Elements from Nautical Almanac. Mean sun's R. A. 16th. 3h 35m 22-14 Moon's semi. 18th. 3 43 15 25 Noon, 16 43 58 58.8 9 57 Moon's decl. Sun's semi. 18° 51′ 56′′ N. 15′ 50′′ 18 47 43 N. 7′ 50′′. 4.66 13 17 16 N. 15 49 Mid. 16 0 6 58 45 3 9 59 23 58 13 8 8 N. Distance at 3 hours, 89° 31′ 39′′; at 6 hours, 91° 9′ 1′′ Rule LII. Ship time obtained from star's altitude. 1. Get a Greenwich date. 2. Take out of the Nautical Almanac and correct for Greenwich date the following quantities:-star's right ascension and declination; right ascension of mean sun; moon's semidiameter and horizontal parallax. 3. Correct the star's observed altitude for index correction and dip; the result is the star's apparent altitude; from this subtract refraction; the remainder is the star's true altitude, which take from 90° to find star's true zenith distance. 4. Correct the moon's altitude for index correction, dip, semidiameter (augmented), and thus get the moon's apparent altitude; to this add the correction in altitude: the result is the moon's true altitude. Subtract the moon's true altitude from 90°; the remainder is the moon's true zenith distance. 5. When the correction of the moon's altitude is taken out, take out also at the same opening of the book the auxiliary angle A. 6. To find ship mean time. Under star's declination put latitude of ship: take the sum if the names be unlike; but if the names be like take the difference: under the result put the star's true zenith distance: take the sum and difference of the two last quantities put down. Add together the log. secants of the two first quantities in this form (rejecting the tens in the index) and the halves of the log. haversines of the two last; the sum will be the log. haversine of the star's hour angle, to be taken out at the top of the page when the star is west of the meridian, and at bottom when east. To the hour angle thus found add the star's right ascension, and from the sum (increased if necessary by 24 hours) subtract the right ascension of the mean sun; the result (rejecting 24 hours if greater than 24 hours) will be ship mean time. Then proceed as in p. 211, arts. 8, 9. EXAMPLES. June 2, 1849, at 10h 17m P.M., mean time nearly, in lat. 50° 50′ N., and long. by account 41° W., the following lunar observation was taken The height of the eye above the sea was 20 feet; required (203.) Jan. 8, 1851, at 7h 0m P.M., mean time nearly, in lat. 50° 40′ N., and long. by account 4° E., the following lunar was taken : : The height of eye above the sea was 18 feet; required the longitude. Ans., 3° 46′ 30′′ E. mean time nearly, in by account 10° E., the following (204.) Jan. 9, 1851, at 7h 50m P.M., lat. 49° 40' N., and long. lunar was taken : The height of eye above the sea was 18 feet; required the longitude. Ans., 10° 19′ 15′′ E. (205) April 18, 1850, at 9h 40m P.M., mean time nearly, in lat. 56° 10' N., and long. by account 23° E., the following The height of eye above the sea was 20 feet; required the longitude. Ans., 23° 3′ 30′′ E. (206) April 17, 1850, at 8h 45m P.M., mean time nearly, in lat. 51° 20′ N., and long. by account 5° 10' E., the following lunar was taken : The height of eye above the sea was 20 feet; required the longitude. Ans., 5° 8' E. (207.) November 17, 1847, at 2h 50m A.M., mean time nearly, in lat. 44° 30′ N., and long. by account 121° E., the following lunar was taken : The height of eye above the sea was 20 feet; required the longitude. Ans., 121° 5' 45′′ E. (208.) March 20, 1845, at 7h 50m P.M., mean time nearly, in lat. 49° 50' N., and long. by account 1° 30' E., the following lunar was taken : The height of eye above the sea was 20 feet; required the longitude. Ans., 1° 34′ 30′′ E. |