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{{For|the archaic unit of mass|:de:Versine{{!}}Versine}}
{{Redir|versin|the Polish village|Versin (village)}}
{{Trigonometry}}
The '''versine''' or '''versed sine''' is a [[trigonometric function]] found in some of the earliest [[trigonometric table]]s. The versine of an angle is 1 minus its [[cosine]].

There are several related functions, most notably the '''coversine''' and '''haversine'''. The latter, half a versine, is of particular importance in the [[haversine formula]] of navigation.
[[Image:Unit-circle sin cos tan cot exsec excsc versin cvs.svg|thumb|190px|A [[unit circle]] with [[trigonometric function]]s.<ref name="Haslett_1855"/>]]

=={{anchor|ver|vcs|cvs|cvc|hav|hvc|hcv|hcc|lhav}}Overview==
The '''versine'''<ref name="Inman_1835"/><ref name="Abramowitz_1972"/><ref name="Weisstein_vers"/><ref name="Tapson_2004"/><ref name="Atlas_2009"/><ref name="Beebe_2017"/> or '''versed sine'''<ref name="Hall_1909"/><ref name="Haslett_1855"/><ref name="Clagett_1969"/><ref name="Precalc_1999"/><ref name="Korn_2000"/><ref name="Weisstein_vers"/><ref name="Calvert_2004"/> is a [[trigonometric function]] already appearing in some of the earliest trigonometric tables. It is written as '''versin(''&theta;'')''',<ref name="Weisstein_vers"/><ref name="Clagett_1969"/><ref name="Precalc_1999"/> '''sinver(''&theta;'')''',<ref name="Braunmühl_1903"/><ref name="Cajori_1929"/> '''vers(''&theta;'')''',<ref name="Inman_1835"/><ref name="Hall_1909"/><ref name="Abramowitz_1972"/><ref name="Weisstein_vers"/><ref name="Korn_2000"/><ref name="Tapson_2004"/><ref name="Atlas_2009"/> '''ver(''&theta;'')'''<ref name="Shaneyfelt"/> or '''siv(''&theta;'')'''.<ref name="Cauchy_1821"/><ref name="Bradley_2009"/> In [[Latin]], it is known as the ''sinus versus''<ref name="Cauchy_1821"/><ref name="Bradley_2009"/> (flipped sine), ''versinus'', ''versus'' or the ''sagitta'' (arrow).<ref name="Brummelen_2013"/>

Expressed in terms of the meanwhile more commonly used "vertical" [[sine]]s (''sinus rectus'') and [[cosine]]s (''cosinus rectus'') functions, the versine is equal to {{nowrap|1 − cos(''θ'')}}, or 2&nbsp;sin<sup>2</sup>{{big|(}}{{sfrac|''θ''|2}}{{big|)}}.

There are several related functions corresponding to the versine:

* The '''versed cosine''',<ref name="Weisstein_vercos"/><ref group="nb" name="vercosine vs. coversine"/> or '''vercosine''',<ref name="Weisstein_vercos"/><ref group="nb" name="vercosine vs. coversine"/> written '''vercosin(''&theta;'')''', '''vercos(''&theta;'')'''<ref name="Weisstein_vercos"/> or '''vcs(''&theta;'')'''<ref name="Shaneyfelt"/>
* The '''coversed sine''',<ref group="nb" name="vercosine vs. coversine"/><ref name="Hall_1909"/> '''coversine''',<ref name="Abramowitz_1972"/><ref name="Tapson_2004"/><ref name="Atlas_2009"/><ref name="Clagett_1969"/><ref name="Precalc_1999"/><ref name="Weisstein_covers"/><ref name="Beebe_2017"/> '''cosinus versus'''<ref name="Cauchy_1821"/><ref name="Bradley_2009"/><ref group="nb" name="vercosine vs. coversine"/> or '''coversinus''', written '''coversin(''&theta;'')''',<ref name="Weisstein_hacoversin"/><!-- in contrast to the dedicated reference, this one mentions coversin() as well. --> '''covers(''&theta;'')''',<ref name="Hall_1909"/><ref name="Abramowitz_1972"/><ref name="Tapson_2004"/><ref name="Atlas_2009"/><ref name="Korn_2000"/><ref name="Cajori_1929"/><ref name="Weisstein_covers"/><ref name="Ludlow_1891"/><ref name="Wentworth_1903"/><ref name="Kenyon_1913"/> '''cosiv(''&theta;'')'''<ref name="Cauchy_1821"/><ref name="Bradley_2009"/><ref group="nb" name="vercosine vs. coversine"/> or '''cvs(''&theta;'')'''<ref name="Precalc_1999"/><ref name="Cajori_1929"/><ref name="Shaneyfelt"/><ref name="Anderegg_1896"/>
* The '''coversed cosine'''<ref name="Weisstein_covercos"/> or '''covercosine''',<ref name="Weisstein_covercos"/> written '''covercosin(''&theta;'')''' or '''covercos(''&theta;'')'''<ref name="Weisstein_covercos"/> or '''cvc(''&theta;'')'''<ref name="Shaneyfelt"/>

In full analogy to the above-mentioned four functions another set of four "half-value" functions exists as well:

* The '''haversed sine''',<ref name="Weisstein_hav"/> '''haversine'''<ref name="Inman_1835"/><ref name="Abramowitz_1972"/><ref name="Tapson_2004"/><ref name="Atlas_2009"/><ref name="Korn_2000"/><ref name="Clagett_1969"/><ref name="Weisstein_hav"/><ref name="Beebe_2017"/> or '''semiversus''',<ref name="Fulst_1972"/><ref name="Sauer_2015"/> written '''haversin(''&theta;'')''', '''semiversin(''&theta;'')''', '''semiversinus(''&theta;'')''', '''havers(''&theta;'')''',<ref name="Inman_1835"/> '''hav(''&theta;'')''',<ref name="Inman_1835"/><ref name="Abramowitz_1972"/><ref name="Tapson_2004"/><ref name="Atlas_2009"/><ref name="Korn_2000"/><ref name="Cajori_1929"/><ref name="Shaneyfelt"/><ref name="Weisstein_hav"/><ref name="Rider_1923"/><ref name="Wolfram_hav"/> '''hvs(''&theta;'')''',<ref group="nb" name="hvs"/> '''sem(''&theta;'')'''<ref name="Sauer_2015"/> or '''hv(''&theta;'')''',<ref name="Rudzinski_2015"/> most famous from the [[haversine formula]] used historically in [[navigation]]
* The '''haversed cosine'''<ref name="Weisstein_havercos"/> or '''havercosine''',<ref name="Weisstein_havercos"/> written '''havercosin(''&theta;'')''', '''havercos(''&theta;'')''',<ref name="Weisstein_havercos"/> '''hac(''&theta;'')'''<!-- found in Russian WP --> or '''hvc(''&theta;'')'''<ref name="Shaneyfelt"/>
* The '''hacoversed sine''',<ref name="Weisstein_hacoversin"/> also called '''hacoversine'''<ref name="Weisstein_hacoversin"/> or '''cohaversine'''<ref name="Weisstein_hacoversin"/><ref name="Beebe_2017"/> and written '''hacoversin(''&theta;'')''',<ref name="Weisstein_hacoversin"/> '''semicoversin(''&theta;'')''', '''hacovers(''&theta;'')''', '''hacov(''&theta;'')'''<ref name="Vlijmen_2005"/> or '''hcv(''&theta;'')'''<ref name="Shaneyfelt"/>
* The '''hacoversed cosine''',<ref name="Weisstein_hacovercos"/> also called '''hacovercosine'''<ref name="Weisstein_hacovercos"/> or '''cohavercosine'''<ref name="Weisstein_hacovercos"/> and written '''hacovercosin(''&theta;'')''', '''hacovercos(''&theta;'')'''<ref name="Weisstein_hacovercos"/> or '''hcc(''&theta;'')'''<ref name="Shaneyfelt"/>

==History and applications==

===Versine and coversine===
[[Image:Versine.svg|right|thumb|Sine, cosine, and versine of angle ''&theta;'' in terms of a [[unit circle]] with radius 1, centered at ''O''. This figure also illustrates the reason why the versine was sometimes called the ''sagitta'', Latin for [[arrow]].<ref name="Brummelen_2013"/><ref name="OED_Sagitta"/> If the arc ''ADB'' of the double-angle ''&Delta;''&nbsp;= 2''&theta;'' is viewed as a "[[bow (weapon)|bow]]" and the [[Chord (geometry)|chord]] ''AB'' as its "string", then the versine ''CD'' is clearly the "arrow shaft".]]

