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The versed sine, also called the versine and, in Latin, the sinus versus ("flipped sine") or the sagitta ("arrow"), is a trigonometric function versin(θ) (sometimes further abbreviated "vers") defined by the equation:

There are also three corresponding functions:

• the coversed sine (the versed sine of the complementary angle π/2 − θ, or coversine):

• the haversed sine or haversine (half the versed sine):

• the hacoversed sine (half the coversed sine, also called the hacoversine, the cohaversine, and the havercosine):

Another similar function is the exsecant (sec θ − 1).

Historically, the versed sine was considered one of the most important trigonometric functions, but it has fallen from popularity in modern times due to the availability of computers and scientific calculators. 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. (Even with a 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 its logarithm is defined everywhere except for the single angle (θ = 0, 2π,...) 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 accurately compute distances on a sphere given angular positions (e.g., longitude and latitude). (One could also use sin²(θ/2) directly, but having a table of the haversine removed the need to compute squares and square roots.) The term haversine was, apparently, coined in a navigation text for just such an application (see references).

In fact, the earliest surviving trigonometric table, from the 4th–5th century Siddhantas from India, was a table of values for the sine and versed sine only (in 3.75° increments from 0 to 90°). This is, perhaps, even less surprising considering that the versine appears as an intermediate step in the application of the half-angle formula sin²(θ/2) = versin(θ)/2, derived by Ptolemy, that was used to construct such tables.

Sine, cosine, and versine of θ in terms of a unit circle, centered at O

As for sine, the etymology derives from a 12th century mistranslation of the Sanskrit jiva via Arabic. To contrast it with the versed sine (sinus versus), the ordinary sine function was sometimes historically called the sinus rectus ("vertical sine"). 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) while the versine is flipped on its side (versus); 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, from the Arabic usage sahem of the same meaning. 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).

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.

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 8v/L² goes to the instantaneous curvature.

This usage is especially common in rail transport, where it describes measurements of the straightness of the rail tracks (Nair, 1972).