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Navigation Essay, Research Paper
Navigation is the science of conducting a vessel to a certain destination. The two types of
navigation that I will discuss are global positioning using satellites and celestial navigation.
For centuries sailors have guided their ships across the oceans by celestial navigation or nautical
astronomy. This is the art of finding position by observing the sun, moon, stars and planets.
As they journey for some distance, travelers observe that the celestial bodies appear to change
their paths across the sky and to rise and set at new points along the horizon. Since the apparent
positions of celestial bodies thus change with time and with changes in an observer’s position on
the nearly spherical earth, the location of a ship or other craft may be determined by careful
observations of celestial bodies.
Celestial bodies – such as the stars – are so far from the earth that they appear to be located on
the inside surface of an imaginary hollow sphere. This sphere – which has an infinite radius – is
called the celestial sphere. Its center coincides with the center of the earth. All points on the
earth’s surface are considered to be projected onto the celestial sphere, as are the equator, the
parallels of latitude, and the meridians.
For the purpose of navigation, a system of coordinates is required on the celestial sphere in order
that the position of a celestial body at any time may be accurately described. One such system is
the celestial equator or equinoctial system.
In this system the celestial equator or equinoctial is the base – or primary – circle. It corresponds
to the earth’s equator. At right angles to the celestial equator are the hour circles. An hour circle
is a great circle on the celestial sphere that passes through the poles and through a celestial body
or point. Each meridian of the celestial sphere is identical with an hour circle.
The declination (dec.) of any point on the celestial sphere is its angular distance north or south
from the celestial equator, measured along the hour circle that passes through the point.
Declination on the celestial sphere corresponds to latitude on the earth’s surface.
The Greenwich hour angle (GHA) of any point or body is the angle – measured at the pole of the
celestial sphere – between the celestial meridian of Greenwich and the hour circle of the point.
The angle is measured along the celestial equator westward from the Greenwich celestial
meridian – from 0? through 360?. The GHA differs from longitude on the earth’s surface in that
longitude is measured east or west – from 0? through 180? – and remains constant. The GHA of
a body increases through each day as the earth rotates.
The GPS satellites orbit the earth twice a day, 11,000 miles above the earth, transmitting their
precise signal position and elevation. The GPS receiver acquires the signal, then measures the
interval between transmission and receipt of the to determine the distance between the receiver
and the satellite. Once the receiver has calculated this data for at least 3 satellites, its location on
the earth’s surface can be determined.
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