(199). Feb. 20, 1850, at 3h 50m P.M., mean time nearly, in lat. 10° 20' N., and long. by account 7° W., the following lunar was taken : The height of the eye above the sea was 20 feet; required the longitude. Ans., 7° 4' W. (200.) Jan. 9, 1851, at 2h 50m P.M., mean time nearly, in lat. 56° 10′ 20′′ N., and long. by account 20° 40′ E., the following lunar was taken: The height of the eye above the sea was 20 feet; required Distance at 3 hours, 111° 33′ 34′′; at 6 hours, 112° 57′ 16′′. Feb. 6. 15 42 19 S. 7.15 23 . . 14 22:51 add. 14 55.0. . . 54 44.3. . 16 14 34 16 S. 4.4. 58 59-2.. 16 9.6. 59 180 16 11 Distance at 3 hours, 100° 7′ 50%; at 6 hours, 101° 45' 50". 2 S. 16 17 Ship time obtained from moon's altitude. When the sun or star is near the meridian, the ship meam time must be obtained by computing the hour angle of the moon and deducing from thence the ship mean time. This may be done by the following rule. Rule LI. Objects observed, moon and sun. Altitudes taken.. Ship mean time obtained from moon's altitude. 1. Get a Greenwich date. 2. Take out of Nautical Almanac and correct for Greenwich date the following quantities:-Right ascension of mean sun and sun's semidiameter; right ascension and declination of moon; semidiameter and horizontal parallax. of moon. 3. Correct the sun's altitude for index correction, dip, semidiameter, and thus get the apparent altitude: from the apparent altitude subtract correction in altitude; the result is sun's true altitude, which subtract from 90° for sun's true zenith distance. 4. Correct the moon's altitude for index correction, dip, semidiameter (augmented); the result is the moon's apparent altitude. To the apparent altitude add the correction in altitude, the result subtract from 90° for the moon's true zenith distance. 5. When the moon's correction in altitude is taken out, take out also at the same opening of the book the auxiliary angle A. 6. Correct the observed distance for index and semidiameter. 7. To find ship mean time. Under the moon's declination put the latitude of ship: take the difference if the names be alike, but their sum if L the names be unlike: under the result put the moon's zenith distance, and take the sum and difference. Add together the log. secants of the two first quantities in this form (rejecting the tens in index) and the halves of the log. haversines of the two last; the sum is the log. haversine of the moon's hour angle, to be taken out at the top of the page if the moon is west of the meridian, but at the bottom of the page if the moon is east of meridian. To the hour angle thus found add the moon's right ascension, and from right sum (increased if necessary by 24 hours) subtract the ascension of the mean sun; the remainder (rejecting 24 hours if greater than 24 hours) is ship mean time at the instant of observation. 8. Then proceed as in p. 211, arts. 8, 9. EXAMPLES. May 22, 1844, at 11h 15m A.M., mean time nearly, in lat. 50° 48′ N., and long. by account, 1° W., the following lunar observation was taken : The height of eye above the sea was 24 feet; required the (201.) May 16, 1850, at 0h 50m P.M., mean time nearly, in lat. 42° 30′ N., and long. by account 29° 6′ W., the following lunar was taken :— 4 18.5 1° 4' 37" W. Ans., 29° 5' 0" W. · The height of the eye above the sea was 20 feet; required the longitude. (202.) May 18, 1850, at 1h 0m P.M., mean time nearly, in lat. 43° N., and long. by account 41° 36′ W., the following lunar was taken :— The height of the eye above the sea was 15 feet; required the longitude. Ans., 41° 33′ 45′′ W. |