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# Arcs and Triangles Help (page 2)

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By — McGraw-Hill Professional
Updated on Oct 3, 2011

## Spherical Angles

The sides of any spherical triangle are curves, not straight lines. The interior angles of a spherical triangle are called spherical angles. A spherical angle can be symbolized ∠ sph . There are two ways to define this concept.

Definition 1. Consider the three planes defined by the vertices of the spherical triangle and the center of the sphere. In Fig. 11-4, these are plane PQR, plane PQS, and plane PRS.

Fig. 11–4. Each side of a spherical triangle is an arc of a great circle spanning less than 180°. The vertices of the spherical triangle, along with the center of the sphere itself, define planes as discussed in the text.

The angles between the arcs are defined like this:

• The angle between planes PQR and PQS, which intersect in line PQ, defines the angle between arcs r and s, whose vertex is at point Q
• The angle between planes PQR and PRS, which intersect in line PR, defines the angle between arcs q and s, whose vertex is at point R
• The angle between planes PQS and PRS, which intersect in line PS, defines the angle between arcs q and r, whose vertex is at point S

How do we express the measure of an angle between two planes, also known as a dihedral angle? It’s easy to intuit, but hard to explain. Figure 11-5 illustrates the concept. Suppose two planes X and Y intersect in a common line L. Consider line M in plane X and line N in plane Y, such that line M is perpendicular to line L, line N is perpendicular to line L, and lines M and N both meet somewhere on line L. The angle between the intersecting planes X and Y can be represented in two ways. The first angle, whose measure is denoted by u, is the smaller angle between lines M and N. The second angle, whose measure is denoted by v, is the larger angle between lines M and N. The smaller angle is acute, and the larger angle is obtuse. When we talk about the angle at the vertex of a spherical triangle, we must pay attention to whether it is acute or obtuse!

Fig. 11–5. The angle between two intersecting planes X and Y can be represented by u , the acute angle between lines M and N , or by v, the obtuse angle between lines M and N.

Definition 2 . This definition is less rigorous than the first, but it is easier for some people to envision. Let’s use a real-world example. On the surface of the earth, suppose two “shortwave” radio signals arrive from two different directions after having traveled partway around the planet along great-circle arcs. If the receiving station uses a directional antenna to check the azimuth bearings (compass directions) of the signals, the curvature of the earth is not considered. The observation is made locally, over a region small enough so that the earth’s surface can be considered flat within it. The angle between two great-circle arcs on any sphere that intersect at a point Q can be defined similarly. It is the angle as measured within a circle on the sphere surrounding point Q , such that the circle is so small with respect to the sphere that the circle is essentially a flat disk (Fig. 11-6).

Fig. 11–6. The angle between two intersecting great-circle arcs, as considered within a circular region on the sphere so small that it is essentially a flat disk.

Then the great-circle arcs seem to be straight rays running off to infinity, and the angle between them can be expressed as if they lie in the geometric plane tangent to the surface of the sphere at point Q .

## Angular Sides & Equilateral Spherical Triangle Principles

### Angular Sides

The sides q , r , and s of the spherical triangle of Fig. 11-4 are often defined in terms of their arc angles (∠ SPR , ∠ QPS , and ∠ RPQ , respectively), rather than in terms of their actual lengths in linear units. When this is done, it is customary to express the arc angles in radians.

Fig. 11–4. Each side of a spherical triangle is an arc of a great circle spanning less than 180°. The vertices of the spherical triangle, along with the center of the sphere itself, define planes as discussed in the text.

If we know the radius of a sphere (call it r sph ), then the length of an arc on the sphere, in the same linear units as we use to measure the radius of the sphere, is equal to the angular measure of the arc (in radians) multiplied by r sph . Suppose we let | q |, | r |, and | s | represent the lengths of the arcs q , r , and s of Δ sph QRS in linear units, while their extents in angular radians are denoted q , r , and s . Then the following formulas hold:

In the case of the earth, the linear lengths (in kilometers) of the sides of the spherical triangle Δ sph QRS are therefore:

### Equilateral Spherical Triangle Principles

In plane geometry, if a triangle has sides that are all of the same length, then the interior angles are all of the same measure. The converse also holds true: If the interior angles of a triangle are all of equal measure, then the sides all have the same length.

There is an analogous principle for equilateral triangles on a sphere. If a spherical triangle has sides all of the same angular length, then the interior spherical angles are all of equal measure. The converse is also true: If the interior spherical angles of a spherical triangle all have the same measure, then the angular lengths of the sides are all the same. These principles are important to the solving of the two problems that follow.

## The Case Of The Expanding Triangle

Imagine what happens to an equilateral spherical triangle that starts out tiny and grows larger. (An equilateral spherical triangle has sides of equal angular length and interior spherical angles of equal measure.) An equilateral spherical triangle on the earth that measures 1 arc second on a side is almost exactly the same as a plane equilateral triangle whose sides are 30.88 meters long. The sum of the interior spherical angles, if we measure them with a surveyor’s apparatus, appears to be 180°, and each interior spherical angle appears to be an ordinary angle that measures 60°. The interior area and the perimeter can be calculated using the formulas used for triangles in a plane.

As the equilateral spherical triangle grows, the measure of each interior spherical angle increases. When each side has a length that is ¼ of a great circle (the angular length of each side is π /2 rad), then each interior spherical angle measures 90°, and the sum of the measures of the interior spherical angles is three times this, or 270°. An example is shown in Fig. 11-7B. As the spherical triangle expands further, it eventually attains a perimeter equal to the circumference of the earth. Then each side has an angular length of 2 π /3 rad. The spherical triangle has become a great circle. Its interior area has grown to half the surface area of the planet. The formulas for the perimeter and interior area of a plane triangle do not work for a spherical triangle that is considerable with respect to the size of the globe.

Fig. 11–7. Solution 2. At A, points Q and R are on the equator, and point S is at the north pole. At B, points Q and R are on the equator and are separated by 90° of longitude, while point S is at the north pole.

Now think about what happens if the equilateral spherical triangle continues to “grow” beyond the size at which it girdles the earth. The lengths of the sides get shorter, not longer, even though the measures of the interior spherical angles, and the interior area of the spherical triangle, keep increasing. Ultimately, our equilateral spherical triangle becomes so “large” that the three vertices are close together again, perhaps only 1 arc second apart. We have what looks like a triangle similar to the one we started out with—but wait! There are differences. The perimeter is the same, but the interior area is almost that of the whole earth. The inside of this triangle looks like the outside, and the outside looks like the inside. The interior spherical angles are not close to 60°, as they were in the beginning, but instead are close to 300°. They must be measured “the long way around”. The sum of their measures is approximately 900°.

This is a bizarre sort of triangle, but in theory, it’s entirely “legal.” In fact, we can keep on going past a complete circle, letting the interior area and the measures of the interior angles keep growing while the perimeter cycles between zero and the circumference of the earth, over and over. Not many people can envision such a “triangle” after six or eight trips around the world. It’s definable, but it’s also incomprehensible.

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