http://www.ck12.org Chapter 9. Circles
9.1 Parts of Circles
Here you’ll learn the vocabulary associated with the parts of circles.
What if you were asked to geometrically consider the ancient astronomical clock in Prague, pictured below? It has
a large background circle that tells the local time and the “ancient time” and then the smaller circle rotates around
on the orange line to show the current astrological sign. The yellow point is the center of the larger clock. How does
the orange line relate to the small and larger circle? How does the hand with the moon on it (black hand with the
circle) relate to both circles? Are the circles concentric or tangent? After completing this Concept, you’ll be able to
use your knowledge of parts of circles to answer questions like these.
For more information on this clock, see: http://en.wikipedia.org/wiki/Prague_Astronomical_Clock
Watch This
MEDIA
Click image to the left for more content.CK-12 Foundation: Chapter9PartsofCirclesA
Watch the first half of this video.
MEDIA
Click image to the left for more content.James Sousa:Introduction toCircles
Guidance
Acircleis the set of all points in the plane that are the same distance away from a specific point, called thecenter.
The center of the circle below is pointA. We call this circle “circleA,” and it is labeled
⊙
A.
Important Circle Parts
Radius:The distance from the center of the circle to its outer rim.
Chord:A line segment whose endpoints are on a circle.
Diameter:A chord that passes through the center of the circle. The length of a diameter is two times the length of
a radius.
Secant:A line that intersects a circle in two points.