The circumference of a circle = \(\pi d\) or \(2\pi r\). Look at the sector of the circle shown below. To calculate the length of the arc, we need to know what fraction of the circle is shown.

Circles are 2D shapes with one side and no corners. The circumference is always the same distance from the centre - the radius. Sectors, segments, arcs and chords are different parts of a circle.

It's been used to help calculate the orbit of planets in the solar system and examine how ripples in rivers carry energy. This year is the 37th annual Pi Day celebration at the Exploratorium .