The ordinary ''[[sine]]'' function ([[History of trigonometric functions#Etymology|see note on etymology]]) was sometimes historically called the ''sinus rectus'' ("straight sine"), to contrast it with the versed sine (''sinus versus'').<ref name="Boyer_1991"/> The meaning of these terms is apparent if one looks at the functions in the original context for their definition, a [[unit circle]]:

For a vertical [[Chord (geometry)|chord]] ''AB'' of the unit circle, the sine of the angle ''&theta;'' (representing half of the subtended angle ''&Delta;'') is the distance ''AC'' (half of the chord). On the other hand, the versed sine of ''&theta;'' is the distance ''CD'' from the center of the chord to the center of the arc. Thus, the sum of cos(''&theta;'') (equal to the length of line ''OC'') and versin(''&theta;'') (equal to the length of line ''CD'') is the radius ''OD'' (with length 1). Illustrated this way, the sine is vertical (''rectus'', literally "straight") while the versine is horizontal (''versus'', literally "turned against, out-of-place"); both are distances from ''C'' to the circle.

This figure also illustrates the reason why the versine was sometimes called the ''sagitta'', Latin for [[arrow]],<ref name="Brummelen_2013"/><ref name="OED_Sagitta"/> from the Arabic usage ''sahem''<ref name="Miller_2007"/> of the same meaning. This itself comes from the Indian word 'sara' (arrow){{fact|date=April 2019}} that was commonly used to refer to "[[Jyā, koti-jyā and utkrama-jyā|utkrama-jya]]". If the arc ''ADB'' of the double-angle ''&Delta;''&nbsp;= 2''&theta;'' is viewed as a "[[bow (weapon)|bow]]" and the chord ''AB'' as its "string", then the versine ''CD'' is clearly the "arrow shaft".

In further keeping with the interpretation of the sine as "vertical" and the versed sine as "horizontal", ''sagitta'' is also an obsolete synonym for the [[abscissa]] (the horizontal axis of a graph).<ref name="OED_Sagitta"/>

In 1821, [[Augustin-Louis Cauchy|Cauchy]] used the terms ''sinus versus'' (''siv'') for the versine and ''cosinus versus'' (''cosiv'') for the coversine.<ref name="Cauchy_1821"/><ref name="Bradley_2009"/><ref group="nb" name="vercosine vs. coversine"/><!-- Mentioned here merely to give a snapshot in time as Cauchy does not discuss the vercosine, covercosine, or any kind of haversines, so they might originate later in time. -->

[[Image:Circle-trig6.svg|right|thumb|320px|The trigonometric functions can be constructed geometrically in terms of a [[unit circle]] centered at ''O''.]]

Historically, the versed sine was considered one of the most important trigonometric functions.<ref name="Calvert_2004"/><ref name="Boyer_1991"/><ref name="Miller_2007"/>

As ''&theta;'' goes to zero, versin(''&theta;'') is the difference between two nearly equal quantities, so a user of a [[trigonometric table]] for the cosine alone would need a very high accuracy to obtain the versine in order to avoid [[catastrophic cancellation]], making separate tables for the latter convenient.<ref name="Calvert_2004"/> Even with a calculator or computer, [[round-off error]]s make it advisable to use the sin<sup>2</sup> formula for small&nbsp;''&theta;''.

Another historical advantage of the versine is that it is always non-negative, so its [[logarithm]] is defined everywhere except for the single angle (''&theta;'' = 0, 2{{pi}}, …) where it is zero&mdash;thus, one could use [[logarithmic table]]s for multiplications in formulas involving versines.

In fact, the earliest surviving table of sine (half-[[Chord (geometry)|chord]]) values (as opposed to the [[Ptolemy's table of chords|chords tabulated by Ptolemy]] and other Greek authors), calculated from the [[Surya Siddhanta|Surya Siddhantha]] of India dated back to the 3rd century BC, was a table of values for the sine and versed sine (in 3.75° increments from 0 to 90°).<ref name="Boyer_1991"/>

The versine appears as an intermediate step in the application of the [[half-angle formula]] sin<sup>2</sup>{{big|(}}{{sfrac|''θ''|2}}{{big|)}} = {{sfrac|1|2}}versin(''&theta;''), derived by [[Ptolemy]], that was used to construct such tables.

===Haversine===
The haversine, in particular, was important in [[navigation]] because it appears in the [[haversine formula]], which is used to reasonably accurately compute distances on an astronomic [[spheroid]] (see issues with the [[equatorial bulge|earth's radius vs. sphere]]) given angular positions (e.g., [[longitude]] and [[latitude]]). One could also use sin<sup>2</sup>{{big|(}}{{sfrac|''θ''|2}}{{big|)}} directly, but having a table of the haversine removed the need to compute squares and square roots.<ref name="Calvert_2004"/>

An early utilization by [[José de Mendoza y Ríos]] of what later would be called haversines is documented in 1801.<!-- [[Law of haversines]] of spherical trigonometry, perhaps even 1795 --><ref name="Cajori_1929"/><ref name="Ríos_1795"/>

The first known English equivalent to a [[table of haversines]] was published by James Andrew in 1805.<ref name="RCA_1945"/><ref name="Andrew_1805"/><ref name="Brummelen_2013"/>

In 1835,<!-- Cajori, 1929, even mentions the original 1821 issue of Inman's book, but does not state, that Inman invented the term haversine himself. --> the term ''[[haversine]]'' (notated naturally as ''hav.'' or [[base-10 logarithm]]ically as ''log. haversine'' or ''log. havers.'') was coined<ref name="OED_1989_Haversine"/> by [[James Inman]]<ref name="Cajori_1929"/><ref name="White_1926-02"/><ref name="White_1926-07"/> in the third edition of his work ''Navigation and Nautical Astronomy: For the Use of British Seamen'' to simplify the calculation of distances between two points on the surface of the earth using [[spherical trigonometry]] for applications in navigation.<ref name="Inman_1835"/><ref name="OED_1989_Haversine"/> Inman also used the terms ''nat.<!-- natural --> versine'' and ''nat.<!-- natural --> vers.'' for versines.<ref name="Inman_1835"/>

Other high-regarded tables of haversines were those of Richard Farley in 1856<ref name="RCA_1945"/><ref name="Farley_1856"/> and John Caulfield Hannyngton in 1876.<ref name="RCA_1945"/><ref name="Hannyngton_1876"/>

The haversine continues to be used in navigation and has even found new applications in recent decades, as in Bruce D. Stark's method for clearing [[lunar distance (navigation)|lunar distance]]s utilizing [[Gaussian logarithm]]s since 1995<ref name="Stark_1997"/><ref name="Kalivoda_2003"/> or in a more compact method for [[sight reduction]] since 2014.<ref name="Rudzinski_2015"/>

===Modern uses===
Whilst the usage of the versine, coversine and haversine as well as their [[#Inverse functions|inverse function]]s can be traced back centuries, the names for the other five [[cofunction]]s appear to be of much younger origin.

One period (0 < ''&theta;'' < {{sfrac|{{pi}}|2}}) of a versine or, more commonly, a haversine (or havercosine) waveform is also commonly used in [[signal processing]] and [[control theory]] as the shape of a [[pulse (signal processing)|pulse]] or a [[window function]] (including [[Hann window|Hann]], [[Hann–Poisson window|Hann–Poisson]] and [[Tukey window]]s), because it smoothly ([[continuous function|continuous]] in value and [[slope]]) "turns on" from [[0 (number)|zero]] to [[1 (number)|one]] (for haversine) and back to zero.<ref group="nb" name="hvs"/> In these applications, it is named [[Hann function]] or [[raised-cosine filter]]. Likewise, the havercosine is used in [[raised-cosine distribution]]s in [[probability theory]] and [[statistics]].

