# Theorem 19 - Angles in a circle, standing on an arc.

- Author:
- SeamusMcCabe

This demonstrates that the angle at the

**centre**of a circle, standing on an arc is twice any angle**at the circumference**, standing on the**same arc**.Drag the point E around the circle to see that the angle at E doesn't change.
Drag A or C to change the angle at O. Notice that the angle at E is always half the angle at O.
Drag A and C so that the blue arc is longer than the red arc. Notice that the angle at the centre is now

**reflex.**