Modern navigation; A text-book of navigation and nautical astronomy suitable for the examinations of the Royal Navy and the Board of Education (South Kensington)

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Modern navigation; A text-book of navigation and nautical astronomy suitable for the examinations of the Royal Navy and the Board of Education (South Kensington)

Book Description: 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. 1904 Excerpt: ...blunders, which are sure to occur. Fig. 42. 220. Ambiguity in value of Hour-Angle.--Whichever formula is used, there will always be a choice as to the value of H-A, for sin A = sin (180-A).-. hav h = hav (24h-A) or sin A = sin J(24h-A) and so we might equally well take out A = 4h or A = 20h h = 6h or h = 18h Refer to the figures 42, which show a Westerly and an Easterly body. Since H-A is measured continuously Westwards from 0h to 24h, it follows that Westerly H-A is less than 12h Easterly H-A is greater than 12u so that we must always refer to the known position of the body, East or West, to decide whether to take the value of h or 12h. No ambiguity can occur in Az since it is never greater than 180. It is named from the Lat, N. or S.; and from the known position of the body, E. or W. 221. Ex.--In Lat 28 21' S., body's Dec 12 18' N., Z-D 66 37' the body bearing West, find H-A and Az. The work is arranged as follows:--We use five-figure Logs to find H-A, as this gives the nearest second of time accurately; and four-figure Logs for Az, as this gives the Az closer than it could be observed on board ship. A tenth of a degree (-1) is close enough. When the ratios as " Sec " " Hav " are printed in small capitals it implies that they are tabular logarithmic and not natural ratios. 222. To illustrate the method, we shall work the last example again, changing Lat from South to North and the Az from West to East. We omit all explanation and merely write down those figures which a practical worker would use. Notice, as a valuable check on accuracy, that one of the "half-log-haversines" must be the same in the work of H-A and of Az. A blunder is often detected by their non-agreement. 223. Finding Longitude.--After finding the HourAngle, t...