In the form of sin<sup>2</sup>(''&theta;'') the haversine of the double-angle ''&Delta;'' describes the relation between [[spread (rational trigonometry)|spread]]s and angles in [[rational trigonometry]], a proposed reformulation of [[metric space|metrical]] [[plane geometry|planar]] and [[solid geometry|solid geometries]] by [[Norman John Wildberger]] since 2005.<ref name="Wildberger_2005"/>

As sagitta and cosagitta, double-angle ''&Delta;'' variants of the haversine and havercosine have also found new uses in describing the [[correlation]] and [[anti-correlation]] of correlated [[photon]]s in [[quantum mechanics]].<ref name="Stavek_2013"/>

==Mathematical identities==

===Definitions===
{| class="wikitable"
|-
| <math>\textrm{versin} (\theta) := 2\sin^2\!\left(\frac{\theta}{2}\right) = 1 - \cos (\theta) \,</math><ref name="Abramowitz_1972"/>
|| [[File:Versin plot 2.svg|300px]]
|-
| <math>\textrm{coversin}(\theta) := \textrm{versin}\!\left(\frac{\pi}{2} - \theta\right) = 1 - \sin(\theta) \,</math><ref name="Abramowitz_1972"/>
|| [[File:Coversin plot 2.svg|300px]]
|-
| <math>\textrm{vercosin} (\theta) := 2\cos^2\!\left(\frac{\theta}{2}\right) = 1 + \cos (\theta) \,</math><ref name="Weisstein_vercos"/>
|| [[File:Vercosin plot 2.svg|300px]]
|-
| <math>\textrm{covercosin}(\theta) := \textrm{vercosin}\!\left(\frac{\pi}{2} - \theta\right) = 1 + \sin(\theta) \,</math><ref name="Weisstein_covercos"/>
|| [[File:Covercosin plot 2.svg|300px]]
|-
| <math>\textrm{haversin}(\theta) := \frac {\textrm{versin}(\theta)} {2} = \sin^2\!\left(\frac{\theta}{2}\right) = \frac{1 - \cos (\theta)}{2} \,</math><ref name="Abramowitz_1972"/>
|| [[File:Haversin plot 2.svg|300px]]
|-
| <math>\textrm{hacoversin}(\theta) := \frac {\textrm{coversin}(\theta)} {2} = \frac{1 - \sin (\theta)}{2} \,</math><ref name="Weisstein_hacoversin"/>
|| [[File:Hacoversin plot 2.svg|300px]]
|-
| <math>\textrm{havercosin}(\theta) := \frac {\textrm{vercosin}(\theta)} {2} = \cos^2\!\left(\frac{\theta}{2}\right) = \frac{1 + \cos (\theta)}{2} \,</math><ref name="Weisstein_havercos"/>
|| [[File:Havercosin plot 2.svg|300px]]
|-
| <math>\textrm{hacovercosin}(\theta) := \frac {\textrm{covercosin}(\theta)} {2} = \frac{1 + \sin (\theta)}{2} \,</math><ref name="Weisstein_hacovercos"/>
|| [[File:Hacovercosin plot 2.svg|300px]]
|}

===Circular rotations===
The functions are circular rotations of each other.
:<math>\begin{align}
  \mathrm{versin}(\theta)
    &= \mathrm{coversin}\left(\theta + \frac{\pi}{2}\right)
     = \mathrm{vercosin}\left(\theta + \pi\right) 
     = \mathrm{covercosin}\left(\theta + \frac{3\pi}{2}\right) \\
  \mathrm{haversin}(\theta)
    &= \mathrm{hacoversin}\left(\theta + \frac{\pi}{2}\right)
     = \mathrm{havercosin}\left(\theta + \pi\right) 
     = \mathrm{hacovercosin}\left(\theta + \frac{3\pi}{2}\right) 
\end{align}</math>

===Derivatives and integrals===
{| class="wikitable"
|-
|  <math>\frac{\mathrm{d}}{\mathrm{d}x}\mathrm{versin}(x) = \sin{x}</math><ref name="Weisstein_vers"/>
|| <math>\int\mathrm{versin}(x) \,\mathrm{d}x = x - \sin{x} + C</math><ref name="Abramowitz_1972"/><ref name="Weisstein_vers"/>
|-
|  <math>\frac{\mathrm{d}}{\mathrm{d}x}\mathrm{vercosin}(x) = -\sin{x}</math>
|| <math>\int\mathrm{vercosin}(x) \,\mathrm{d}x = x + \sin{x} + C</math>
|-
|  <math>\frac{\mathrm{d}}{\mathrm{d}x}\mathrm{coversin}(x) = -\cos{x}</math><ref name="Weisstein_covers"/>
|| <math>\int\mathrm{coversin}(x) \,\mathrm{d}x = x + \cos{x} + C</math><ref name="Weisstein_covers"/>
|-
|  <math>\frac{\mathrm{d}}{\mathrm{d}x}\mathrm{covercosin}(x) = \cos{x}</math>
|| <math>\int\mathrm{covercosin}(x) \,\mathrm{d}x = x - \cos{x} + C</math>
|-
|  <math>\frac{\mathrm{d}}{\mathrm{d}x}\mathrm{haversin}(x) = \frac{\sin{x}}{2}</math><ref name="Weisstein_hav"/>
|| <math>\int\mathrm{haversin}(x) \,\mathrm{d}x = \frac{x - \sin{x}}{2} + C</math><ref name="Weisstein_hav"/>
|-
|  <math>\frac{\mathrm{d}}{\mathrm{d}x}\mathrm{havercosin}(x) = \frac{-\sin{x}}{2}</math>
|| <math>\int\mathrm{havercosin}(x) \,\mathrm{d}x = \frac{x + \sin{x}}{2} + C</math>
|-
|  <math>\frac{\mathrm{d}}{\mathrm{d}x}\mathrm{hacoversin}(x) = \frac{-\cos{x}}{2}</math>
|| <math>\int\mathrm{hacoversin}(x) \,\mathrm{d}x = \frac{x + \cos{x}}{2} + C</math>
|-
|  <math>\frac{\mathrm{d}}{\mathrm{d}x}\mathrm{hacovercosin}(x) = \frac{\cos{x}}{2}</math>
|| <math>\int\mathrm{hacovercosin}(x) \,\mathrm{d}x = \frac{x - \cos{x}}{2} + C</math>
|}

==={{anchor|aver|avcs|acvs|acvc|ahav|ahvc|ahcv|ahcc}}Inverse functions===
Inverse functions like '''arcversine'''<ref name="Vlijmen_2005"/> (arcversin, arcvers,<ref name="Hall_1909"/><ref name="Vlijmen_2005"/> avers,<ref name="Simpson_2001"/><ref name="Doel_2010"/> aver), '''arcvercosine''' (arcvercosin, arcvercos, avercos, avcs), '''arccoversine'''<ref name="Vlijmen_2005"/> (arccoversin, arccovers,<ref name="Hall_1909"/><ref name="Vlijmen_2005"/> acovers,<ref name="Simpson_2001"/><ref name="Doel_2010"/> acvs), '''arccovercosine''' (arccovercosin, arccovercos, acovercos, acvc), '''archaversine''' (archaversin, archav,<ref name="Vlijmen_2005"/> haversin<sup>−1</sup>,<ref name="Plus_hav"/> invhav,<ref name="Vlijmen_2005"/><ref name="Skvarc_1999"/><ref name="Skvarc_2014"/><ref name="Ballew_2007"/> ahav,<ref name="Vlijmen_2005"/><ref name="Simpson_2001"/><ref name="Doel_2010"/> ahvs, ahv, hav<sup>−1</sup><ref name="Weisstein_ahav"/><ref name="Wolfram_ahav"/>), '''archavercosine''' (archavercosin, archavercos, ahvc), '''archacoversine''' (archacoversin, ahcv) or '''archacovercosine''' (archacovercosin, archacovercos, ahcc) exist as well:

{| class="wikitable"
|-
|| <math>\operatorname{arcversin}(y) = \arccos\left(1-y\right)\,</math><ref name="Vlijmen_2005"/><ref name="Simpson_2001"/><ref name="Doel_2010"/>
|-
|| <math>\operatorname{arcvercos}(y) = \arccos\left(y-1\right)\,</math>
|-
|| <math>\operatorname{arccoversin}(y) = \arcsin\left(1-y\right)\,</math><ref name="Vlijmen_2005"/><ref name="Simpson_2001"/><ref name="Doel_2010"/>
|-
|| <math>\operatorname{arccovercos}(y) = \arcsin\left(y-1\right)\,</math>
|-
|| <math>\operatorname{archaversin}(y) = 2\arcsin\left(\sqrt{y}\right) = \arccos\left(1-2y\right)\,</math><ref name="Vlijmen_2005"/><ref name="Simpson_2001"/><ref name="Doel_2010"/><ref name="Plus_hav"/><ref name="Skvarc_1999"/><ref name="Skvarc_2014"/><ref name="Weisstein_ahav"/><ref name="Wolfram_ahav"/>
|-
|| <math>\operatorname{archavercos}(y) = 2\arccos\left(\sqrt{y}\right) = \arccos\left(2y-1\right)</math>
|-
|| <math>\operatorname{archacoversin}(y) = \arcsin\left(1-2y\right)\,</math>
|-
|| <math>\operatorname{archacovercos}(y) = \arcsin\left(2y-1\right)\,</math>
|}

