What Is a Circle?
A circle is one of the most fundamental shapes in mathematics — and you see them everywhere, from wheels and coins to clocks and pizzas!
A circle is a perfectly round shape where every point on the edge is exactly the same distance from the centre. This simple rule is what makes a circle so special and symmetrical.
A closed curve where every point on the curve is the same distance from a fixed point called the centre.
Unlike polygons (triangles, squares, etc.), a circle has no straight sides and no corners. But it has many important parts that we need to learn — and each one has a specific name.
Circles appear constantly in the world around us: the cross-section of a cylindrical cell under a microscope, the roughly circular orbit of planets, the shape of a lens in optics, and the rings formed when a droplet hits water. Understanding circle vocabulary is a key tool in both mathematics and science.
MYP Mathematics connection: This topic sits within the Geometry and Trigonometry strand of MYP Mathematics. The key concept at work here is Form — the idea that shapes and structures have properties that can be precisely described and communicated. The related concept is Space — understanding how figures occupy and define regions.
Centre and Radius
Let's start with the two most basic parts of any circle.
The fixed point in the exact middle of a circle. Every point on the circle is the same distance from the centre. It is often labelled O.
A straight line segment from the centre of the circle to any point on the circumference (edge). The plural of radius is radii.
Think of the radius as a spoke on a bicycle wheel — it connects the hub (centre) to the rim (circumference). You can draw a radius in any direction from the centre, and it will always be the same length.
Imagine you're standing in the middle of a round room holding a rope. If you stretch the rope out to touch the wall, that rope is the radius. No matter which direction you turn, the rope reaches the wall at the same length.
When using a compass to draw a circle, the distance between the compass point and the pencil is the radius. So if you want a circle with radius 5 cm, set your compass opening to 5 cm.
11 more sections in this topic
Pick this up in your Library: it holds the whole topic, notes, cheatsheet and questions.