Nautical Astronomy, Latitude, Longitude & Azimuth, Sumner's Method, Marcq St. Hilaire's Method, Ocean Meteorology, International Rules & Signals (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1908 Excerpt: ...article, it was assumed that the ship had been stationary. Suppose, however, that the ship has changed her position; that from the time of observation in the morning until noon she has sailed, or steamed, on a course, say, true N E 40 miles, and that the latitude by meridian altitude at noon was 11 35' N. Then, in order to find the position at noon, the method of procedure would be as follows: From the point r, the position calculated from the morning observation, or, in fact, from any point on tu, lay off the course' and distance run in the interval, or N E 40 miles, and through the extremity b of this line draw a line /, //, parallel to the original Sumner line tu. The point d, where this second line /, u, intersects the latitude parallel of 11 35' N, is the true position of the ship at noon. By the position thus found, the longitude in at noon is readily determined by inspection of the chart--in this case 54 40' W, nearly (see Fig. 7). If it is desired to know what the exact position of the ship was at the morning observation, simply draw, toward tu, a line from d parallel to the course sailed; the point c', where this line intersects the original Sumner line tu, was the exact position of the ship at that time. The latitude used in computing the hour angle was therefore nearly 18' wrong, to the south. Thus, it should be evident that by using a Sumner line in the manner shown there is no particular necessity of recalculating, the longitude with the new value of latitude obtained at noon (corrected for run) and then, by dead reckoning, carrying it up to noon. Instead, the longitude in at noon may be quickly and accurately found by mere inspection. 9. Intersecting Sumner Lines.--Thus far only a single Sumner line has been considered, and it has been shown w...