Circular segment

In geometry, a circular segment (symbol: ), also known as a disk segment, is a region of a disk which is "cut off" from the rest of the disk by a secant or a chord. More formally, a circular segment is a region of two-dimensional space that is bounded by a circular arc (of less than π radians by convention) and by the circular chord connecting the endpoints of the arc.

A circular segment (in green) is enclosed between a secant/chord (the dashed line) and the arc whose endpoints equal the chord's (the arc shown above the green area).

Formulae

Let R be the radius of the arc which forms part of the perimeter of the segment, θ the central angle subtending the arc in radians, c the chord length, s the arc length, h the sagitta (height) of the segment, and a the area of the segment.

Usually, chord length and height are given or measured, and sometimes the arc length as part of the perimeter, and the unknowns are area and sometimes arc length. These can't be calculated simply from chord length and height, so two intermediate quantities, the radius and central angle are usually calculated first.

${\displaystyle R={\tfrac {h}{2}}+{\tfrac {c^{2}}{8h}}}$ [1]

The central angle is

${\displaystyle \theta =2\arcsin {\tfrac {c}{2R}}}$

Chord length and height

The chord length and height can be back-computed from radius and central angle by:

The chord length is

${\displaystyle c=2R\sin {\tfrac {\theta }{2}}=R{\sqrt {2(1-\cos \theta )}}}$
${\displaystyle c=2{\sqrt {R^{2}-(R-h)^{2}}}=2{\sqrt {2Rh-h^{2}}}}$

The sagitta is

${\displaystyle h=R-{\sqrt {R^{2}-{\frac {c^{2}}{4}}}}=R(1-\cos {\tfrac {\theta }{2}})=R\left(1-{\sqrt {\tfrac {1+\cos \theta }{2}}}\right)={\frac {c}{2}}\tan {\frac {\theta }{4}}}$

Arc length and area

The arc length, from the familiar geometry of a circle, is

${\displaystyle s={\theta }R}$

The area a of the circular segment is equal to the area of the circular sector minus the area of the triangular portion (using the double angle formula to get an equation in terms of ${\displaystyle \theta }$ ):

${\displaystyle a={\tfrac {R^{2}}{2}}\left(\theta -\sin \theta \right)}$

In terms of R and h,

${\displaystyle a=R^{2}\arccos \left(1-{\frac {h}{R}}\right)-\left(R-h\right){\sqrt {R^{2}-\left(R-h\right)^{2}}}}$

Unfortunately, ${\displaystyle a}$  is a transcendental function of ${\displaystyle c}$  and ${\displaystyle h}$  so no algebraic formula in terms of these can be stated. But what can be stated is that as the central angle gets smaller (or alternately the radius gets larger), the area a rapidly and asymptotically approaches ${\displaystyle {\tfrac {2}{3}}c\cdot h}$ . If ${\displaystyle \theta \ll 1}$ , ${\displaystyle a={\tfrac {2}{3}}c\cdot h}$  is a substantially good approximation.

As the central angle approaches π, the area of the segment is converging to the area of a semicircle, ${\displaystyle {\tfrac {\pi R^{2}}{2}}}$ , so a good approximation is a delta offset from the latter area:

${\displaystyle a\approx {\tfrac {\pi R^{2}}{2}}-(R+{\tfrac {c}{2}})(R-h)}$  for h>.75R

As an example, the area is one quarter the circle when θ ~ 2.31 radians (132.3°) corresponding to a height of ~59.6% and a chord length of ~183% of the radius.[clarification needed]

Etc.

The perimeter p is the arclength plus the chord length,

${\displaystyle p=c+s=c+\theta R}$

As a proportion of the whole area of the disc, ${\displaystyle A=\pi R^{2}}$ , you have

${\displaystyle {\frac {a}{A}}={\frac {\theta -\sin \theta }{2\pi }}}$

Applications

The area formula can be used in calculating the volume of a partially-filled cylindrical tank laying horizontally.

In the design of windows or doors with rounded tops, c and h may be the only known values and can be used to calculate R for the draftsman's compass setting.

One can reconstruct the full dimensions of a complete circular object from fragments by measuring the arc length and the chord length of the fragment.

To check hole positions on a circular pattern. Especially useful for quality checking on machined products.

For calculating the area or centroid of a planar shape that contains circular segments.

1. ^ The fundamental relationship between R, c, and h derivable directly from the Pythagorean theorem among R, C/2 and r-h components of a right-angled triangle is: ${\displaystyle R^{2}=({\tfrac {c}{2}})^{2}+(R-h)^{2}}$  which may be solved for R, c, or h as required.