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The Celestial LOP
General
Information
You have seen how lines of position, obtained through bearings on
terrestrial objects, are used to fix a ship’s position in piloting. You
know that a line of position (LOP) is a locus of possible positions of the
ship. In other words, the ship’s position must be somewhere along that
line. A fix, by definition, is a relatively accurate determination of
latitude and longitude. In practice, this position is the intersection of
two or more lines of position; but often it is not the ship’s exact position
because you can always assume some errors in observation, plotting, and
the like.
The celestial navigator must establish lines of position by applying the
results of observations of heavenly bodies. A line of position obtained
at one time may be used at a later time. All you need to do is move the
line parallel to itself, a distance equal to the run of the ship in the
interim, and in the same direction as the run. Such a line of position
cannot be as accurate as a new line because the amount and direction of
its movement can be determined only by the usual DR methods. If two
new lines cannot be obtained, however, an old line, advanced and
intersected with a new one, may be the only possible way of establishing
a fix. Naturally, the distance an old line may be advanced without a
substantial loss of accuracy depends on how closely the run can be
reckoned.
In celestial navigation, as in piloting, you essentially are trying to
establish the intersection of two or more lines of position. A single
observation and the resulting LOP is insufficient to obtain a fix.
The most accurate method of obtaining a celestial fix is to take sights on
many bodies in a short time. For example, it is quite common to take
sights on six or more stars in a period of 15 minutes or less. Taking
sights on many bodies allows the observer to identify and throw out
LOPS with obvious errors.
Determining the
You might be entitled to complain that much has been said concerning
LOP
what an LOP tells you, but very little has been told about how you
determine it in the first place. We are coming to that part now.
The first item is to take on a heavenly body or bodies and then reduce
the sights. Reducing the sights taken gives you the information you
need to plot the LOP. The LOPS then gives you the resulting fix.
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