is advisable to ascertain the correct Error when opportunity offers at ports the ship may visit. Rating a chronometer" means finding the rate at which it gains or loses per day by dividing the sum or difference of the errors on different dates by the number of days elapsed. Examples. 1. A chronometer was found to be 4m 30' fast of mean noon at Greenwich on January 15th, 1906, and on March 30th, 1906, it was 7" 35' fast of mean noon at Greenwich. Required the rate. m. 8. Fast 4 30 on January 15. 3 5 gained in 74 days. 74)185(25 gain per day. 148 370 370 2. A chronometer was 14m 25' fast of mean noon at Greenwich on September 1st, 1906, and at mean noon at Greenwich on December 4th, 1906, it was 13" 145 fast. Required the rate. m. 8. Fast 14 25 on September 1. 1 10 5 lost in 94 days. 94)70-5(75′ losing daily. 470 470 3. A chronometer was 1" 42" slow of M.T.G. at mean noon on January 1st, 1906, and on March 31st at mean noon Greenwich time it was 6" 9" slow. Required the rate. m. 8. Slow 1 42 on January 1. 4 27 lost in 89 days. 89)267(3 losing daily. 4. On leaving Liverpool, January 19th, 1904, the ship's chronometer was 4 25 slow of mean noon at Greenwich, and at Sydney on May 28th it was found to be 3m 10'5 slow of mean noon at Greenwich. Required the rate. m. 8. Slow 4 2.5 on January 19. 52.0 gained in 130 days. 130)520(4 gaining daily. 52.0 5. At New York on February 27th, 1904, a ship's chronometer was 18' slow at mean noon Greenwich time, and at Southampton on March 6th, at Greenwich mean noon it was 14' fast. Required the rate. 8. Slow 18 on February 27. Fast 14 on March 6. 32 gained in 8 days. 8)32 4' gaining daily. 6. A chronometer was fast of mean noon at Greenwich 2m 17′ on January 14th, 1906, and on June 1st it was 3m 55'5 slow of mean noon at Greenwich. Required the rate. m. 8. Fast 2 17 on January 14. Slow 3 555 on June 1. 6 12.5 lost in 138 days. 138)372-5(27 losing daily. 276 965 ART. 41. The time by chronometer being given, and two errors or one error and daily rate, to determine the correct G.M.T. Find the rate as explained above, multiply this by the number of days and decimals elapsed between the date of second error, and date of chronometer time. This gives the accumulated rate. Now take the time by chronometer expressed astronomically, being careful to make it correspond as far as possible with the Greenwich Time deduced from the Ship Time and approximate longitude, apply the second error and accumulated rate to obtain the correct G.M.T. Examples. 1. February 24th, about 8h 20m A.M. at ship in long. by account 74° 15′ W., the chronometer showed 1 12 205, and had been found 13 35' slow of mean noon at Greenwich on October 30th, and on December 24th it was 10m 555 slow. Find G.M.T. 2. October 18th, at 2h 30m A. M., M.T. ship nearly in long. by D.R. 50° E.; the chronometer showed 11" 3" 22", which had been found 4" 137 fast on July 24th, and losing daily 3'4. Find G.M.T. 3. February 3rd, approximate ship time 1 8m A.M., in long. about 31° W., when a chronometer, known to be 43" 19" fast on January 13th, and gaining daily 2′ 5, showed 3" 55" 30'5. Find correct G.M.T. EXERCISES. Find the chronometer "Rate" from the following errors on Greenwich Mean Noon: 9. November 27th, at about 10 A.M., at ship in long. 123° 15′ E., when a chronometer showed 1 53 346, which was slow for G.M.N. 3 13' on September 10th, and fast 2 30′ on October 20th. Find correct G.M.T. 10. October 31st, about 2 15" P.M., at ship in long. 124° W., when a chronometer showed 10 7 33, which was slow 10m 46' 8 on September 29th, and gaining 1'01 daily. Find the G.M.T. 11. March 20th, about 4" P.M., at ship in long. 63° 45′ W., when a chronometer showed 7" 12" 1', which was slow 1h 2m 4' on January 2nd, and on February 1st was slow 1 3 54. Find the G. M.T. 12. December 10th, about 2h 15" P.M., at ship in long. 161° W., when a chronometer showed 2 1m 23′, which was fast 59m 16' on August 12th, and on October 31st was fast 1 2m 16' on G.M.N. Find G.M.T. 13. July 4th, at 7h 17 A. M., at ship in long. 75° W., when a chronometer showed 12 o'clock, it was fast of G.M. noon 38' on April 15th, and gained 75' daily since. Find G.M.T. 14. November 27th, at 7 A.M., at ship in long. 4° 45′ W., when a chronometer showed 7h 17m 585, which had been found 6 57' slow for G.M. noon on August 21st, and slow 6" 375 on September 20th. Find G.M.T. 15. December 14th, about 4" 5" P.M., M.T.S. in long. 11° 6' W., when a chronometer showed 5 20m 44', which was fast of G.M. noon 2m 38*1 on November 10th, and on November 27th was correct. Find G.M.T. 16. May 1st, about 6h 30m P.M., at ship in long. 91° E., when a chronometer showed 125, and was slow 24" 54' on January 11th for G.M. noon, and slow 24" 34' on March 2nd. Find G.M.T. 17. April 16th, about 4 P.M., at ship in long. 131° 25′ E., when a chronometer showed 7h 18m 56', which was slow of G.M. noon on February 9th 9 215, and was slow 3m 19"-5 on March 17th. Find G.M.T. CHAPTER X. ELEMENTS FROM THE NAUTICAL ALMANAC." ART. 42.-A great part of the labour involved in the calculation of problems in Nautical Astronomy is the Reduction of the Elements from the "Nautical Almanac;" therefore it is most important that the student should master this part of the work at the outset. The advantage of a thorough acquaintance with the "Nautical Almanac " is incalculable. The necessary extracts for working all the problems in this Book will be found in the Appendix. In the case of the stars the "mean position" only is given, and this is the one used in the calculations, as the introduction of the "apparent places" would considerably enlarge the Appendix without giving a corresponding advantage. The Sun's Elements.-These are given for Noon at Greenwich, therefore it is always necessary to know the Greenwich Astronomical Time. When Apparent Time is given, the elements are taken from p. I. of the month; but when Mean Time is given, they are taken from p. II. The variations or changes in one hour are on p. I. All minutes and seconds in the Greenwich time are reduced to decimals of an hour. (1) Declination. Examples. 1. Find the sun's declination for August 16th 8h 36m P.M. apparent time at Greenwich. The correction is subtracted because the declination is decreasing. |