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Practical N avigation Gui ' : y oye | e-~-VI Determining Latitude by Ex-Meridian Observation— Explanation of the Sumner Line and How It Is Used interfere with the observation of the sun at the instant it reached meridian altitude. In this case, the sight would be ruined. A _ navigator, however,« takes an ex-meridian alti- tude the moment the clouds have passed and if the interval of time is not great this can be conveniently worked out with the use of tables 26 and 27 in Bowditch. These tables give the variations up to an interval of 26 minutes. Observations taken at greater intervals than 26 minutes be- fore or after the sun passes the meridian altitude are not sufficiently accurate for practical purposes. First the Greenwich mean time as given by the chronometer is reduced to Greenwich apparent time; longitude in time is added to obtain the local apparent time. This will be a few minutes before or after noon, the ex- meridian time. Then the observed altitude of the sun is reduced to the true altitude in the usual way and the declination is taken from the nautical almanac. With the latitude obtained by dead reckon- ing as the vertical argument and the declination of the sun as the horizontal argu- ment, the variation in altitude is taken from Bowditch Table 26. This result is then taken as the vertical argument and the ex- meridian time as the horizontal argument to be applied to Bow- ditch Table 27, from which is taken the re- duction to be applied to altitudes. This re- ‘sult added to the true altitude at the place of observation will give the true noon altitude, from which the zenith I MAY happen that clouds wiil ‘distance is taken. This zenith distance and the declination com- ‘bined will be the lati- tude of the ship. But this resulting latitude is that of the ship at the instant of observa- tion. To bring it up to noon the run must ‘be applied. Take the BY V. G. IDEN case of a ship which on July 12, 1919, was in latitude 50 degrees North and longitude 40 degrees West, by dead reckoning. The ex-meridian altitude of the sun was observed to be 61 degrees 48 minutes 30 seconds.. The chronometer time was 2 hours, 41 minutes and 39 seconds. The chro- nometer correction was 2 minutes and 30 seconds minus. The _ indicated error was minus 3 minutes 0 seconds. The height of the eye was 15 feet. To find the exact latitude, study the example worked out in Table X. Sumner Line Navigators have found the plotting of position by means of the so-called Sumner line a great convenience. This method is named after its dis- coverer, Capt. Thomas E. Sumner, who first put it to use in 1837. In its essentials, it is a simple method of finding the position although there have been many variations of the principle for which different merits have been claimed. The chronometer time is known but the latitude has been worked out by dead reckoning and is uncertain. FiG. 14—DIAGRAMMATIC INSIGI{T INTO SUMNER PRINCIPLE 549 An observation of the sun is taken and two longitudes are worked out for the latitude nearest north and the latitude nearest south of that found by dead reckoning. These two posi- tions are plotted on the chart and joined by a straight line. It is as- sumed that the ship is at some point on this line. These two positions have been found by using the same altitude and the same polar distance. From this line the true course the ship is making is laid off. When the sun has changed its bearing a few points another observation is taken, and with the assumed latitudes, two more longitudes are figured out. These two points are also joined by a straight line. By laying out a third line parallel to the first in the direc- tion of the course the ship has taken and the distance from it the ship has | sailed in the interim, this third and second line may be made to intersect upon proper extension of each. Where they intersect is the position of the ship. In Fig. 14, the first two positions plotted would be at A and B which have been joined by a straight line. The second two. posi- tions would be C and D which have also been joined by a straight line. In the interim, the ship has sailed 50 miles due east. A third line is drawn parallel to AB 50 miles east and - where this intersects CD at P is the position of the ship. One application of the Sumner method which seems to be quite popular because it neces- sitates the taking of but one observation is that where the azimuth an- gle is used. In this case the position is worked up with «the latitude found by dead reckoning and from this position the line of. the azimuth of the sun is plotted on the chart. A second line drawn at right angles to this through the position of the ship will be the Sumner line, upon. some e