228 



Ocean Magnetic Observations, 1905-16 



Longitude Observations (Single Altitudes). 



Specimen observations and computations of astronomic longitudes by single altitudes 

 are showTi on Form 41 below, which is designed for both Sun and star altitudes, the por- 

 tions not required in either case being marked in the specimen form "for star observations," 

 or "for Sun observations." The hour angle, t, is computed from the latitude <p, the polar 

 distance p, and the altitude h, by means of the equation 



sin^ n < = sec <p cosec p cos s sin (s — h) 



in which s = -^{h + cp + p). Six altitudes are measured in quick succession, three of the 



lower limb and three of the upper hmb when the Sun is observed, as indicated in the speci- 

 men form. The times are noted by a recorder who enters them with the corresponding 



Geographic Positions at Sea: Longitude Observations {Single Altitudes) 



(Form 41) 



Date: Sat., Aug. 23, 1913, A. M. 

 Watch: M 



Vessel: Carnegie 

 Obs'r: H. R. S. 



Com'd'r: W. J. P. 

 Conip'r: H. R. S. 



Object 



Time by 

 Watch M 



Observed 

 Altitude 



Chronometer Comparisons 



Before 



After 



Astronomic Elements 



S 



S 

 5 

 S 



h m s 



10 34 52 



35 10 



35 22 



35 41 



35 56 



36 12 



29 

 29 



12 30 



16 00 



29 18 20 



29 54 20 



29 57 00 



30 00 00 



Chron'r 53862 

 Chron'r corr'n 

 G. M. T. 

 Watch M 



Watch M corr'n 



TO s 



19 20 



+36.6 

 19 56.6 

 18 36.0 



h m s 

 10 47 30 



+36.6 

 10 48 06.6 

 10 46 45.2 



+01 20.0 



+01 21.4 



Means 

 Corr'ns 



10 35 32.2 

 +01 21.4 



29° 36'. 4 

 -05.6 



Starb'd log 94.8 

 Port log 



Decl'n at G. M. N. 

 Hourly diff.: -0'.85 

 TimefromG. M. N. 



-IM 

 Correction: 



(-1.4) (-0.85) 



Decl'n at obs'n 



G. M. T. 

 R. A. M. S. 

 Tab. Ill 



G. S. T. 



R. A. of star 



H. A. from Gr. 



10 36 53.6 



for star obs'ns 



43 58.0 = (s-ft) 



Log sec ip 

 Log CSC p 



Log COS s 

 Log sin {s-h) 



Corrections to Obs. Alt. 



Sex. No. 2617 



Semi-diameter 

 Refraction and Par. 

 Dip of horizon 

 Index corrrection 

 Eccentricity 



Total 



-1.5 



-4.1 



0.0 



-5.6 



Log sin' j( 

 h 

 12 -« 



L." M. T. 



Longitude in time 

 Longitude in arc 

 Correction for assumed lat : 



(-3:7) (-0.2) 

 Corr'd longitude (\\ est) 



0.10976 

 0.00895 



9.45385 

 9.84151 



Eq. timeatG. M.N 

 Hourly diff.: -0=64 

 TimefromG. M.N. 



-1^4 

 Correction : 



(-1.4) (-0.64) 



Eq. time at obs'n 



9.41407 

 h TO 

 7 55 

 +02 

 7 57 

 2 39 

 39' 



3 



01.4 

 38.9 

 40.3 

 13.3 

 48'.3 



R. A. of star at 



G. M. N. 

 Hourly diff.; 

 TimefromG. M.N. 

 Corr'n: 

 Right asc'n at obs'n 



+ 11 34.5 



+01.2 



+ 11 35.7 



TO S 



+2 38.0 



+00.9 



+2 38.9 



•for star obs'ns 



39 



+0.7 

 49.0 



Remarks 

 Position: bridge Ht. of eye: 18 ft. 

 Horizon: good Wind: SSE 

 Barom'r: 771 mm. Therm'r; 25?5 C. 

 Direc. of obj.: N. 99° E 



altitudes. In the specimen above the times were taken from the watch M, which had 

 been compared with the chronometer 53862, the entries being made in the allotted space 

 on the form at the time the comparisons were made. Greenwich mean time is indicated 

 by the letters G. M. T. The right ascension of the mean Sun, Greenwich sidereal tune, 

 the right ascension, the hour angle, and equation of time are indicated respectively by the 

 abbreviations R. A. M. S., G. S. T., R. A., t, and E. "Tab. Ill" refers to the table of 

 the American Ephemeris which gives the acceleration of sidereal on mean time. With 

 these explanations the steps of the computation are easily followed to the longitude. 



