The centre of a circle is the point in the very middle.
The radius of a circle is a line from the centre of the circle to a point on the side. The radius is the same length all the way around the circle. Mathematicians use the letter r for the length of a circle’s radius.
The diameter of a circle is a straight line that goes from one side to the other, passing right through the centre of the circle. Diameter means ‘all the way across’. Mathematicians use the letter d for the length of this line.
The diameter of a circle is equal to twice its radius. (d = 2 x r or d = 2r)
The circumference of a circle is the distance all the way around it. Circumference means ‘all the way around’. Mathematicians use the letter c for the length of this line.
An arc is part of the circumference of a circle. The full arc of a circle measures 360 degrees.
A chord is a straight line joining two points on a circle. The diameter is an example of a chord.
A semicircle is a shape that forms half a circle.
Circles have a high level of symmetry.
Good to know!
The circle shape is a favourite of humans and can be seen in many designs.
The invention of the wheel (a circle shape) was one of the most important in human history.