# Positional Astronomy: Spherical trigonometry

A great-circle arc, on the sphere, is the analogue of a straight line, on the plane.

Where two such arcs intersect, we can define the spherical angle
either as angle between the tangents to the two arcs, at the point of intersection,
or as the angle between the planes of the two great circles where they intersect at the centre of the sphere.
(Spherical angle is only defined where arcs of great circles meet.)

A spherical triangle is made up of three arcs of great circles, all less than 180°.
The sum of the angles is not fixed, but will always be greater than 180°.
If any side of the triangle is exactly 90°, the triangle is called quadrantal.

There are many formulae relating the sides and angles of a spherical triangle.
In this course we use only two: the sine rule and the cosine rule.

Consider a triangle ABC on the surface of a sphere with radius = 1.

(See note)

We use the capital letters A, B, C to denote the angles at these corners;
we use the lower-case letters a, b, c to denote the opposite sides.
(Remember that, in spherical geometry, the side of a triangle is the arc of a great circle,
so it is also an angle.)

Turn the sphere so that A is at the "north pole",
and let arc AB define the "prime meridian".

Set up a system of rectangular axes OXYZ:
O is at the centre of the sphere;
OZ passes through A;
OX passes through arc AB (or the extension of it);
OY is perpendicular to both.
Find the coordinates of C in this system:
x = sin(b) cos(A)
y = sin(b) sin(A)
z = cos(b)

Now create a new set of axes,
keeping the y-axis fixed and moving the "pole" from A to B
(i.e. rotating the x,y-plane through angle c).
The new coordinates of C are
x' = sin(a) cos(180-B) = - sin(a) cos(B)
y' = sin(a) sin(180-B) =    sin(a) sin(B)
z' = cos(a)

The relation between the old and new systems
is simply a rotation of the x,z-axes through angle c:
x' = x cos(c) - z sin(c)
y' = y
z' = x sin(c) + z cos(c)

That is:
- sin(a) cos(B) = sin(b) cos(A) cos(c) - cos(b) sin(c)
sin(a) sin(B)  = sin(b) sin(A)
cos(a) = sin(b) cos(A) sin(c) + cos(b) cos(c)

These three equations give us the formulae for solving spherical triangles.

The first equation is the transposed cosine rule,
which is sometimes useful but need not be memorised.

The second equation gives the sine rule. Rearrange as:
sin(a)/sin(A) = sin(b)/sin(B)
Similarly,
sin(b)/sin(B) = sin(c)/sin(C), etc.

So the sine rule is usually expressed as:
sin(a)/sin(A) = sin(b)/sin(B) = sin(c)/sin(C)

The third equation gives the cosine rule:
cos(a) = cos(b) cos(c) + sin(b) sin(c) cos(A)
and similarly:
cos(b) = cos(c) cos(a) + sin(c) sin(a) cos(B)
cos(c) = cos(a) cos(b) + sin(a) sin(b) cos(C)

Here they are together:

sine rule:
sin(a)/sin(A) = sin(b)/sin(B) = sin(c)/sin(C)

cosine rule:
cos(a) = cos(b) cos(c) + sin(b) sin(c) cos(A)
cos(b) = cos(c) cos(a) + sin(c) sin(a) cos(B)
cos(c) = cos(a) cos(b) + sin(a) sin(b) cos(C)

The cosine rule will solve almost any triangle if it is applied often enough.
The sine rule is simpler to remember but not always applicable.

Note that both formulae can suffer from ambiguity:
E.g. if the sine rule yields
sin(x) = 0.5,
then x may be 30° or 150°.
Or, if the cosine rule yields
cos(x) = 0.5,
then x may be 60° or 300° (-60°).
In this case, there is no ambiguity if x is a side of the triangle, as it must be less than 180°,
but there could still be uncertainty if an angle of the triangle was positive or negative.

So, when applying either formula, check to see if the answer is sensible.
If in doubt, recalculate using the other formula, as a check.

Exercise:

Alderney, in the Channel Islands, has longitude 2°W, latitude 50°N.
Winnipeg, in Canada, has longitude 97°W, latitude 50°N.
How far apart are they, in nautical miles, along a great-circle arc?

If you set off from Alderney on a great-circle route to Winnipeg,
in what direction (towards what azimuth) would you head?

Click here for the answer.

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