*Kiddle Encyclopedia.*

# Oval facts for kids

An **oval** (from Latin *ovum*, "egg") is a closed curve in a plane which "loosely" resembles the outline of an egg. The term is not very specific, but in some areas (projective geometry, technical drawing, etc.) it is given a more precise definition, which may include either one or two axes of symmetry. In common English, the term is used in a broader sense: any shape which reminds one of an egg. The three-dimensional version of an oval is called an **ovoid**.

## Oval in geometry

The term **oval** when used to describe curves in geometry is not well-defined, except in the context of projective geometry. Many distinct curves are commonly called ovals or are said to have an "oval shape". Generally, to be called an oval, a plane curve should *resemble* the outline of an egg or an ellipse. In particular, these are common traits of ovals:

- they are differentiable (smooth-looking), simple (not self-intersecting), convex, closed, plane curves;
- their shape does not depart much from that of an ellipse, and
- an oval would generally have an axis of symmetry, but this is not required.

Here are examples of ovals described elsewhere:

- Cassini ovals
- portions of some elliptic curves
- Moss's Egg
- superellipse
- Cartesian oval
- stadium

An **ovoid** is the surface in 3-dimensional space generated by rotating an oval curve about one of its axes of symmetry. The adjectives **ovoidal** and **ovate** mean having the characteristic of being an ovoid, and are often used as synonyms for "egg-shaped".

## Egg shape

The shape of an egg is approximated by the "long" half of a prolate spheroid, joined to a "short" half of a roughly spherical ellipsoid, or even a slightly oblate spheroid. These are joined at the equator and share a principal axis of rotational symmetry, as illustrated above. Although the term *egg-shaped* usually implies a lack of reflection symmetry across the equatorial plane, it may also refer to true prolate ellipsoids. It can also be used to describe the 2-dimensional figure that, if revolved around its major axis, produces the 3-dimensional surface.

## Technical drawing

In technical drawing, an **oval** is a figure constructed from two pairs of arcs, with two different radii (see image on the right). The arcs are joined at a point in which lines tangential to both joining arcs lie on the same line, thus making the joint smooth. Any point of an oval belongs to an arc with a constant radius (shorter or longer), but in an ellipse, the radius is continuously changing.

## In common speech

In common speech, "oval" means a shape rather like an egg or an ellipse, which may be two-dimensional or three-dimensional. It also often refers to a figure that resembles two semicircles joined by a rectangle, like a cricket infield, speed skating rink or an athletics track. However, this is more correctly called a stadium. Sometimes, it can even refer to any rectangle with rounded corners.

In Australia an oval can be a ground for Australian Rules football such as Adelaide Oval.

The terms "ellipse" and "oblong" are often used interchangibly with oval, despite not being precise synonyms.