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time are covered fully in appropriate chapters. These terms, and others similar to them, represent definite
quantities or conditions which may be measured in several different ways. For example, the position of
an aircraft may be expressed in coordinates such as a certain latitude and longitude. The position may
also be expressed as 10 miles south of a certain city. The study of navigation demands the navigator
learn how to measure quantities such as those just defined and how to apply the units by which they are
expressed.
Section 1B— The Earth
1.3. Shape and Size. For most navigational purposes, the earth is assumed to be a perfect sphere,
although in reality it is not. Inspection of the earth's crust reveals there is a height variation of
approximately 12 miles from the top of the tallest mountain to the bottom of the deepest point in the
ocean. A more significant deviation from "round" is caused by a combination of the earth's rotation and
its structural flexibility. When you take the ellipsoidal shape of the planet into account, mountains seem
rather insignificant. The peaks of the Andes are much farther from the center of the earth than Mount
Everest.
1.3.1. Measured at the equator, the earth is approximately 6,378,137 meters in diameter, while the polar
diameter is approximately 6,356,752.3142 meters. The difference in these diameters is 21,384.6858
meters, and this difference may be used to express the ellipticity of the earth. The ratio between this
difference and the equatorial diameter is:
1.3.2. Since the equatorial diameter exceeds the polar diameter by only 1 part in 298, the earth is nearly
spherical. A symmetrical body having the same dimensions as the earth, but with a smooth surface, is
called an ellipsoid. The ellipsoid is sometimes described as a spheroid, or an oblate spheroid.
1.3.3. In Figure 1.1, Pn, E, Ps, and W represent the surface of the earth, and Pn-Ps represents the axis of
rotation. The earth rotates from W to E. All points in the hemisphere Pn, W, Ps approach the reader,
while those in the opposite hemisphere recede from the reader. The circumference W-E is called the
equator, which is defined as that imaginary circle on the surface of the earth whose plane passes through
the center of the earth and is perpendicular to the axis of rotation.
AFPAM11-216 1 MARCH 2001 21
Figure 1.1. Schematic Representation of the Earth Showing Axis of Rotation and Equator.
1.4. Great Circles and Small Circles. A great circle is defined as a circle on the surface of a sphere
whose center and radius are those of the sphere itself. It is the largest circle that can be drawn on the
sphere; it is the intersection with the surface of the earth of any plane passing through the earth's center.
1.4.1. The arc of a great circle is the shortest distance between two points on a sphere, just as a straight
line is the shortest distance between two points on a plane. On any sphere, an indefinitely large number
of great circles may be drawn through any point, though only one great circle may be drawn through any
two points not diametrically opposite. Several great circles are shown in Figure 1.2.
1.4.2. Circles on the surface of the sphere other than great circles may be defined as small circles. A
small circle is a circle on the surface of the earth whose center and/or radius are not that of the sphere. A
set of small circles, called latitude, is discussed later.
1.4.3. In summary, the intersection of a sphere and a plane is a great circle if the plane passes through the
center of the sphere and a small circle if it does not.
1.5. Latitude and Longitude. The nature of a sphere is such that any point on it is exactly like any other
point. There is neither beginning nor ending as far as differentiation of points is concerned. In order that
points may be located on the earth, some points or lines of reference are necessary so that other points
may be located with regard to them. The location of New York City with reference to Washington DC
can be stated as a number of miles in a certain direction from Washington. Any point on the earth can be
located in this manner.
1.5.1. Imaginary Reference Lines. Such a system, however, does not lend itself readily to navigation,
because it would be difficult to locate a point precisely in midocean without any nearby geographic
22 AFPAM11-216 1 MARCH 2001
features to use for reference. We use a system of coordinates to locate positions on the earth by means of
imaginary reference lines. These lines are known as parallels of latitude and meridians of longitude.
Figure 1.2. A Great Circle Is the Largest Circle in a Sphere.
1.5.2. Latitude. Once a day, the earth rotates on its north-south axis which is terminated by the two
　
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