===Other properties===
These functions can be extended into the [[complex plane]].<ref name="Weisstein_vers"/><ref name="Weisstein_covers"/><ref name="Weisstein_hav"/>

[[Maclaurin series]]:<ref name="Weisstein_hav"/>
: <math>\begin{align}
    \operatorname{versin}(z) &= \sum_{k=1}^\infty \frac{(-1)^{k-1} z^{2k}}{(2k)!} \\
  \operatorname{haversin}(z) &= \sum_{k=1}^\infty \frac{(-1)^{k-1} z^{2k}}{2(2k)!}
\end{align}</math>

: <math>\lim_{\theta \to 0} \frac{\operatorname{versin}(\theta)}{\theta} = 0</math><ref name="Hall_1909"/>

: <math>\begin{align}
  \frac{\operatorname{versin}(\theta) + \operatorname{coversin}(\theta)}
       {\operatorname{versin}(\theta) - \operatorname{coversin}(\theta)} -
  \frac{\operatorname{exsec}(\theta) + \operatorname{excsc}(\theta)}
       {\operatorname{exsec}(\theta) - \operatorname{excsc}(\theta)}
    &= \frac{2 \operatorname{versin}(\theta) \operatorname{coversin}(\theta)}
            {\operatorname{versin}(\theta) - \operatorname{coversin}(\theta)} \\[3pt]
  [\operatorname{versin}(\theta) + \operatorname{exsec}(\theta)]\,
       [\operatorname{coversin}(\theta) + \operatorname{excsc}(\theta)]
    &= \sin(\theta) \cos(\theta)
\end{align}</math><ref name="Hall_1909"/>

==Approximations==
[[File:Comparison of Versine Approximations for Angles of 0 to 2 Pi.svg|thumb|Comparison of the versine function with three approximations to the versine functions, for angles ranging from 0 to 2''&pi;'']]

[[File:Comparison of Versine Approximations for Angles of 0 to Pi divided by 2.svg|thumb|Comparison of the versine function with three approximations to the versine functions, for angles ranging from 0 to ''&pi;''/2]]

When the versine ''v'' is small in comparison to the radius ''r'', it may be approximated from the half-chord length ''L'' (the distance ''AC'' shown above) by the formula
:<math>v \approx \frac{L^2}{2r}</math>.<ref name="Woodward_1978"/>

Alternatively, if the versine is small and the versine, radius, and half-chord length are known, they may be used to estimate the arc length ''s'' (''AD'' in the figure above) by the formula
:<math>s\approx L+\frac{v^2}{r}</math>
This formula was known to the Chinese mathematician [[Shen Kuo]], and a more accurate formula also involving the sagitta was developed two centuries later by [[Guo Shoujing]].<ref name="Needham_1959"/>

A more accurate approximation used in engineering<ref name="Boardman_1930"/> is
:<math>v\approx \frac{s^\frac{3}{2} L^\frac{1}{2}}{8r}</math>

==Arbitrary curves and chords==
The term ''versine'' is also sometimes used to describe deviations from straightness in an arbitrary planar curve, of which the above circle is a special case. Given a chord between two points in a curve, the perpendicular distance ''v'' from the chord to the curve (usually at the chord midpoint) is called a ''versine'' measurement. For a straight line, the versine of any chord is zero, so this measurement characterizes the straightness of the curve. In the [[limit (mathematics)|limit]] as the chord length ''L'' goes to zero, the ratio {{sfrac|8''v''|''L''<sup>2</sup>}} goes to the instantaneous [[curvature]]. This usage is especially common in [[rail transport]], where it describes measurements of the straightness of the [[rail tracks]]<ref name="Nair_1972"/> and it is the basis of the [[Hallade method]] for [[rail surveying]].

The term ''[[sagitta (optics)|sagitta]]'' (often abbreviated ''sag'') is used similarly in [[optics]], for describing the surfaces of [[lens (optics)|lens]]es and [[mirror]]s.

==See also==
* [[List of trigonometric identities#Historic shorthands|Trigonometric identities]]
* [[Exsecant and excosecant]]
* [[Versiera]] ([[Witch of Agnesi]])
* [[Exponential minus 1]]<!-- similar type of function to increase accuracy -->
* [[Natural logarithm plus 1]]<!-- similar type of function to increase accuracy -->

==Notes==
{{reflist|group="nb"|refs=
<ref group="nb" name="vercosine vs. coversine">{{anchor|Bradley-Errata|Korn-Errata}}Some English sources confuse the versed cosine with the coversed sine. Historically (f.e. in [[#Cauchy-1821|Cauchy, 1821]]), the ''sinus versus'' (versine) was defined as siv(''&theta;'')&nbsp;=&nbsp;1−cos(''&theta;''), the ''cosinus versus'' (what is now also known as coversine) as cosiv(''&theta;'')&nbsp;=&nbsp;1−sin(''&theta;''), and the vercosine as vcs''&theta;''&nbsp;=&nbsp;1+cos(''&theta;''). However, in their 2009 English translation of Cauchy's work, [[#Bradley-2009|Bradley and Sandifer]] associate the ''cosinus versus'' (and cosiv) with the ''versed cosine'' (what is now also known as vercosine) rather than the ''coversed sine''. Similarly, in their 1968/2000 work, [[#Korn-2000|Korn and Korn]] associate the covers(''&theta;'') function with the ''versed cosine'' instead of the ''coversed sine''.</ref>
<ref group="nb" name="hvs">The abbreviation ''hvs'' sometimes used for the haversine function in signal processing and filtering is also sometimes used for the unrelated [[Heaviside step function]].</ref>
}}

