Spherical law of cosines

Summary

In spherical trigonometry, the law of cosines (also called the cosine rule for sides[1]) is a theorem relating the sides and angles of spherical triangles, analogous to the ordinary law of cosines from plane trigonometry.

Spherical triangle solved by the law of cosines.

Given a unit sphere, a "spherical triangle" on the surface of the sphere is defined by the great circles connecting three points u, v, and w on the sphere (shown at right). If the lengths of these three sides are a (from u to v), b (from u to w), and c (from v to w), and the angle of the corner opposite c is C, then the (first) spherical law of cosines states:[2][1]

Since this is a unit sphere, the lengths a, b, and c are simply equal to the angles (in radians) subtended by those sides from the center of the sphere. (For a non-unit sphere, the lengths are the subtended angles times the radius, and the formula still holds if a, b and c are reinterpreted as the subtended angles). As a special case, for C = π/2, then cos C = 0, and one obtains the spherical analogue of the Pythagorean theorem:

If the law of cosines is used to solve for c, the necessity of inverting the cosine magnifies rounding errors when c is small. In this case, the alternative formulation of the law of haversines is preferable.[3]

A variation on the law of cosines, the second spherical law of cosines,[4] (also called the cosine rule for angles[1]) states:

where A and B are the angles of the corners opposite to sides a and b, respectively. It can be obtained from consideration of a spherical triangle dual to the given one.

Proofs

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First proof

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Let u, v, and w denote the unit vectors from the center of the sphere to those corners of the triangle. The angles and distances do not change if the coordinate system is rotated, so we can rotate the coordinate system so that   is at the north pole and   is somewhere on the prime meridian (longitude of 0). With this rotation, the spherical coordinates for   are   where θ is the angle measured from the north pole not from the equator, and the spherical coordinates for   are   The Cartesian coordinates for   are   and the Cartesian coordinates for   are   The value of   is the dot product of the two Cartesian vectors, which is  

Second proof

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Let u, v, and w denote the unit vectors from the center of the sphere to those corners of the triangle. We have u · u = 1, v · w = cos c, u · v = cos a, and u · w = cos b. The vectors u × v and u × w have lengths sin a and sin b respectively and the angle between them is C, so

sin a sin b cos C = (u × v) · (u × w) = (u · u)(v · w) − (u · v)(u · w) = cos c − cos a cos b,

using cross products, dot products, and the Binet–Cauchy identity (p × q) · (r × s) = (p · r)(q · s) − (p · s)(q · r).

Third proof

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Let u, v, and w denote the unit vectors from the center of the sphere to those corners of the triangle. Consider the following rotational sequence where we first rotate the vector v to u by an angle   followed by another rotation of vector u to w by an angle   after which we rotate the vector w back to v by an angle   The composition of these three rotations will form an identity transform.[clarification needed] That is, the composite rotation maps the point v to itself. These three rotational operations can be represented by quaternions:

 
where     and   are the unit vectors representing the axes of rotations, as defined by the right-hand rule, respectively. The composition of these three rotations is unity,   Right multiplying both sides by conjugates   we have   where   and   This gives us the identity[5][6]

 

The quaternion product on the right-hand side of this identity is given by

 

Equating the scalar parts on both sides of the identity, we have

 

Here   Since this identity is valid for any arc angles, suppressing the halves, we have

 

We can also recover the sine law by first noting that   and then equating the vector parts on both sides of the identity as

 

The vector   is orthogonal to both the vectors   and   and as such   Taking dot product with respect to   on both sides, and suppressing the halves, we have   Now   and so we have   Dividing each side by   we have

 

Since the right-hand side of the above expression is unchanged by cyclic permutation, we have

 

Rearrangements

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The first and second spherical laws of cosines can be rearranged to put the sides (a, b, c) and angles (A, B, C) on opposite sides of the equations:

 

Planar limit: small angles

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For small spherical triangles, i.e. for small a, b, and c, the spherical law of cosines is approximately the same as the ordinary planar law of cosines,

 

To prove this, we will use the small-angle approximation obtained from the Maclaurin series for the cosine and sine functions:

 

Substituting these expressions into the spherical law of cosines nets:

 

or after simplifying:

 

The big O terms for a and b are dominated by O(a4) + O(b4) as a and b get small, so we can write this last expression as:

 

History

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Something equivalent to the spherical law of cosines was used (but not stated in general) by al-Khwārizmī (9th century), al-Battānī (9th century), and Nīlakaṇṭha (15th century).[7]

See also

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Notes

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  1. ^ a b c W. Gellert, S. Gottwald, M. Hellwich, H. Kästner, and H. Küstner, The VNR Concise Encyclopedia of Mathematics, 2nd ed., ch. 12 (Van Nostrand Reinhold: New York, 1989).
  2. ^ Romuald Ireneus 'Scibor-Marchocki, Spherical trigonometry, Elementary-Geometry Trigonometry web page (1997).
  3. ^ R. W. Sinnott, "Virtues of the Haversine", Sky and Telescope 68 (2), 159 (1984).
  4. ^ Reiman, István (1999). Geometria és határterületei. Szalay Könyvkiadó és Kereskedőház Kft. p. 83.
  5. ^ Brand, Louis (1947). "§186 Great Circle Arccs". Vector and Tensor Analysis. Wiley. pp. 416–417.
  6. ^ Kuipers, Jack B. (1999). "§10 Spherical Trignometry". Quaternions and Rotation Sequences. Princeton University Press. pp. 235–255.
  7. ^ Van Brummelen, Glen (2012). Heavenly mathematics: The forgotten art of spherical trigonometry. Princeton University Press. p. 98. Bibcode:2012hmfa.book.....V.