Circle,
in geometry, plane curve such that each point on the curve is the same distance from a fixed point. This point is called the center of the circle. The circle belongs to the class of curves known as conic sections because a circle can be described as the intersection of a right circular cone with a plane that is perpendicular to the axis of the cone.

Any line segment that passes through the center and is terminated by the circle is called a diameter of the circle. A radius is a line segment from the center of the circle to the perimeter of the circle.

A chord is any straight-line segment that is intercepted by the circle. An arc of a circle is a portion lying between two points on the circle. A central angle is an angle with the vertex at the center of the circle and with sides forming radii of the circle. A central angle is subtended by the arc that lies between the points at which the central angle's sides intersect the circle.

The center of a circle is a point of symmetry, and any diameter of a circle is an axis of symmetry. Concentric circles-that is, circles having different perimeters but the same center-never intersect. The area of a circle is equal to pi multiplied by the square of the circle's radius.

An arc of a circle is proportional to the angle subtended at the center, and conversely; this property forms the basis of angular measure. There are 360° in a circle.

One cannot find a beginning nor end in a circle, direction nor orientation. Anatomy of a Circle Parts of a circle include its diameter, circumference, radius, chord, arc, and sector. To explore the properties of a circle, wrap a string around a soda can and measure the length of the string (the circumference of the circle formed by the string). Then lay the string directly across the top of the soda can so that it divides the top in half.

Probably the most important and most widely spread geometric symbol, partially due to the fact on which pattern the sun and moon follows. The circle, according to the expressive and neoexpressive philosophers the most perfect of all shapes; Apollo's legendary temple at the time of the Hyperboreans is also circle shaped. (a reference to the prehistoric Stonehenge in the south of England?) and the city of kings in Plato's "Atlantis Island" as a system of concentric land- and waterrings.

Measure the string's length (the diameter of the circle formed by the top of the can). Divide the circumference (C) by the diameter (D). Do this for several circles. You should find that the ratio of C/D is about 3:1 for all of your circles, both large and small. This ratio is often represented by the symbol p. Another experiment compares running around the circumference of a circular track to heading directly across along its diameter. For every 3 steps around the circular track, a runner only needs to take 1 step directly across.