==References==
{{reflist|refs=
<ref name="Braunmühl_1903">{{cite book |title=Vorlesungen über Geschichte der Trigonometrie - Von der Erfindung der Logarithmen bis auf die Gegenwart |language=German |trans-title=Lectures on history of trigonometry - from the invention of logarithms up to the present |author-first=Anton |author-last=Edler von Braunmühl |author-link=Johann Anton Edler von Braunmühl |volume=2 |date=1903 |publisher=[[B. G. Teubner]] |location=Leipzig, Germany |page=231 |url=https://books.google.com/books/about/Vorlesungen_%C3%BCber_Geschichte_der_Trigono.html?id=2Kc_AQAAIAAJ |access-date=2015-12-09}}</ref>
<ref name="Shaneyfelt">{{cite web |title=德博士的 Notes About Circles, ज्य, & कोज्य: What in the world is a hacovercosine? |author-first=Ted V. |author-last=Shaneyfelt |publisher=[[University of Hawaii]] |location=Hilo, Hawaii |url=http://www2.hawaii.edu/~tvs/trig.html |access-date=2015-11-08 |url-status=live |archive-url=https://web.archive.org/web/20150919053929/http://www2.hawaii.edu/~tvs/trig.html |archive-date=2015-09-19}}</ref>
<ref name="Cauchy_1821">{{anchor|Cauchy-1821}}{{cite book |author-first=Augustin-Louis |author-last=Cauchy |author-link=Augustin-Louis Cauchy |title=Cours d'Analyse de l'Ecole royale polytechnique |title-link=Cours d'Analyse |language=French |volume=1 |chapter=Analyse Algébrique |date=1821 |publisher=L'Imprimerie Royale, Debure frères, Libraires du Roi et de la [[Bibliothèque du Roi]]}}[https://books.google.com/books?id=UrT0KsbDmDwC&pg=PA11&lpg=PA11<!-- access-date=2015-11-07-->] (reissued by [[Cambridge University Press]], 2009; {{isbn|978-1-108-00208-0}})</ref>
<ref name="Bradley_2009">{{anchor|Bradley-2009}}{{cite book |author-first1=Robert E. |author-last1=Bradley |author-first2=Charles Edward |author-last2=Sandifer |editor-first1=J. Z. |editor-last1=Buchwald |others=[[Augustin-Louis Cauchy|Cauchy, Augustin-Louis]] |title=Cauchy’s Cours d’analyse: An Annotated Translation |work=Sources and Studies in the History of Mathematics and Physical Sciences |orig-year=2009 |date=2010-01-14 |publisher=[[Springer Science+Business Media, LLC]] |id=1441905499, 978-1-4419-0549-9 |isbn=978-1-4419-0548-2 |doi=10.1007/978-1-4419-0549-9 |lccn=2009932254 |pages=10, 285 |url=https://books.google.com/books?id=M0or-HGe7D0C
|access-date=2015-11-09}} (See [[#Bradley-Errata|errata]].)</ref>
<ref name="Weisstein_vercos">{{cite web |author-first=Eric Wolfgang |author-last=Weisstein |author-link=Eric Wolfgang Weisstein |title=Vercosine |work=[[MathWorld]] |publisher=[[Wolfram Research, Inc.]] |url=http://mathworld.wolfram.com/Vercosine.html |access-date=2015-11-06 |url-status=live |archive-url=https://web.archive.org/web/20140324181952/http://mathworld.wolfram.com/Vercosine.html |archive-date=2014-03-24}}</ref>
<ref name="Tapson_2004">{{cite web |title=Background Notes on Measures: Angles |author-first=Frank |author-last=Tapson |date=2004 |version=1.4 |publisher=Cleave Books |url=http://www.cleavebooks.co.uk/dictunit/notesa.htm#others |access-date=2015-11-12 |url-status=live |archive-url=https://web.archive.org/web/20070209051219/http://www.cleavebooks.co.uk/dictunit/notesa.htm |archive-date=2007-02-09}}</ref>
<ref name="Cajori_1929">{{anchor|Cajori-1929}}{{cite book |author-first=Florian |author-last=Cajori |author-link=Florian Cajori |title=A History of Mathematical Notations |volume=2 |orig-year=March 1929 |publisher=[[Open court publishing company]] |location=Chicago, USA |date=1952 |edition=2 (3rd corrected printing of 1929 issue) |page=172 |isbn=978-1-60206-714-1 |id=1602067147 |url=https://books.google.com/books?id=bT5suOONXlgC |access-date=2015-11-11 |quote=The haversine first appears in the tables of logarithmic versines of [[José de Mendoza y Rios]] (Madrid, 1801, also 1805, 1809), and later in a treatise on navigation of [[James Inman]] (1821). See J. D. White in ''[[The Nautical Magazine|Nautical Magazine]]'' ([[#White-1926-02|February]] and [[#White-1926-07|July 1926]]).}} (NB. ISBN and link for reprint of 2nd edition by Cosimo, Inc., New York, USA, 2013.)</ref>
<ref name="Weisstein_covers">{{cite web |author-first=Eric Wolfgang |author-last=Weisstein |author-link=Eric Wolfgang Weisstein |title=Coversine |work=[[MathWorld]] |publisher=[[Wolfram Research, Inc.]] |url=http://mathworld.wolfram.com/Coversine.html |access-date=2015-11-06 |url-status=live |archive-url=https://web.archive.org/web/20051127184403/http://mathworld.wolfram.com/Coversine.html |archive-date=2005-11-27}}</ref>
<ref name="Ludlow_1891">{{cite book |title=Elements of Trigonometry with Logarithmic and Other Tables |author-first1=Henry Hunt |author-last1=Ludlow |author-first2=Edgar Wales |author-last2=Bass |date=1891 |edition=3 |publisher=[[John Wiley & Sons]] |location=Boston, USA |page=33 |url=https://books.google.com/books/about/Elements_of_Trigonometry_with_Logarithmi.html?id=s7iBAAAAIAAJ |access-date=2015-12-08}}</ref>
<ref name="Wentworth_1903">{{cite book |title=Plane Trigonometry|url=https://archive.org/details/planetrigonomet06wentgoog<!-- There are several books by the same author dated 1903 with variations on the title ("Plane Trigonometry", "Plane Trigonometry and Tables", "Plane and Spherical Trigonometry", "Plane Trigonometry", "Surveying and Tables"), some mention a coauthor George Anthony Hill. It is unclear, which book was meant specifically. Florian Cajori called it just "Trigonometry". --> |author-first=George Albert |author-last=Wentworth |date=1903 |orig-year=1887 |edition=2 |publisher=[[Ginn and Company]] |location=Boston, USA |page=5}}</ref>
<ref name="Kenyon_1913">{{cite book |title=Trigonometry |author-first1=Alfred Monroe |author-last1=Kenyon |author-first2=Louis |author-last2=Ingold |date=1913 |publisher=[[The Macmillan Company]] |location=New York, USA |pages=8–9 |url=https://books.google.com/books?id=RsxHAAAAIAAJ |access-date=2015-12-08}}</ref>
<ref name="Anderegg_1896">{{cite book |title=Trigonometry: For Schools and Colleges |author-first1=Frederick |author-last1=Anderegg |author-first2=Edward Drake |author-last2=Roe<!-- sometimes written Rowe --> |date=1896 |publisher=[[Ginn and Company]] |location=Boston, USA |page=10 |url=https://books.google.com/books/about/Trigonometry.html?id=deZHAAAAIAAJ |access-date=2015-12-08}}</ref>
<ref name="Weisstein_covercos">{{cite web |author-first=Eric Wolfgang |author-last=Weisstein |author-link=Eric Wolfgang Weisstein |title=Covercosine |work=[[MathWorld]] |publisher=[[Wolfram Research, Inc.]] |url=http://mathworld.wolfram.com/Covercosine.html |access-date=2015-11-06 |url-status=live |archive-url=https://web.archive.org/web/20140328110051/http://mathworld.wolfram.com/Covercosine.html |archive-date=2014-03-28}}</ref>
<ref name="Fulst_1972">{{cite book |title=Nautische Tafeln |author-first1=Otto |author-last1=Fulst |editor-first1=Johannes |editor-last1=Lütjen |editor-first2=Walter |editor-last2=Stein |editor-first3=Gerhard |editor-last3=Zwiebler |date=1972 |edition=24 |publisher=Arthur Geist Verlag |location=Bremen, Germany |language=German |chapter=17, 18 <!-- |id=ASIN:B002M2RD3G -->}}</ref>
<ref name="Sauer_2015">{{cite web |author-first=Frank |author-last=Sauer |title=Semiversus-Verfahren: Logarithmische Berechnung der Höhe |date=2015 |orig-year=2004 |publisher=Astrosail |location=Hotheim am Taunus, Germany |language=German |url=http://www.astrosail.de/de/static/tutorial/semi2.php?cat=41 |access-date=2015-11-12 |url-status=live |archive-url=https://web.archive.org/web/20130917060448/http://www.astrosail.de/de/static/tutorial/semi2.php?cat=41 |archive-date=2013-09-17}}</ref>
<ref name="Weisstein_hav">{{cite web |author-first=Eric Wolfgang |author-last=Weisstein |author-link=Eric Wolfgang Weisstein |title=Haversine |work=[[MathWorld]] |publisher=[[Wolfram Research, Inc.]] |url=http://mathworld.wolfram.com/Haversine.html |access-date=2015-11-06 |url-status=live |archive-url=https://web.archive.org/web/20050310194740/http://mathworld.wolfram.com/Haversine.html |archive-date=2005-03-10}}</ref>
<ref name="Rider_1923">{{cite book |title=Plane Trigonometry |author-first1=Paul Reece |author-last1=Rider |author-first2=Alfred |author-last2=Davis |date=1923 |publisher=[[D. Van Nostrand Company]] |location=New York, USA |page=42 |url=https://books.google.com/books/about/Plane_trigonometry.html?id=G4O4AAAAIAAJ |access-date=2015-12-08}}</ref>
<ref name="Wolfram_hav">{{cite web |title=Haversine |work=Wolfram Language & System: Documentation Center |date=2008 |version=7.0 |url=http://reference.wolfram.com/language/ref/Haversine.html |access-date=2015-11-06 |url-status=live |archive-url=https://web.archive.org/web/20140901065044/http://reference.wolfram.com/language/ref/Haversine.html |archive-date=2014-09-01}}</ref>
<ref name="Rudzinski_2015">{{cite journal |title=Ultra compact sight reduction |author-first1=Greg |author-last1=Rudzinski |others=Ix, Hanno |journal=Ocean Navigator |publisher=Navigator Publishing LLC |location=Portland, ME, USA |date=July<!-- /August --> 2015 |issue=227 |issn=0886-0149 |pages=42–43 |url=http://issuu.com/navigatorpublishing/docs/on227_download_edition |access-date=2015-11-07}}</ref>
<ref name="Weisstein_havercos">{{cite web |author-first=Eric Wolfgang |author-last=Weisstein |author-link=Eric Wolfgang Weisstein |title=Havercosine |work=[[MathWorld]] |publisher=[[Wolfram Research, Inc.]] |url=http://mathworld.wolfram.com/Havercosine.html |access-date=2015-11-06 |url-status=live |archive-url=https://web.archive.org/web/20140329054218/http://mathworld.wolfram.com/Havercosine.html |archive-date=2014-03-29}}</ref>
<ref name="Weisstein_hacoversin">{{cite web |author-first=Eric Wolfgang |author-last=Weisstein |author-link=Eric Wolfgang Weisstein |title=Hacoversine |work=[[MathWorld]] |publisher=[[Wolfram Research, Inc.]] |url=http://mathworld.wolfram.com/Hacoversine.html |access-date=2015-11-06 |url-status=live |archive-url=https://web.archive.org/web/20140329060147/http://mathworld.wolfram.com/Hacoversine.html |archive-date=2014-03-29}}</ref>
