SELECTING BODIES FOR OBSERVATION
Before going further into problems and tables, mention should be made of a few items concerned with selecting astronomical bodies for observation.
Observing two heavenly bodies in rapid succession is the most convenient method of finding two lines of position necessary to establish a fix. Noting three bodies gives three lines, and these three define the fix more accurately (as in piloting). Accuracy of the fix established by intersecting lines of position depends upon the angle between the lines. The nearer this angle approaches 90°, the more accurate is the fix.
Actually, sights seldom are taken on two or more bodies simultaneously. Instead, the navigator decides which bodies to observe, then takes a round of sights, each one timed exactly. Resulting lines of position are advanced or retarded the amount of the ship's run between the time of observation and the time of the desired fix. The ideal situation for lines of position established by observing three bodies would be that wherein the bodies lie 120° apart in azimuth. An ideal fix using four bodies would include two north - south lines and two east - west lines of position to form a box. As already mentioned, lines , perpendicular to the course are frequently valuable for checking the run. Those lines parallel to it are helpful in deciding the accuracy of the course made good.
Concerning altitude, best results are obtained by observation of bodies whose altitudes are between 15° and 65° In general, observations are taken from bodies whose altitudues are between 10° and 80°.
ADVANCING AND RETARDING THE LOP
Several methods may be used to advance a fine of position. The most frequent method consists of advancing the AP in the direction of your course and for the distance of the run, and drawing the new LOP. To retard a LOP, just go the reciprocal of your course and for the distance run.
If appreciable time has elapsed since the determination of the last fix of the Ship's position at sea, the error in the DR plot may change where the ship's actual position is well away from her DR plot. A single line of position can be useful in establishing an estimated position. If an accurate line is obtained, the actual position is somewhere on this line. In the absence of better information, a perpendicular from the previous DR position or EP to the line of position establishes the new EP. The foot of the perpendicular from the AP has no significance in this regard, since it is used only to locate the line of position.
The establishment of a good EP is dependent upon accurate interpretation of all information available. Generally, such ability can be acquired only by experience. If, in the judgment of the navigator, the course has been made good, but the speed has been uncertain, the best estimate of the position might be at the intersection of the course line and the LOP. If the speed since the last fix is considered accurate, but the course is considered uncertain, the EP might be at the intersection of the line of position and an arc centered on the previous fix and of radius equal to distance traveled.
