There are several related functions, most notably thecoversine andhaversine. The latter, half a versine, is of particular importance in thehaversine formula of navigation.
Theversine[3][4][5][6][7] orversed sine[8][9][10][11][12] is atrigonometric function already appearing in some of the earliest trigonometric tables. It is symbolized in formulas using the abbreviationsversin,sinver,[13][14]vers, orsiv.[15][16] InLatin, it is known as thesinus versus (flipped sine),versinus,versus, orsagitta (arrow).[17]
Expressed in terms of commontrigonometric functions sine, cosine, and tangent, the versine is equal to
There are several related functions corresponding to the versine:
Thecoversed sine orcoversine[19] (in Latin,cosinus versus orcoversinus), abbreviatedcoversin,covers,[20][21][22]cosiv, orcvs[23]
Special tables were also made of half of the versed sine, because of its particular use in thehaversine formula used historically innavigation.
Thehaversed sine[24] orhaversine (Latinsemiversus),[25][26] abbreviatedhaversin,semiversin,semiversinus,havers,hav,[27][28]hvs,[nb 2]sem, orhv.[29] It is defined as
Sine, cosine, and versine of angleθ in terms of aunit circle with radius 1, centered atO. This figure also illustrates the reason why the versine was sometimes called thesagitta, Latin forarrow.[17][30] If the arcADB of the double-angleΔ = 2θ is viewed as a "bow" and thechordAB as its "string", then the versineCD is clearly the "arrow shaft".Graphs of historical trigonometric functions compared with sin and cos – inthe SVG file, hover over or click a graph to highlight it
The ordinarysine function (see note on etymology) was sometimes historically called thesinus rectus ("straight sine"), to contrast it with the versed sine (sinus versus).[31] The meaning of these terms is apparent if one looks at the functions in the original context for their definition, aunit circle:
For a verticalchordAB of the unit circle, the sine of the angleθ (representing half of the subtended angleΔ) is the distanceAC (half of the chord). On the other hand, the versed sine ofθ is the distanceCD from the center of the chord to the center of the arc. Thus, the sum of cos(θ) (equal to the length of lineOC) and versin(θ) (equal to the length of lineCD) is the radiusOD (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 fromC to the circle.
This figure also illustrates the reason why the versine was sometimes called thesagitta, Latin forarrow.[17][30] If the arcADB of the double-angleΔ = 2θ is viewed as a "bow" and the chordAB as its "string", then the versineCD 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 theabscissa (the horizontal axis of a graph).[30]
In 1821,Cauchy used the termssinus versus (siv) for the versine andcosinus versus (cosiv) for the coversine.[15][16][nb 1]
The trigonometric functions can be constructed geometrically in terms of aunit circle centered atO.
Asθ goes to zero, versin(θ) is the difference between two nearly equal quantities, so a user of atrigonometric table for the cosine alone would need a very high accuracy to obtain the versine in order to avoidcatastrophic cancellation, making separate tables for the latter convenient.[12] Even with a calculator or computer,round-off errors make it advisable to use the sin2 formula for small θ.
Another historical advantage of the versine is that it is always non-negative, so itslogarithm is defined everywhere except for the single angle (θ = 0, 2π, …) where it is zero—thus, one could uselogarithmic tables for multiplications in formulas involving versines.
In fact, the earliest surviving table of sine (half-chord) values (as opposed to thechords tabulated by Ptolemy and other Greek authors), calculated from theSurya 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°).[31]
The versine appears as an intermediate step in the application of thehalf-angle formula sin2(θ/2) =1/2versin(θ), derived byPtolemy, that was used to construct such tables.
The haversine, in particular, was important innavigation because it appears in thehaversine formula, which is used to reasonably accurately compute distances on an astronomicspheroid (see issues with theEarth's radius vs. sphere) given angular positions (e.g.,longitude andlatitude). One could also use sin2(θ/2) directly, but having a table of the haversine removed the need to compute squares and square roots.[12]
The first known English equivalent to atable of haversines was published by James Andrew in 1805, under the name "Squares of Natural Semi-Chords".[33][34][17]
In 1835, the termhaversine (notated naturally ashav. orbase-10 logarithmically aslog. haversine orlog. havers.) was coined[35] byJames Inman[14][36][37] in the third edition of his workNavigation and Nautical Astronomy: For the Use of British Seamen to simplify the calculation of distances between two points on the surface of the Earth usingspherical trigonometry for applications in navigation.[3][35] Inman also used the termsnat. versine andnat. vers. for versines.[3]
Other high-regarded tables of haversines were those of Richard Farley in 1856[33][38] and John Caulfield Hannyngton in 1876.[33][39]
The haversine continues to be used in navigation and has found new applications in recent decades, as in Bruce D. Stark's method for clearinglunar distances utilizingGaussian logarithms since 1995[40][41] or in a more compact method forsight reduction since 2014.[29]
While the usage of the versine, coversine and haversine as well as theirinverse functions can be traced back centuries, the names for the other fivecofunctions appear to be of much younger origin.
