covercosine

The versine or versed sine is a trigonometric function found in some of the earliest (Sanskrit ''Aryabhatia'',The Āryabhaṭīya by Āryabhaṭa

Section I) trigonometric tables. 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.

Overview

The versine or versed sine is a trigonometric function already appearing in some of the earliest trigonometric tables. It is symbolized in formulas using the abbreviations , , , or . In Latin, it is known as the ''sinus versus'' (flipped sine), ''versinus'', ''versus'', or ''sagitta'' (arrow).

Expressed in terms of common trigonometric functions sine, cosine, and tangent, the versine is equal to
$\operatorname\theta = 1 - \cos \theta = 2\sin^\frac\theta2 = \sin\theta\,\tan\frac\theta2$

There are several related functions corresponding to the versine:

* The versed cosine, or vercosine, abbreviated , , or .
* The coversed sine or coversine (in Latin, ''cosinus versus'' or ''coversinus''), abbreviated , , , or
* The coversed cosine or covercosine, abbreviated , , or

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

* The haversed sine or haversine (Latin ''semiversus''), abbreviated , , , , , , , or , most famous from the haversine formula used historically in navigation
* The haversed cosine or havercosine, abbreviated , , or
* The hacoversed sine, hacoversine, or cohaversine, abbreviated , , , or
* The hacoversed cosine, hacovercosine, or cohavercosine, abbreviated , or

History and applications

Versine and coversine

The ordinary '' sine'' function ( see note on etymology) was sometimes historically called the ''sinus rectus'' ("straight sine"), to contrast it with the versed sine (''sinus versus''). 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 ''AB'' of the unit circle, the sine of the angle ''θ'' (representing half of the subtended angle ''Δ'') 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'') (equal to the length of line ''OC'') and versin(''θ'') (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, from the Arabic usage ''sahem'' 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'' of the double-angle ''Δ'' = 2''θ'' 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).

In 1821, Cauchy used the terms ''sinus versus'' (''siv'') for the versine and ''cosinus versus'' (''cosiv'') for the coversine.

Historically, the versed sine was considered one of the most important trigonometric functions.

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 in order to avoid catastrophic cancellation, making separate tables for the latter convenient. 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 its logarithm is defined everywhere except for the single angle (''θ'' = 0, 2, …) where it is zero—thus, one could use logarithmic tables for multiplications in formulas involving versines.

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 the 3rd century BC, was a table of values for the sine and versed sine (in 3.75° increments from 0 to 90°).

The versine appears as an intermediate step in the application of the half-angle formula sin² = versin(''θ''), 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 earth's radius vs. sphere) given angular positions (e.g., longitude and latitude). One could also use sin² directly, but having a table of the haversine removed the need to compute squares and square roots.

An early utilization by José de Mendoza y Ríos of what later would be called haversines is documented in 1801.

The first known English equivalent to a table of haversines was published by James Andrew in 1805, under the name "Squares of Natural Semi-Chords".

In 1835, the term ''haversines_in_probability_theory">+\cos\_...