<ref name="Vlijmen_2005">{{cite web |title=Goniology |language=English |work=Eenheden, constanten en conversies<!-- |trans-work=Units of measurement, constants and conversions --> |author-first=Oscar |author-last=van Vlijmen |date=2005-12-28 |orig-year=2003 |url=http://home.kpn.nl/vanadovv/Gonio.html |access-date=2015-11-28 |url-status=live |archive-url=https://web.archive.org/web/20091028065825/http://home.hetnet.nl/~vanadovv/Gonio.html |archive-date=2009-10-28}}</ref>
<ref name="Weisstein_hacovercos">{{cite web |author-first=Eric Wolfgang |author-last=Weisstein |author-link=Eric Wolfgang Weisstein |title=Hacovercosine |work=[[MathWorld]] |publisher=[[Wolfram Research, Inc.]] |url=http://mathworld.wolfram.com/Hacovercosine.html |access-date=2015-11-06 |url-status=live |archive-url=https://web.archive.org/web/20140329094237/http://mathworld.wolfram.com/Hacovercosine.html |archive-date=2014-03-29}}</ref>
<ref name="OED_Sagitta">{{OED|sagitta}}</ref>
<ref name="Miller_2007">{{cite web |author-first=Jeff |author-last=Miller <!-- |publisher=Gulf High School--> |title=Earliest Known Uses of Some of the Words of Mathematics (V) |date=2007-09-10 |location=New Port Richey, Florida, USA |url=http://jeff560.tripod.com/v.html |access-date=2015-11-10 |url-status=live |archive-url=https://web.archive.org/web/20150905141835/http://jeff560.tripod.com/v.html |archive-date=2015-09-05}}</ref>
<ref name="Boyer_1991">{{cite book |author-first1=Carl Benjamin |author-last1=Boyer |author-link1=Carl Benjamin Boyer |author-first2=Uta C. |author-last2=Merzbach |author-link2=Uta Merzbach |title=A History of Mathematics |edition=2 |date=1991-03-06 |orig-year=1968 |publisher=[[John Wiley & Sons]] |location=New York, USA |isbn=978-0471543978 |id=0471543977 |url=https://archive.org/details/historyofmathema00boye |access-date=2019-08-10}}</ref>
<ref name="Calvert_2004">{{cite web |author-first=James B. |author-last=Calvert |title=Trigonometry |orig-year=2004-01-10 |date=2007-09-14 |url-status=live |archive-url=https://web.archive.org/web/20071002214133/http://mysite.du.edu/~jcalvert/math/trig.htm |archive-date=2007-10-02 |url=http://www.du.edu/~jcalvert/math/trig.htm |access-date=2015-11-08}}</ref>
<ref name="Ríos_1795">{{cite book |author-first=Joseph |author-last=de Mendoza y Ríos |author-link=José de Mendoza y Ríos |title=Memoria sobre algunos métodos nuevos de calcular la longitud por las distancias lunares: y aplication de su teórica á la solucion de otros problemas de navegacion |language=Spanish |url=https://books.google.com/books?id=030t0OqlX2AC |date=1795 |publisher=Imprenta Real |location=Madrid, Spain}}</ref>
<ref name="RCA_1945">{{cite book |author-first=Raymond Clare |author-last=Archibald |title=Recent Mathematical Tables |chapter=197: <!-- Paul R. Rider, Charles A. Hutchinson: -->Natural and Logarithmic Haversines |type=Review |journal=[[Mathematical Tables and other Aids to Computation]] (MTAC) |publisher=[[National Research Council (United States)|The National Research Council]], Division of Physical Sciences, Committee on Mathematical Tables and Other Aids to Computation; [[American Mathematical Society]] |doi=10.1090/S0025-5718-45-99080-6 |date=1945-07-11 |volume=1 |pages=421–422 |url=http://www.ams.org/journals/mcom/1945-01-011/S0025-5718-45-99080-6/S0025-5718-45-99080-6.pdf |access-date=2015-11-19 |url-status=live |archive-url=https://web.archive.org/web/20151119015033/http://www.ams.org/journals/mcom/1945-01-011/S0025-5718-45-99080-6/S0025-5718-45-99080-6.pdf |archive-date=2015-11-19}} [http://www.ams.org/journals/mcom/1945-01-011/S0025-5718-45-99080-6/]</ref>
<ref name="Andrew_1805">{{cite book |author-first=James |author-last=Andrew |title=Astronomical and Nautical Tables with Precepts for finding the Latitude and Longitude of Places |location=London |date=1805 |volume=T. XIII |pages=29–148}} (A 7-place [[haversine]] table from 0° to 120° in intervals of 10".)</ref>
<ref name="Brummelen_2013">{{cite book |author-first=Glen Robert |author-last=van Brummelen |author-link=Glen Robert van Brummelen |title=Heavenly Mathematics: The Forgotten Art of Spherical Trigonometry |date=2013 |publisher=[[Princeton University Press]] |isbn=9780691148922 |id=0691148929 |url=https://books.google.com/books?id=0BCCz8Sx5wkC&pg=PR7 |access-date=2015-11-10}}</ref>
<ref name="White_1926-02">{{anchor|White-1926-02}}{{cite journal |author-first=J. D. |author-last=White |title=(unknown title)<!-- TBD --> |journal=[[The Nautical Magazine|Nautical Magazine]] |date=February 1926}} (NB. According to [[#Cajori-1929|Cajori, 1929]], this journal has a discussion on the origin of haversines.)</ref>
<ref name="White_1926-07">{{anchor|White-1926-07}}{{cite journal |author-first=J. D. |author-last=White |title=(unknown title)<!-- TBD. --> |journal=[[The Nautical Magazine|Nautical Magazine]] |date=July 1926}} (NB. According to [[#Cajori-1929|Cajori, 1929]], this journal has a discussion on the origin of haversines.)</ref>
<ref name="Inman_1835">{{cite book |author-first=James |author-last=Inman<!-- Rev. D.D. --> |author-link=James Inman |title=Navigation and Nautical Astronomy: For the Use of British Seamen |date=1835 |orig-year=1821 |edition=3 |publisher=W. Woodward, C. & J. Rivington |location=London, UK |url=https://books.google.com/books/about/Navigation_and_Nautical_Astronomy_for_th.html?id=-fUOnQEACAAJ |access-date=2015-11-09}} (Fourth edition: [https://books.google.com/books?id=MK8PAAAAYAAJ].)</ref>
<ref name="OED_1989_Haversine">{{OED2|haversine}}</ref>
<ref name="Farley_1856">{{cite book |author-first=Richard |author-last=Farley |title=Natural Versed Sines from 0 to 125°, and Logarithmic Versed Sines from 0 to 135° |location=London |date=1856}} (A [[haversine]] table from 0° to 125°/135°.)</ref>
<ref name="Hannyngton_1876">{{cite book |author-first=John Caulfield |author-last=Hannyngton |title=Haversines, Natural and Logarithmic, used in Computing Lunar Distances for the Nautical Almanac |location=London |date=1876}} (A 7-place [[haversine]] table from 0° to 180°, [[log. haversine]]s at intervals of 15", [[nat. haversine]]s at intervals of 10".)</ref>
<ref name="Stark_1997">{{cite book |title=Stark Tables for Clearing the Lunar Distance and Finding Universal Time by Sextant Observation Including a Convenient Way to Sharpen Celestial Navigation Skills While On Land |author-first=Bruce D. |author-last=Stark |publisher=Starpath Publications |edition=2 |date=1997 |orig-year=1995 |isbn=978-0914025214 |id=091402521X |url=http://www.starpath.com/catalog/books/1875.htm |access-date=2015-12-02}} (NB. Contains a table of [[Gaussian logarithm]]s [[Decimal logarithm|lg]](1+10<sup>-x</sup>).)</ref>
<ref name="Kalivoda_2003">{{cite web |title=Bruce Stark - Tables for Clearing the Lunar Distance and Finding G.M.T. by Sextant Observation (1995, 1997) |type=Review |author-first=Jan |author-last=Kalivoda |date=2003-07-30 |location=Prague, Czech Republic |url=http://www.starpath.com/catalog/books/StarkTables.htm |access-date=2015-12-02 |url-status=live |archive-url=https://web.archive.org/web/20040112233843/http://web.dkm.cz/kalivoda/StarkTables.htm |archive-date=2004-01-12}}[http://fer3.com/arc/m2.aspx/Tables-for-clearing-Lunar-Distances-from-Bruce-Stark-Kalivoda-jul-2003-w10812][https://web.archive.org/web/20040704084227/http://www.starpath.com/catalog/books/StarkTables.htm]</ref>
<ref name="Wildberger_2005">{{cite book |author-first=Norman John |author-last=Wildberger |author-link=Norman John Wildberger |title=Divine Proportions: Rational Trigonometry to Universal Geometry |publisher=Wild Egg Pty Ltd |location=Australia |date=2005 |edition=1 |isbn=0-9757492-0-X |url=https://www.researchgate.net/publication/266738365_Divine_Proportions_Rational_Trigonometry_to_Universal_geometry |access-date=2015-12-01}}</ref>
<ref name="Stavek_2013">{{cite journal |author-first=Jiří |author-last=Stávek |title=On the Trigonometric Loophole |date=2013-10-18 |doi=10.5539/apr.v5n6p48 |journal=Applied Physics Research |publisher=Canadian Center of Science and Education |location=Prague, CZ |volume=5 |issue=6 |issn=1916-9639 |eissn=1916-9647 |url=http://www.ccsenet.org/journal/index.php/apr/article/viewFile/31357/18340 |access-date=2015-11-19 |url-status=live |archive-url=https://web.archive.org/web/20151119015133/http://www.ccsenet.org/journal/index.php/apr/article/viewFile/31357/18340 |archive-date=2015-11-19}}</ref>
<ref name="Abramowitz_1972">{{AS ref|4.3.147: Elementary Transcendental Functions - Circular functions|78|first1=Ruth|last1=Zucker}}</ref>
<ref name="Weisstein_vers">{{cite web |author-first=Eric Wolfgang |author-last=Weisstein |author-link=Eric Wolfgang Weisstein |title=Versine |work=[[MathWorld]] |publisher=[[Wolfram Research, Inc.]] |url=http://mathworld.wolfram.com/Versine.html |access-date=2015-11-05 |url-status=live |archive-url=https://web.archive.org/web/20100331150251/http://mathworld.wolfram.com/Versine.html |archive-date=2010-03-31}}</ref>
<ref name="Atlas_2009">{{cite book |title=An Atlas of Functions: with Equator, the Atlas Function Calculator |author-first1=Keith B. |author-last1=Oldham |author-first2=Jan C. |author-last2=Myland |author-first3=Jerome |author-last3=Spanier |publisher=[[Springer Science+Business Media, LLC]] |edition=2 |date=2009 |orig-year=1987 |chapter=32.13. The Cosine cos(x) and Sine sin(x) functions - Cognate functions |page=322 |isbn=978-0-387-48806-6 |doi=10.1007/978-0-387-48807-3 |lccn=2008937525}}</ref>
<ref name="Korn_2000">{{anchor|Korn-2000}}{{cite book |title=Mathematical handbook for scientists and engineers: Definitions, theorems, and formulars for reference and review |author-first1=Grandino Arthur |author-last1=Korn |author-first2=Theresa M. |author-last2=Korn |edition=3<!-- (based on 1968 edition by McGrawHill, Inc.) --> |date=2000 |orig-year=1961 |publisher=[[Dover Publications, Inc.]] |location=Mineola, New York, USA |chapter=Appendix B: B9. Plane and Spherical Trigonometry: Formulas Expressed in Terms of the Haversine Function |pages=892–893 |isbn=978-0-486-41147-7}} (See [[#Korn-Errata|errata]].)</ref>
