# Dictionary Definition

cardioid n : an epicycloid in which the rolling circle equals the fixed circle

# User Contributed Dictionary

## English

### Noun

1. An epicycloid with exactly one cusp; the plane curve with polar equation \rho = 1 + \cos\,\theta - having a shape supposedly heart-shaped

#### Translations

1. Having this characteristic shape

# Extensive Definition

In geometry, the cardioid is an epicycloid with one cusp. That is, a cardioid is a curve that can be produced as the path (or locus) of a point on the circumference of a circle as that circle rolls around another fixed circle with the same radius.
The cardioid is also a special type of limaçon: it is the limaçon with one cusp. The cusp is formed when the ratio of a to b in the equation is equal to one.
The name comes from the heart shape of the curve (Greek kardioeides = kardia:heart + eidos:shape). Compared to the heart symbol (♥), though, a cardioid only has one sharp point (or cusp). It is rather shaped more like the outline of the cross section of a plum.
The cardioid is an inverse transform of a parabola.
The large central figure in the Mandelbrot set is a cardioid.
Caustics can take the shape of cardioids. The caustic seen at the bottom of a coffee cup, for instance, may be a cardioid. The specific curve depends on the angle the light source makes relative to the bottom of the cup. The shape can be a nephroid, which looks quite similar.

## Equations

Since the cardioid is an epicycloid with one cusp, in cartesian coordinates it has parametric equations
x(t) = 2r \left( \cos t - \cos 2 t \right),
y(t) = 2r \left( \sin t - \sin 2 t \right)
where r is the radius of the circles which generate the curve, and the fixed circle is centered at the origin. The cusp is at (r,0).
\rho(\theta) = 2r(1 - \cos \theta). \
yields a cardioid with the same shape. It is the same curve as the cardioid given above, shifted to the left r units, so the cusp is at the origin.
For a proof, see cardioid proofs.

## Graphs

Four graphs of cardioids oriented in the four cardinal directions, with their respective polar equations.

## Area

The area of a cardioid with polar equation
\rho(\theta) = a(1 - \cos \theta)
is
A = \pi a^2 .
See proof.

## References

cardioid in Afrikaans: Kardioïed
cardioid in Bulgarian: Кардиоида
cardioid in Catalan: Cardioide
cardioid in German: Kardioide
cardioid in Spanish: Cardioide
cardioid in French: Cardioïde
cardioid in Korean: 하트방정식
cardioid in Italian: Cardioide
cardioid in Lombard: Cardiòit
cardioid in Japanese: カージオイド
cardioid in Polish: Kardioida
cardioid in Portuguese: Cardióide
cardioid in Romanian: Cardioidă
cardioid in Russian: Кардиоида
cardioid in Turkish: Kardiyoit