ADVANCING OR RETARDING A LINE OF POSITION
In celestial navigation, lines of position are rarely obtained simultaneously. This is true during the day when the sun may be the only available celestial body. A celestial line of position may be advanced for 3 or 4 hours to obtain a celestial running fix. It may also be advanced by advancing the AP in direction and distance an amount consistent with the ship's travel during the interval between two successive observations. In this procedure, the azimuth line is drawn through the advanced AP without any change in direction. The advanced LOP is drawn perpendicular to the azimuth, a distance from the AP equal to the intercept, and toward or away from the GP.
PLOITING THE CELESTIAL FIX
At morning and evening twilight, the navigator may observe the altitudes of a number of celestial bodies in a few minutes and establish a celestial fix. If 2 or more minutes elapse between observations, the navigator must consider:
a. elapsed time
b. speed of ship
c. scale of the chart or plotting sheet
To determine whether or not a more accurate fix can be obtained by advancing AP's to a common time. It is possible during the day to obtain a celestial fix rather than a celestial running fix if two or more of the three following bodies are visible:
a. Sun
b. Moon
c. Venus
LOP FROM CELESTIAL OBSERVATIONS
Lines of position, obtained through bearings on terrestrial objects, are used to fix a ship's position in piloting. A ship has many possible locations on a line of positions. The ship's position must be somewhere along that line. A fix, by position is the intersection of two or more lines of position, but this is not the ship's exact position, because one can always assume some errors in observation, and plotting.
The celestial navigator must establish the lines of position by applying the results of the 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, the old line is advanced and intersected with a new one, it may be the only possible way of establishing a fix. The distance an old line may be advanced without a loss of accuracy depends on how closely the run can be.
In celestial navigation, as in piloting, you are trying to establish the intersection of two or more lines of position. A single observation will not give you a fix, it could be used with a loran line, etc. to provide a fix.
TWO CIRCLES
Observation of two bodies at the same time gives the navigator two circles of equal altitude. The circles intersect each other at two points, because the ship is somewhere on each one of them, you must be at one or the other points of intersection.