<ref name="Clagett_1969">{{cite book |title=Critical Problems in the History of Science |editor-first=Marshall |editor-last=Clagett |author-first=Carl Benjamin |author-last=Boyer |author-link=Carl Benjamin Boyer |chapter=5: Commentary on the Paper of [[E. J. Dijksterhuis]] (The Origins of Classical Mechanics from Aristotle to Newton) |pages=185–190 |date=1969 |orig-year=1959 |edition=3 |publisher=[[University of Wisconsin Press, Ltd.]] |location=Madison, Milwaukee, and London |isbn=0-299-01874-1 |id=9780299018740 |lccn=59-5304 |url=https://books.google.com/books?id=WboPReSZ668C |access-date=2015-11-16}}</ref>
<ref name="Precalc_1999">{{cite book |title=Precalculus: A Study of Functions and Their Applications |chapter=5 (Trigonometric Functions) |author-first1=Todd |author-last1=Swanson<!-- Hope College --> |author-first2=Janet |author-last2=Andersen<!-- Hope College --> |author-first3=Robert |author-last3=Keeley<!-- Calvin College --> |date=1999 |publisher=[[Harcourt Brace & Company]] |page=344 |url=http://math.hope.edu/swanson/text/chapter5.pdf |access-date=2015-11-12 |url-status=live |archive-url=https://web.archive.org/web/20030617045723/http://math.hope.edu/swanson/text/chapter5.pdf |archive-date=2003-06-17}}</ref>
<ref name="Skvarc_1999">{{cite web |title=identify.py: An asteroid_server client which identifies measurements in MPC format |author-first=Jure |author-last=Skvarc |work=Fitsblink |type=[[Python (programming language)|Python]] source code |url=http://www.fitsblink.net/software/clients/identify.py |date=1999-03-01 |access-date=2015-11-28 |url-status=live |archive-url=https://web.archive.org/web/20081120083747/http://www.fitsblink.net/software/clients/identify.py |archive-date=2008-11-20}}</ref>
<ref name="Skvarc_2014">{{cite web |title=astrotrig.py: Astronomical trigonometry related functions |author-first=Jure |author-last=Skvarc |type=[[Python (programming language)|Python]] source code |publisher=Telescope Vega, [[University of Ljubljana]] |location=Ljubljana, Slovenia |url=http://astro.ago.fmf.uni-lj.si/podatki/2014/V2014-10-27/astro/bojan@arix |date=2014-10-27 |access-date=2015-11-28 |url-status=live |archive-url=https://web.archive.org/web/20151128200410/http://astro.ago.fmf.uni-lj.si/podatki/2014/V2014-10-27/astro/bojan@arix |archive-date=2015-11-28}}</ref>
<ref name="Ballew_2007">{{cite web |title=Versine |work=Math Words, page 4 |at=Versine |author-first=Pat |author-last=Ballew |date=2007-02-08 |orig-year=2003 |url=http://www.pballew.net/arithme4.html#versine |access-date=2015-11-28 |url-status=live |archive-url=https://web.archive.org/web/20070208190009/http://www.pballew.net/arithme4.html#versine |archive-date=2007-02-08}}</ref>
<ref name="Simpson_2001">{{cite web |title=AUXTRIG |author-first=David G. |author-last=Simpson |publisher=[[NASA Goddard Space Flight Center]] |location=Greenbelt, Maryland, USA |date=2001-11-08 |type=[[Fortran 90]] source code |url=http://www.davidgsimpson.com/software/auxtrig_f90.txt |access-date=2015-10-26 |url-status=live |archive-url=https://web.archive.org/web/20080616185313/http://www.davidgsimpson.com/software/auxtrig_f90.txt |archive-date=2008-06-16}}</ref>
<ref name="Doel_2010">{{cite web |title=jass.utils Class Fmath |author-first=Kees |author-last=van den Doel |work=JASS - Java Audio Synthesis System |date=2010-01-25 |version=1.25 |url=http://www.cs.ubc.ca/~kvdoel/jass/doc/jass/utils/Fmath.html#aexsec%28double%29 |access-date=2015-10-26 |url-status=live |archive-url=https://web.archive.org/web/20070902035859/http://www.cs.ubc.ca/~kvdoel/jass/doc/jass/utils/Fmath.html |archive-date=2007-09-02}}</ref>
<ref name="Plus_hav">{{cite news |author=mf344 |title=Lost but lovely: The haversine |date=2014-07-04 |work=Plus magazine |publisher=maths.org |url=http://plus.maths.org/content/lost-lovely-haversine |access-date=2015-11-05 |url-status=live |archive-url=https://web.archive.org/web/20140718121825/http://plus.maths.org/content/lost-lovely-haversine |archive-date=2014-07-18}}</ref>
<ref name="Weisstein_ahav">{{cite web |author-first=Eric Wolfgang |author-last=Weisstein |author-link=Eric Wolfgang Weisstein |title=Inverse Haversine |work=[[MathWorld]] |publisher=[[Wolfram Research, Inc.]] |url=http://mathworld.wolfram.com/InverseHaversine.html |access-date=2015-10-05 |url-status=live |archive-url=https://web.archive.org/web/20080608014248/http://mathworld.wolfram.com/InverseHaversine.html |archive-date=2008-06-08}}</ref>
<ref name="Wolfram_ahav">{{cite web |title=InverseHaversine |work=Wolfram Language & System: Documentation Center |date=2008 |version=7.0 |url=http://reference.wolfram.com/language/ref/InverseHaversine.html |access-date=2015-11-05}}</ref>
<ref name="Woodward_1978">{{cite book |title=Geometry - Plane, Solid & Analytic Problem Solver |author-first=Ernest |author-last=Woodward<!-- professor of mathematics at Austin Peay State University, Clarkesville, Tennessee, USA, not to be confused with (Ernest) [[Llewellyn Woodward]] --> |series=Problem Solvers Solution Guides |publisher=[[Research & Education Association]] (REA) |date=December 1978 |isbn=978-0-87891-510-1 |page=359 |url=https://books.google.com/books?id=4iNvcGB3M9sC&pg=PA359}}</ref>
<ref name="Needham_1959">{{cite book |title=Science and Civilisation in China: Mathematics and the Sciences of the Heavens and the Earth |author-first=Noel Joseph Terence Montgomery |author-last=Needham |author-link=Noel Joseph Terence Montgomery Needham |publisher=[[Cambridge University Press]] |date=1959 |volume=3 |isbn=9780521058018 |page=39 |url=https://books.google.com/books?id=jfQ9E0u4pLAC&pg=PA39}}</ref>
<ref name="Boardman_1930">{{cite book |title=Table For Use in Computing Arcs, Chords and Versines |author-first=Harry |author-last=Boardman |publisher=[[Chicago Bridge and Iron Company]] |date=1930 |page=32}}</ref>
<ref name="Nair_1972">{{cite journal |author-first=<!-- Padmasree -->P. N. Bhaskaran |author-last=Nair |author-link=P. N. Bhaskaran Nair |title=Track measurement systems—concepts and techniques |journal=Rail International |publisher=International Railway Congress Association, [[International Union of Railways]] |oclc=751627806 |volume=3 |issue=3 |pages=159–166 |date=1972 |issn=0020-8442}}</ref>
<ref name="Hall_1909">{{cite book |title=Trigonometry |volume=Part I: Plane Trigonometry |author-first1=Arthur Graham |author-last1=Hall |author-first2=Fred Goodrich |author-last2=Frink |date=January 1909 |location=Ann Arbor, Michigan, USA |chapter=Review Exercises [100] Secondary Trigonometric Functions |publisher=[[Henry Holt and Company]] / Norwood Press / J. S. Cushing Co. - Berwick & Smith Co., Norwood, Massachusetts, USA |publication-place=New York, USA |pages=125-127 |url=https://archive.org/stream/planetrigonometr00hallrich#page/125/mode/1up |access-date=2017-08-12 |url-status=live}}</ref>
<ref name="Haslett_1855">{{cite book |title=The Mechanic's, Machinist's, Engineer's Practical Book of Reference: Containing tables and formulæ for use in superficial and solid mensuration; strength and weight of materials; mechanics; machinery; hydraulics, hydrodynamics; marine engines, chemistry; and miscellaneous recipes. Adapted to and for the use of all classes of practical mechanics. Together with the Engineer's Field Book: Containing formulæ for the various of running and changing lines, locating side tracks and switches, &c., &c. Tables of radii and their logarithms, natural and logarithmic versed sines and external secants, natural sines and tangents to every degree and minute of the quadrant, and logarithms from the natural numbers from 1 to 10,000 |author-first=Charles |author-last=Haslett |editor-first=Charles W. |editor-last=Hackley |date=September 1855 |publisher=James G. Gregory, successor of W. A. Townsend & Co. (Stringer & Townsend) |location=New York, USA |url=https://books.google.com/books?id=WVkOAAAAYAAJ |access-date=2017-08-13 |url-status=live |quote=[…] Still there would be much labor of computation which may be saved by the use of tables of [[external secant]]s and [[#ver|versed sine]]s, which have been employed with great success recently by the Engineers on the [[Ohio and Mississippi Railroad]], and which, with the formulas and rules necessary for their application to the laying down of curves, drawn up by Mr. Haslett, one of the Engineers of that Road, are now for the first time given to the public. […] In presenting this work to the public, the Author claims for it the adaptation of a new principle in trigonometrical analysis of the formulas generally used in field calculations. Experience has shown, that versed sines and external secants as frequently enter into calculations on curves as sines and tangents; and by their use, as illustrated in the examples given in this work, it is believed that many of the rules in general use are much simplified, and many calculations concerning curves and running lines made less intricate, and results obtained with more accuracy and far less trouble, than by any methods laid down in works of this kind. The examples given have all been suggested by actual practice, and will explain themselves. […] As a book for practical use in field work, it is confidently believed that this is more direct in the application of rules and facility of calculation than any work now in use. In addition to the tables generally found in books of this kind, the author has prepared, with great labor, a Table of Natural and Logarithmic Versed Sines and External Secants, calculated to degrees, for every minute; also, a Table of Radii and their Logarithms, from 1° to 60°. […]}} [https://books.google.com/books?id=KvLR97_I_RcC 1856 edition]</ref>
<ref name="Beebe_2017">{{cite book |author-first=Nelson H. F. |author-last=Beebe |title=The Mathematical-Function Computation Handbook - Programming Using the MathCW Portable Software Library |chapter=Chapter 11.1. Sine and cosine properties |date=2017-08-22 |location=Salt Lake City, UT, USA |publisher=[[Springer International Publishing AG]] |edition=1 |lccn=2017947446 |isbn=978-3-319-64109-6 |doi=10.1007/978-3-319-64110-2 |page=301}}</ref>
}}

