BOOK IX.

SPHERICAL GEOMETRY

Definitions.

1. A sphere is a solid bounded by a curved surface, all the points of which are equally distant from a point within, callod the center.

The sphere may be conceived to be described by the revolution of a semicircle. ADB, about its diameter AB, which remains unmoved.

2. The radius of a sphere, is a straight line drawn from the center to any point of the surface. The diameter, or axis, is a line passing through the center, and terminated. each way by the surface.

D

A

B

All the radii of a sphere are equal; all the diameters are also equal, and each double of the radius.

3. It will be shown (Prop. I.), that every section of a sphere made by a plane is a circle. A great circle is a section made by a plane which passes through the center of the sphere. Any other section made by a plane is called a smalt circle.

4. A plane touches a sphere, when it meets the sphere, but, being produced, does not cut it.

5. The pole of a circle of a sphere, is a point in the surface equally distant from every point in the circumference of this circle. It will be shown (Prop. V.), that every circle, whether great or small, has two poles.

6. A spherical triangle is a part of the surface of a sphere, bonded by three arcs of great circles, each of which is less than a semicircumference. These arcs are called the sides of the triangle; and the angles which their planes make with each other, are the angles of the triangle.

7. A spherical triangle is called right-angled, isosceles or equilateral, in the same cases as a plane triangle.

8. A spherical polygon is a part of the surface of a sphere bounded by several arcs of great circles.

9. A lune is a part of the surface of a sphere included between the halves of two great circles.

10. A spherical wedge, or ungula, is that portion of the sphere included between the same semicircles, and has the lune for its base.

11. A spherical pyramid is a portion of the sphere included between the planes of a solid angle, whose vertex is at the center. The base of the pyramid is the spherical polygon intercepted by those planes.

12. A zone is a part of the surface of a sphere included between two parallel planes. 13. A spherical segment is a portion of the sphere included between two parallel planes. 14. The bases of the segment are the sections of the sphere; the altitude of the segment, or zone, is the distance between the

A

sections. One of the two planes may touch the sphere, in which case the segment has but one base. 15. A spherical sector is a solid described by the revolution of a circular sector, in the same manner as the sphere is described by the revolution of a semicircle.

While the semicircle ADB, revolving round its diameter AB, describes a sphere, every circular sector, as ACE or ECD, describes a spherical sector.

16. Two angles which are together equal to two right angles; or two arcs

D

E

B

which are together equal to a semicircumference, are called the supplements of each other.

PROPOSITION I. THEOREM.

Every section of a sphere, made by i plane, is a circle

Let ABD be a section, made by a plane, in a sphere whose center is C. From the point C draw CE perpendicu- A lar to the plane ABD; and draw lines CA, CB, CD, &c., to different points of the curve ABD which bounds the section.

The oblique lines CA, CB, CD are equal, because they are radii of the

D

B

sphere; therefore they are equally distant from the perpen. dicular CE (Prop. V., Cor., B. VII.). Hence all the lines EA, EB, ED are equal; and, consequently, the section ABD is a circle, of which E is the center. Therefore, every section, &c.

Cor. 1. If the section passes through the center of the sphere, its radius will be the radius of the sphere; hence all great circles of a sphere are equal to each other.

Cor. 2. Two great circles always bisect each other; for, since they have the same center, their common section is a diameter of both, and therefore bisects both.

Cor. 3. Every great circle divides the sphere and its surface into two equal parts. For if the two parts are separated and applied to each other, base to base, with their convexities turned the same way, the two surfaces must coincide; otherwise there would be points in these surfaces unequally distant from the center.

Cor. 4. The center of a small circle, and that of the sphere, are in a straight line perpendicular to the plane of the small circle.

Cor. 5. The circle which is furthest from the center is the least; for the greater the distance CE, the less is the chord AB, which is the diameter of the small circle ABD.

Cor. 6. An arc of a great circle may be made to pass. through any two points on the surface of a sphere; for the two given points, together with the center of the sphere, make three points which are necessary to determine the position of a plane. If, however, the two given points were sit uated at the extremities of a diameter, these two points and the center would then be in one straight line, and any num ber of great circles might be made to pass through them.

PROPOSITION II. THEOREM.

Any two sides of a spherical triangle are greater than the third.

Let ABC be a spherical triangle; any two sides as, AB, BC, are together greater A than the third side AC.

B

Let D be the center of the sphere; and join AD, BD, CD. Conceive the planes ADB, BDC, CDA to be drawn, forming a solid angle at D. The angles ADB, BDC, CDA will be measured by AB, BC, CA, the sides of the spherical triangle. when a solid angle is formed by three plane angles, the sum of any two of them is greater than the third (Prop. XVII., B. VII.); hence any two of the arcs AB, BC, CA must b greater than the third. Therefore, any two sides, &c.

But

D

PROPOSITION III. THEOREM.

The shortest path from one point to another on the surface of a sphere, is the arc of a great circle joining the two given points.

Let A and B be any two points on the surface of a sphere, and let ADB be the arc of a great circle which joins them; then will the line ADB be the shortest path from A to B on the surface of the sphere.

For, if possible, let the shortest path from A to B pass through C, a point situated out of the arc of a great circle ADB. Draw AC, CB, arcs of great circles, and take BD equal to BC.

ла

B

D

By the preceding theorem, the arc ADB is less than AC+ CB. Subtracting the equal arcs BD and BC, there will remain AD less than AC. Now the shortest path from B tọ C, whether it be an arc of a great circle, or some other line, is equal to the shortest path from B to D; for, by revolving BC around B, the point C may be made to coincide with D, and thus the shortest path from B to C must coincide with the shortest path from B to D. But the shortest path from A to B was supposed to pass through C; hence the shortest path from A to C, can not be greater than the shortest path from A to D.

Now the arc AD has been proved to be less than AC; and therefore if AC be revolved about A until the point C falls on the arc ADB, the point C will fall between D and B. Hence the shortest path from C to A must be greater than. the shortest path from D to A; but it has just been proved not to be greater, which is absurd. Consequently, no point of the shortest path from A to B, can be out of the arc of a great circle ADB. Therefore, the shortest path, &c.

PROPOSITION IV. THEOREM.

The sum of the sides of a spherical polygon, is less than the circumference of a great circle.

Let ABCD be any spherical polygon; then will the sum of the sides AB, BC, CD, DA be less than the circumference of a great circle.

A

D

B

Let E be the center of the sphere, and join AE, BE, CE, DE. The solid angle at E is contained by the plane angles AEB, BEC, CED, DEA, which together are less than four right angles (Prop. XVIII., B. VII.). Hence the sides AB, BC, CD, DA, which are the measures of these angles, are together less than four quadrants described with the radius AE; that is, than the circumference of a great circle. Therefore, the sum of the sides, &c.

E

PROPOSITION V. THEOREM.

The extremities of a diameter of a sphere, are the poles of all circles perpendicular to that diameter.

Let AB be a diameter perpendicular to CDE, a great circle of a sphere, and also to the small circle FGH then will A and B, the extremities of the diameter, be the poles of both these circles.

For, because AB is perpendicular to the plane CDE, it is perpendicular to every straight line CI, DI, EI, &c.. drawn through its foot in the plane;

F

Α

B

hence all the arcs AC, AD, AE, &c., are quarters of the cu

E