the edge, or a region. Click through all eight to build the whole vocabulary. The fixed point that is an equal distance from every point on the circle. Every circle has exactly one center. Eight names, one picture — and the diameter, chord, and secant all turn out to be relatives of each other. One number runs the whole circle Drag the radius in and out. Everything else about the circle is locked to it: the diameter is always twice the radius, and the circumference and area follow from r through . Set r and the circle's whole size is decided. Double the radius and the diameter and circumference double too — but the area, with its r^2 , grows four-fold. The whole circle vocabulary, in one place

This is the written version of the interactive lesson above. See the full Geometry course.