Rene-NASA-Mooned-America

Space Navigation / Chap. 7 p. 58
tude is artificial. It starts with an arbitrarily chosen point and had to wait for the development
of an accurate seagoing clock before it was helpful for navigation. Once the west had an
accurate sea going clock and it's astronomers provided adequate almanacs or ephemeri-des,
we began to use the stars for navigation.
The GP (Geographical Position) of any celestial body is the ground point directly under
a body at its zenith (directly overhead) as shown below.
To determine a GP it is necessary to have a sextant to measure the angle of elevation of
that body (the Sun, planets, selected navigational stars, or the Moon) from our horizon. The
exact time of that "shot" (observation) must also be known. From that data you can find the
geographic position (GP) of the body from an almanac.
An almanac consists of a series of tables prepared by astronomers that show the positions
of the Sun, Moon, major planets, and the navigational stars every hour of every day for
the year. When an observation isn't made on an even hour, the GP can be easily extrapolated
from the tables.
With the advent of electronic navigation systems, celestial navigation became unnecessary.
However, just like the optical method, these systems (like loran) also depend on measuring
your distance from the known geographic locations of land-based transmitters. In
general, each of these systems requires the measurements from three different bodies for a
fix. The exception to this rule is the use of a high noon shot of the Sun. In itself, it gives you
latitude, and if you know the time it also gives you longitude.
Each system generates a single circular line of position (LOP) at a radius equal to your
distance from the observed body's geographic location at the instant the observation or radio
reading was made. It is axiomatic that you could be anywhere on one particular line.
Suppose that you knew you were 35° away from the geographical position of a transmitter
located at 45° N latitude and 30° W longitude. If you had a large globe and a large
NASA MOONED AMERICA! / Rene