==Further reading==
*<!--<ref name="Hawking_2002">-->{{cite book |editor-last=Hawking |editor-first=Stephen William |editor-link=Stephen William Hawking |date=2002 |title=On the Shoulders of Giants: The Great Works of Physics and Astronomy |publisher=[[Running Press]] |location=Philadelphia, USA |lccn=2002100441 |isbn=0-7624-1698-X |url=https://books.google.com/books?id=pb6HR4DAEeMC |access-date=2017-07-31 |url-status=live}}<!--</ref>-->
*<!--<ref name="Stávek_2017">-->{{cite journal |author-first=Jiří |author-last=Stávek |location=Prague, CZ |title=On the Hidden Beauty of Trigonometric Functions |date=2017-03-10 |orig-year=2017-02-26 |journal=Applied Physics Research |publisher=Canadian Center of Science and Education |volume=9 |number=2 |issn=1916-9639 |id={{eISSN|1916-9647}} |doi=10.5539/apr.v9n2p57 |pages=57–64 |url=https://doi.org/10.5539/apr.v9n2p57 |access-date=2017-07-31 |url-status=live |archive-url=https://web.archive.org/web/20170730235721/http://www.ccsenet.org/journal/index.php/apr/article/view/67093 |archive-date=2017-07-31}} [http://www.ccsenet.org/journal/index.php/apr/article/download/67093/36393<!-- https://web.archive.org/web/20170731001724/http://www.ccsenet.org/journal/index.php/apr/article/download/67093/36393 -->]<!--</ref>-->

==External links==
* {{cite web |title=Sagitta, Apothem, and Chord |author-first=Ed |author-last=Pegg, Jr. |author-link=Ed Pegg, Jr. |publisher=[[The Wolfram Demonstrations Project]] |url=http://demonstrations.wolfram.com/SagittaApothemAndChord/}}

[[Category:Trigonometric functions]]