Comparison of the versine function with three approximations to the versine functions, for angles ranging from 0 to 2πComparison of the versine function with three approximations to the versine functions, for angles ranging from 0 toπ/2
When the versinev is small in comparison to the radiusr, it may be approximated from the half-chord lengthL (the distanceAC shown above) by the formula[51]
Alternatively, if the versine is small and the versine, radius, and half-chord length are known, they may be used to estimate the arc lengths (AD in the figure above) by the formulaThis formula was known to the Chinese mathematicianShen Kuo, and a more accurate formula also involving the sagitta was developed two centuries later byGuo Shoujing.[52]
A more accurate approximation used in engineering[53] is
The termversine 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 distancev from the chord to the curve (usually at the chord midpoint) is called aversine measurement. For a straight line, the versine of any chord is zero, so this measurement characterizes the straightness of the curve. In thelimit as the chord lengthL goes to zero, the ratio8v/L2 goes to the instantaneouscurvature. This usage is especially common inrail transport, where it describes measurements of the straightness of therail tracks[54] and it is the basis of theHallade method forrail surveying.
The termsagitta (often abbreviatedsag) is used similarly inoptics, for describing the surfaces oflenses andmirrors.
^abSome English sources confuse the versed cosine with the coversed sine. Historically (f.e. inCauchy, 1821), thesinus versus (versine) was defined as siv(θ) = 1−cos(θ), thecosinus versus (what is now also known as coversine) as cosiv(θ) = 1−sin(θ), and the vercosine as vcsθ = 1+cos(θ). However, in their 2009 English translation of Cauchy's work,Bradley and Sandifer associate thecosinus versus (and cosiv) with theversed cosine (what is now also known as vercosine) rather than thecoversed sine. Similarly, in their 1968/2000 work,Korn and Korn associate the covers(θ) function with theversed cosine instead of thecoversed sine.
^abThe abbreviationhvs sometimes used for the haversine function in signal processing and filtering is also sometimes used for the unrelatedHeaviside step function.
^abcdeHall, Arthur Graham; Frink, Fred Goodrich (January 1909). "Review Exercises [100] Secondary Trigonometric Functions". Written at Ann Arbor, Michigan, USA.Trigonometry. Vol. Part I: Plane Trigonometry. New York, USA:Henry Holt and Company / Norwood Press / J. S. Cushing Co. - Berwick & Smith Co., Norwood, Massachusetts, USA. pp. 125–127. Retrieved2017-08-12.
^Fulst, Otto (1972). "17, 18". In Lütjen, Johannes; Stein, Walter; Zwiebler, Gerhard (eds.).Nautische Tafeln (in German) (24 ed.). Bremen, Germany: Arthur Geist Verlag.
^Andrew, James (1805).Astronomical and Nautical Tables with Precepts for finding the Latitude and Longitude of Places. Vol. T. XIII. London. pp. 29–148. (A 7-placehaversine table from 0° to 120° in intervals of 10".)
^White, J. D. (February 1926). "(unknown title)".Nautical Magazine. (NB. According toCajori, 1929, this journal has a discussion on the origin of haversines.)
^White, J. D. (July 1926). "(unknown title)".Nautical Magazine. (NB. According toCajori, 1929, this journal has a discussion on the origin of haversines.)
^Farley, Richard (1856).Natural Versed Sines from 0 to 125°, and Logarithmic Versed Sines from 0 to 135°. London.{{cite book}}: CS1 maint: location missing publisher (link) (Ahaversine table from 0° to 125°/135°.)
^Hannyngton, John Caulfield (1876).Haversines, Natural and Logarithmic, used in Computing Lunar Distances for the Nautical Almanac. London.{{cite book}}: CS1 maint: location missing publisher (link) (A 7-placehaversine table from 0° to 180°,log. haversines at intervals of 15",nat. haversines at intervals of 10".)
^abcdefvan den Doel, Kees (2010-01-25)."jass.utils Class Fmath".JASS - Java Audio Synthesis System. 1.25.Archived from the original on 2007-09-02. Retrieved2015-10-26.