§ 21 – 23

For the Astro questions: use the ANC ‘Plotting Sheets’ and ‘Sight Forms’ , the Astro-navigation 

Pamphlet and Sight Reduction Tables for Marine Navigation N.P. 401 (H.O. 229).

83. Explain briefly what is meant by the term simultaneous sight.

84. What are Double Sights-and give three examples of the various combinations, which can comprise a double sight.

85. List the Admiralty Plotting Charts, which are available, with a brief description of each.

86. In taking a set of simultaneous sights, why is it desirable to observe four different altitudes rather than two or three.

87. In transferring a P/L for a double sight, why is it desirable to plot the run from a point on the P/L near the vessel’s E.P?

88. A vessel in D.R. Posn. Lat. 44º 58.0 N., Long. 20º 03.0’ W. reduced simultaneous sights: 

ASSUMED POSITION       BODY               BEARING          INTERCEPT

45º N. 20º 15.5’ W.             ✴︎Capella           040º                   15.0 towards

45º N. 19º 36.0’ W.             Venus ♀           102º                    16.5 away

45º N. 20º 23.5’ W.            ✴︎Rigel               157º                    1.0 away

Plot these observations and find the vessel’s observed position.

89. A vessel in D.R. Long. 02º 44.0’.E. obtained a Mer. Alt Sun at 12:17 hrs. which gave Lat. 55º 14.O’.N. She then steered 135º T. at 5.4 kts. At 18:23 hrs. during evening twilight an observation of a star nearing 252º T. gave an intercept of 8.6 towards from an A.P. in Lat. 55º N., Long. 03º 15.8’ E.

What was the vessel’s observed position at 18:23 hrs. if leeway between sights was negligible but is estimated the vessel was influenced by a mean tidal stream setting 200º at 1.2 kts.

90. A navigator having difficulty taking sights at morning twilight owing to cloudy conditions finally completes his observations at 06:20 (Zone +8). His observations gave the following results: –

G.M.T.                             ASSUMED POSITION      BODY        BEARING           INTERCEPT

13h. 49m. 55s. 47º S.   126º 46.0 W.                     ✴︎Sirius      355º T.                 18.5 away

14h. 00m. 10s. 47º S.   126º 03.0 W.                     ✴︎Acrux      281º T.                 9.2 towards

14h. 19m. 49s. 47º S.   126º 26.5 W.                     Venus ♀     060º T.                3.4 away

Plot observations and find observed posn at 06:20 (Zone +8) if vessel steering 115º True. At 18 kts.

91. On Jul 05th 199X, a vessel in D.R. Lat. 50º 28.0’ N., Longitude 06º 18.0’ W. at 08:45 BST found the Sext. Alt. of the Sun’s L.L. to be 29º 11.4’ when the chronometer read 07h.46m.33s. and the log read 106.2. The chronometer was 1m 11s. fast on G.M.T.. the Ht. of E 12ft. and I.E. – 0.8. After steering 195º (T). since the morning observation, Bishop Rock L/Ho. (49º 52’ N.06º 27’ W.) was sighted at 13:15 BST bearing 120º (T)., log reading 142.0.

Find the vessel’s position at 13:15 BST. if the tidal stream was setting 230º at 1.5 kts. For the first two hours, there was slack water for the next hour; the stream set 020º at 0.8 kts. for the remaining time between observations and the vessel made 5º leeway in a SE’ly wind throughout.

92. On Jan 20th 199X the E.P. at Mer. Pass. Sun was Lat. 44º 42.0’ S., Long. 141º 17.0’ E. and the Sext. Mer. Alt. Sun’s L.L. was 65º 12.0’. An afternoon observation of the Sun’s L.L. gave 40º 25.8’ at D.W.T. 06h. 11m. 49s.

Find the vessel’s position at 15:12 (Zone – 9) if the D.W.E. is 08s slow on G.M.T.. the Ht. of. E. 14 ft., the I.E. nil, and the vessel is steering 100º T. at 7.0 kts.