Semidiameter

Ingeometry,thesemidiameterorsemi-diameterof asetofpointsmay be one half of itsdiameter;or, sometimes, one half of its extent along a particular direction.

Special cases

The semi-diameter of asphere,circle,orintervalis the same thing as itsradius— namely, anyline segmentfrom thecenterto itsboundary.

The semi-diameters of a non-circularellipseare the halves of its extents along the twoaxes of symmetry.They are the parametersa,bof theimplicit equation

$\left({\frac {x}{a}}\right)^{2}+\left({\frac {y}{b}}\right)^{2}=1.\,\!$

Likewise, the semi-diameters of anellipsoidare the parametersa,b,andcof its implicit equation

$\left({\frac {x}{a}}\right)^{2}+\left({\frac {y}{b}}\right)^{2}+\left({\frac {z}{c}}\right)^{2}=1.\,\!$

The semi-diameters of asuperellipse,superellipsoid,orsuperquadriccan be identified in the same way.

See also

• Flattening
• Semi-major and semi-minor axes
• Semiperimeter