
Return to Air Navigation The definitions of two elementary terms, the GREAT CIRCLE and the SMALL CIRCLE, are the basis for defining other important terms used in navigation. An intersection of a plane and a sphere defines a circle. GREAT AND SMALL CIRCLES A GREAT CIRCLE is a circle on a sphere's surface whose plane is passing exactly through the center of the sphere. In other words, a great circle divides a sphere into two symmetrical parts. An arc on a great circle represents the shortest distance between two points on a sphere. An indefinite number of great circles may be drawn through any given point on a sphere but only one great circle can pass through two points (except when these points are diametrically opposite). 
SMALL CIRCLE  Any circle on a surface of a sphere which is not a great circle is a SMALL CIRCLE. A SMALL CIRCLE is any circle on a surface of a sphere whose plane is not passing through the sphere's center.
EARTH'S SHAPE AND SIZE
For all practical purposes, the earth is assumed to be
a perfect sphere. The Earth's surface
irregularities such as mountains, valleys, oceans and the slight flatness at the poles
hardly affect the shape of the earth. The largest height variation between the highest
mountain and the deepest point in the bottom of the ocean is about 12 miles. The Earth's
diameter at the equator and the polar diameter respectively are 6,887.91 and 6,864.57
nautical miles. The difference of the equatorial diameter and the polar diameter is 0.003%
which justifies the approximation of the earth as a perfect sphere.
EARTH'S POLES AND THE EQUATOR
The axis of the earth rotation is tilted (approximately
63.5°) relative to earth's orbit plane about the sun. This inclination causes the
differentiation of the sun heat distribution upon the surface of the Earth, thus is the
origin of the weather phenomena including the seasons of the year.
The Earth's POLES are the intersections of the surface of the Earth with the rotation axis. The
poles and the rotation axis provide the prime navigation references. The EQUATOR is a GREAT
CIRCLE whose plane is perpendicular to rotation axis of the Earth.
he circumference of the Earth on the equator is 21639.007 nautical miles yielding to the
following result: 21639.007 NM / 360° = 60.1 Nutical miles or ONE degree arc on the Equator
equals approximately to SIXTY NAUTICAL MILES. It further
follows that ONE MINUTE ARC on the Equator equals approximately to ONE NAUTICAL MILE.
THE EARTH COORDINATES
The nature of a sphere is that any point on its surface can be expressed by using a network
of reference lines. The Earth Coordinates System provides lines of latitude and longitude as
reference. The prime reference lines are the Equator and the Prime meridian. To differentiate
a point on the surface of the Earth its position is given in relation to these two references.
LATITUDE and LONGITUDE
PARALLELS OF LATITUDE, or parallels in short, are called so because they are SMALL CIRCLES
whose plane is parallel to the equator. The Parallels are used to specify the arc, in degrees,
of which a point is located relative to the Equator.
A MERIDIAN (line of longitude) is an arc on a GREAT CIRCLE that extend from one pole to the
opposite pole (Half of a Great Circle). The PRIME MERIDIAN which used in international
navigation was arbitrarily selected in 1884 as the meridian who passes through the Royal
Greenwich Observatory in Greenwich England. The MERIDIANS are used to specify the arc, in
degrees, of which a point is situated relative to the Prime Meridian. Parallels are measured 90° north and 90° south of the Equator while meridians are measured 180° East and West of the ZERO MERIDIAN (Prime Meridian).
The Prime Meridian and the 180° meridian (also called the International Date Line) together,
are forming one great circle whose plane intersects the North and South poles.
LOCATION
The earth's coordinate system provides means to accurately locate any point on the surface of earth. Such position is given in an arc measured in degrees, minutes and seconds. For example: A position report of N31° 15' 00" W86° 20' 00" indicates that this position is situated at 31° ,15' and 00" North of the Equator and 86° 20" and 00" West of the Prime (Greenwich) Meridian as shown in the illustrations.
NOTATION
The most common notation that is used in aviation today is in degrees and minutes. The seconds
are expressed in tenths of a minute. For example S43 30.5 E75 28.2 means that this position
is located 43 degrees, 30 minutes and one half of a minute (30 seconds) south of the Equator,
and 75 degrees, 28 minutes an two tenths of a minute (12 seconds) east of the Prime Meridian.
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