# Versine

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The **versine** or **versed sine**, versin(*θ*), is a trigonometric function equal to 1 − cos(*θ*) and 2sin^{2}(½*θ*). It appeared in some of the earliest trigonometric tables and was once widespread, but it is now little-used.{{ safesubst:#invoke:Unsubst||date=__DATE__ |$B=
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}} There are several related functions, most notably the **haversine**, half the versine, known in the haversine formula of navigation.

It is also written as vers(*θ*) or ver(*θ*). In Latin, it is known as the *sinus versus* (flipped sine) or the *sagitta* (arrow).

## Contents

## Related functions

There are several other related functions:

- The
**versed cosine**, or**vercosine**, written - The
**coversed sine**, or**coversine**, written and sometimes abbreviated to - The
**coversed cosine**, or**covercosine**, written - The
**haversed sine**, or**haversine**, written , most famous from the haversine formula used historically in navigation - The
**haversed cosine**, or**havercosine**, written - The
**hacoversed sine**, also called**hacoversine**or**cohaversine**and written - The
**hacoversed cosine**, also called**hacovercosine**or**cohavercosine**and written - The
**exsecant**, written - The
**excosecant**, written

## Definitions

## Derivatives and Integrals

## History and applications

Historically, the versed sine was considered one of the most important trigonometric functions,^{[2]}^{[3]}^{[4]} but it has fallen from popularity in modern times due to the availability of computers and scientific calculators.^{[5]}Template:Or As θ goes to zero, versin(θ) 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, making separate tables for the latter convenient.^{[4]} Even with a computer, round-off errors make it advisable to use the sin^{2} formula for small θ. Another historical advantage of the versine is that it is always non-negative, so its logarithm is defined everywhere except for the single angle (*θ* = 0, 2*π*,...) where it is zero—thus, one could use logarithmic tables for multiplications in formulas involving versines.

The haversine, in particular, was important in navigation because it appears in the haversine formula, which is used to reasonably accurately compute distances on a sphere (see issues with the Earth`s radius vs. sphere) given angular positions (e.g., longitude and latitude). One could also use sin^{2}(*θ*/2) directly, but having a table of the haversine removed the need to compute squares and square roots.^{[4]} The term *haversine* was, apparently, coined in a navigation text for just such an application.^{[6]}

In fact, the earliest surviving table of sine (half-chord) values (as opposed to the chords tabulated by Ptolemy and other Greek authors), calculated from the Surya Siddhantha of India dated back to 3rd century BC, was a table of values for the sine and versed sine (in 3.75° increments from 0 to 90°).^{[2]} The versine appears as an intermediate step in the application of the half-angle formula sin^{2}(*θ*/2) = versin(*θ*)/2, derived by Ptolemy, that was used to construct such tables.

The ordinary *sine* function (see note on etymology) was sometimes historically called the *sinus rectus* ("vertical sine"), to contrast it with the versed sine (*sinus versus*).^{[2]} The meaning of these terms is apparent if one looks at the functions in the original context for their definition, a unit circle, shown at right. For a vertical chord *AB* of the unit circle, the sine of the angle θ (half the subtended angle) is the distance *AC* (half of the chord). On the other hand, the versed sine of θ is the distance *CD* from the center of the chord to the center of the arc. Thus, the sum of cos(*θ*) = *OC* and versin(θ) = *CD* is the radius *OD* = 1. Illustrated this way, the sine is vertical (*rectus*, lit. "straight") while the versine is horizontal (*versus*, lit. "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,^{[1]} from the Arabic usage *sahem*^{[3]} of the same meaning. This itself comes from the Indian word 'sara' (arrow) that was commonly used to refer to "utkrama-jya". If the arc *ADB* is viewed as a "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).^{[1]}

One period (0 < *θ* < *π*/2) of a versine or, more commonly, a haversine waveform is also commonly used in signal processing and control theory as the shape of a pulse or a window function, because it smoothly (continuous in value and slope) "turns on" from zero to one (for haversine) and back to zero. In these applications, it is given yet another name: raised-cosine filter or Hann function.

## Approximations

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

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

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.^{[8]}

A more accurate approximation used in engineering^{[9]} is

## "Versines" of 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 as the chord length *L* goes to zero, the ratio 8*v*/*L*^{2} goes to the instantaneous curvature. This usage is especially common in rail transport, where it describes measurements of the straightness of the rail tracks^{[10]} and it is the basis of the Hallade method for rail surveying. The term 'sagitta' (often abbreviated *sag*) is used similarly in optics, for describing the surfaces of lenses and mirrors.

## See also

## References

- ↑
^{1.0}^{1.1}^{1.2}Template:OED - ↑
^{2.0}^{2.1}^{2.2}{{#invoke:citation/CS1|citation |CitationClass=book }} - ↑
^{3.0}^{3.1}Template:Cite web - ↑
^{4.0}^{4.1}^{4.2}Template:Cite web - ↑ At one time the versine was important in laying out railroad curves and superelevation. Contemporary high school/college algebra/trig texts do not mention it. See, for example, Beecher, Penna, Bittinger.
*Algebra and Trigonometry.*4th ed. Boston: Addison-Wesley, 2012. - ↑ Template:OED Cites coinage by Prof. Jas. Inman, D. D., in his
*Navigation and Nautical Astronomy*, 3rd ed. (1835). - ↑ {{#invoke:citation/CS1|citation |CitationClass=citation }}.
- ↑ {{#invoke:citation/CS1|citation |CitationClass=citation }}.
- ↑ {{#invoke:citation/CS1|citation |CitationClass=citation }}.
- ↑ {{#invoke:Citation/CS1|citation |CitationClass=journal }}