Shape and also Size
For the majority of navigational reasons, the earth is assumed in order to
be an ideal sphere, although within reality it is not. Inspection
of the actual earth’s crust reveals there typically is a height variation of
approximately 12 miles within the top of the actual tallest mountain
with the bottom of the actual deepest point within the ocean. A more
significant deviation from round can be caused by a combination of
the actual earth’s rotation and even its structural flexibility. When taking
the particular ellipsoidal shape of the planet into account, mountains
seem quite insignificant. The actual peaks of the particular Andes are really much
farther from the center of the actual earth than Mount Everest.
Measured at the particular equator, the actual earth can be approximately 6,378,137
meters with regard to diameter, while the actual polar diameter is approximately
6,356,752.3142 meters. The difference with regard to these diameters
is actually 21,384.6858 meters, plus this particular difference will be chosen to
express the actual ellipticity of the actual earth. The actual ratio between this
difference and also the actual equatorial diameter is:
Ellipticity = 21,384.6858 = 1
6 378,137 298.257223
Since the actual equatorial diameter exceeds the polar diameter by
just 1 part throughout 298, the earth typically is nearly spherical. A symmetrical
human body having the actual same dimensions because the earth, however, with
a smooth surface, can be called an ellipsoid. The particular ellipsoid is
sometimes described because a spheroid, or an oblate spheroid.
polar north (Pn), east (E), polar south (Ps),
and west (W) represent the particular surface of the earth. Pn and in addition Ps
represent the axis of rotation. The earth rotates from west in order to
east. All the points with regard to the hemisphere Pn, W, Ps approach the particular
reader, whilst those with regard to the opposite hemisphere recede from
the particular reader. The actual circumference W-E can be generally known as the actual equator,
that is actually defined because which imaginary circle found on the surface of
the actual earth whose plane passes from the particular center of the actual earth
not to mention typically is perpendicular to be able to the actual axis of rotation.
Great Circles and also Little Circles
A fantastic circle typically is defined as a circle on the surface of a sphere
whose center and also radius usually are those of the sphere itself. It's
the largest circle that can certainly be drawn found on the sphere; it happens to be the actual
intersection with the surface of the particular earth of any plane passing
through the earth’s center. The particular arc of a terrific circle is actually the particular
shortest distance between two points on a sphere, just as a
straight line is actually the particular shortest distance between two points on a
plane.
On any sphere, an indefinitely large number of awesome circles may be drawn by any kind of point, an indefinitely large number of great circle may be drawn through any kind of 2 points not diametrically opposite.
Circles on the surface of the particular sphere, different than good circles, could be defined because little circles. A little circle can be a circle on the particular surface of the actual earth whose center and/or radius are not which of the particular sphere. A set of small circles, generally known as latitude, is actually
discussed later. In summary, the intersection of a sphere as well as a plane is an ideal circle generally if the plane passes from the center of the particular sphere not to mention a small circle when it does not.
Latitude and Longitude
The particular nature of a sphere is such that any point on it is exactly
like any different point. There is actually neither beginning nor ending
as far because differentiation of points can be concerned. Inside order which
points might be found found on the earth, some points or simply lines of
reference tend to be required thus that additional points could be located
with regard in order to them. The actual location of Raw York City with
reference in order to Washington, D.C. can be reported as a number of
miles throughout a certain direction from Washington. Any point on
the earth is found throughout this manner.
Imaginary Reference Lines
Such a system, still, does not lend itself readily to
navigation, because it would be difficult with locate a point
precisely with regard to mid-ocean without any nearby geographic
qualities with use for reference. We all use a system of coordinates
to be able to locate positions found on the earth by means of imaginary
reference lines. Many of these lines are generally well-known as parallels of latitude
and also meridians of longitude.
Latitude
As soon as a day, the earth rotates on its north-south axis, which
can be terminated by the actual 2 poles. The equatorial plane can be
constructed at the midpoint of this axis at right angles in order to it.
[Figure 1-3] An amazing circle drawn through the particular poles typically is called
a meridian, and even an infinite number of really good circles will be
constructed in this particular manner. Every meridian can be divided into four
quadrants by the particular equator not to mention the actual poles. The circle can be arbitrarily
divided into 360°, not to mention each of these quadrants contains 90°.
Longitude
The particular latitude of a point could be shown as 20° N or perhaps 20° S of the
equator, yet right now there can be no means of understanding whether or not specific point is actually
east or west of another. This particular difficulty is actually resolved by utilize of the actual
additional component of the actual coordinate system, longitude, which
is the particular measurement of this particular east-west distance. Longitude,
unlike latitude, has no natural starting point for numbering.
The particular solution has been in order to select an arbitrary starting point.
A wonderful several places currently have been selected, yet when the actual Englishspeaking
people began in order to create charts, they chose the actual
meridian through their principal observatory within Greenwich,
England, as the origin for counting longitude. This particular Greenwich
meridian typically is sometimes generally known as the actual prime meridian, though
actually it's the particular zero meridian. Longitude typically is counted east plus
west from this meridian through 180°. [Figure 1-5] Thereby,
the actual Greenwich meridian typically is the actual 0° longitude on 1 side of
the earth not to mention, after crossing the particular poles, it becomes the particular 180th
meridian (180° east and / or west of the 0° meridian).
Within summary, when a globe has the circles of latitude and also longitude
drawn upon it according with the principles described, any
point could be located found on the globe utilizing these measurements.
Distance
Distance, because previously defined, can be measured by the length of
a line joining two points. The actual standard unit of distance for
navigathon is actually the actual nautical mile (NM). The particular NM is defined
because either 6,076 feet or perhaps 1 minute of latitude. Sometimes it
is essential to be able to convert statute miles (SM) to be able to NM and vice
versa. This particular conversion is actually conveniently completed with the following ratio:
Number of statute miles = 76
Number of nautical miles 66
Direction
Remember, direction can be the actual position of you point throughout space
relative to be able to a different without reference to be able to the distance between
them. The particular time honored point system for specifying a
direction because north, north-northwest, northwest, westnorthwest,
west, etc., is actually not adequate for modern navigation.
It has been replaced for the majority of reasons by a numerical system.
The actual numerical system divides the horizon into
360°, starting with north as 000° and continuing clockwise
by east 090°, south 180°, west 270°, and also back to be able to north.
Great Circle and even Rhumb Line Direction
The actual direction of the amazing circle, shown throughout Figure 1-10, makes
an angle of regarding 40° with the meridian near Washington,
D.C., about 85° with all the meridian near Iceland, and also a still
better angle with the meridian near Moscow. Easily put,
the particular direction of the superb circle can be consistently changing as
progress is actually prepared along the actual route and also can be different at each and every
point along the actual really good circle. Flying such a route requires
constant change of direction and in addition would be difficult with fly
below ordinary conditions. Still, it's the particular many desirable route
because it is the shortest distance between any kind of two points.
A line that makes the actual same angle with every meridian typically is generally known as
a rhumb line. An aircraft carrying a constant true heading
would be flying a rhumb line. Flying this particular sort of path results
inside a better distance traveled, yet it really is easier to steer. In the event that
continued, a rhumb line spirals toward the actual poles in a continual
true direction however, never reaches them. The actual spiral formed typically is
called a loxodrome or perhaps loxodromic curve. Between 2 points on the earth, the fantastic circle is actually shorter
than the rhumb line, however the actual difference typically is negligible for brief
distances (except inside excellent latitudes) or simply in the event that line approximates
a meridian or perhaps the actual equator.
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