§ 18 – 20

Where appropriate, use Astro-navigation Pamphlet and Sight Reduction Tables for Marine Navigation N.P. 401 (H.O. 229). Questions No. 6-10 should be worked on the ANC Sight Forms. In each of the Sight Form questions find the intercept position from which it should be plotted, and the true bearing of the observed celestial body.

77. What is a circle of equal altitude and how is this related to a position circle on the Earth’s surface?

78. Explain why it is not possible to find a vessel’s position when out of sight of land by means of a single-observation.

79. What is an intercept? Explain the Marcq St. Hilaire or Intercept Method of deriving an Astronomical position line.

80. Sight Reduction Tables (N.P.401/H.O. 229). are entered with a whole degree of L.H.A. Lat. and Dec.. Explain with the aid of an example how this  is achieved and why the use of an assumed position is acceptable in position line navigation.

The SRT are entered with a whole degree of L.H.A. Latitude and Declination; as follows:

With the choice of Longitude; to obtain L.H.A. The G.H.A. of the Body at the Date & Time of Observation, obtained from the Nautical Almanac, is taken, with the Assumed Longitude, in such a way as to make the resulting L.H.A. a whole degree.

When the assumed Longitude is Westerly, the minutes of the Longitude are made the same as the minutes of the G.H.A.

When the Longitude is Easterly, the minutes of the Assumed Longitude, (60’ – minutes of the G.H.A.)

i.e. Vessels assumed position Lat 40º 30.0’ N Long 20 10.0’ W – E – GHA of observed body 50º 20.4’

When the Westerly longitude exceeds the GHA, 360 has to be added to the GHA. When the sum of the GHA and the chosen longitude exceeds 360º then 360º must be subtracted to obtain the LHA.

i.e. Vessels assumed position. Lat 50º 10.0’ N Long 162º 10.5’ W – E – GHA of observed body 148º 57.5’ – 254º 52.7’

With the choice of Latitude. The assumed latitude should be the nearest whole degree to the vessels D.R. or estimated position.

With regard to the Declination of the body at the time of observation, as obtained from the Nautical Almanac;

Unless this happens to be by chance an integral degree, nothing can be done; In the majority of cases therefore, an additional correction is necessary to take account of the difference between the body’s actual declination and the nearest integral degree of declination with which the Tables are entered. Odd minutes of declination are accounted for by a First Altitude Difference correction;

Lower whole degree of Declination taken = 20º.

Declination increment of 11.2’ (at the front of the SRT.)

The use of an Assumed Position is acceptable in position line navigation because it lessens the work involved in reducing the sight and, the chances of error are consequently reduced.

The Plotting of simultaneous sights from the same (DR or EP) position can be achieved. But when the plotting of nearly simultaneous sights is done, each from a different longitude, three or more for a fix, three or more Intercepts have to be plotted for different positions.

81. Odd minutes of Dec. are accounted for by a First Altitude Difference correction to the tabulated calculated altitude. When is a Second Altitude Difference correction necessary, and how is it obtained?

In some cases where the altitude exceeds 60, the figures in the ‘d’ column of the SRT, LHA pages have next to them a ‘dot’ this indicates that a small additional correction is made in the interpolation of altitude for declination, this correction being called the Second Difference Correction. This correction always being added, irrespective of the sign of declination.

Where it is necessary to apply the SDC, when extracting the ‘d’ value from the SRT take out the value of ‘d’ immediately above and below the ‘d’ which corresponds to the whole degree of the actual declination.

Subtract one-from-the-other; the difference found is called the Double Second Difference DSD.

The value of the SDC is obtained directly from the critical table on the right-hand side of the interpolation table headed; Double Second Difference and Corr:

Select the two figures in the DSD Column between which the actual DSD lies, read off the correction in the adjacent column.

When the DSD corresponds exactly to one of the tabulated DSD values, use the upper of the two possible corrections.

The component of the DSD table opposite the block of the interpolation table wherein was found the Declination Inc. entry should always be used. i.e. LHA of Body 14º Dec 57º 23.1’ S Lat 49º S.

Question 78 should be worked on the ONC Sight Forms.

82. In each of the following: – find the Intercept, position from which it should be plotted, and the true bearing of the observed celestial body: 

a. 09:08 (Zone -7) on Jan 20th 199X in D.R. Lat. 44º 39.0 s., Long. 101º 07.0 E., D.W.T. 02h. 06m. 55s., D.W.E. 01m. 13s. slow on G.M.T.., Sext. Alt. SUN L.L. 42º 10.5., I.E. + 01.2., Ht. of E. 9 ft.

b. 16:47 (Zone +1) on Jan 03rd 199X in D.R. Lat. 4949º 40.0 N., Long. 09º 24.0 W., D.W.T. 17h. 48m.02s., D.W.E. 42s. fast on G.M.T.., Sext. Alt. of planet Venus ♀21º 52.9., I.E. –0.5., Ht. of E. 16.5 ft.

c. 20:10 (Zone +4) on Jan 01st 199X in D.R. Lat. 53º 55.0 S., Long. 62º 09.0 W., D.W.T. 00h. 08m. 52s., D.W.E. 01m. 12s. slow on G.M.T.., Sext. Alt. of *Acamar 76º 10.5., I.E. nil., Ht. of eye. 8 ft.

d. 08:44 (Zone – 1) on Jul 03rd 199X in D.R. Lat. 58º 07.0 N., Long. 09º 58.0 E., D.W.T. 07h. 44m. 14s., correct on G.M.T.., Sext. Alt. SUN U.L. 38º 05.6., I.E. –01.5., Ht. of eye. 8 ft.

e. 07:08 (Zone +3) on Jul 05th 199X in D.R. Lat. 48º 42.0 S., Long. 37º 35.0 W., D.W.T. 10h. 09m. 36s., D.W.E. 01m. 30s. fast on G.M.T.., Sext. Alt. *Achernar 77º 51.7., I.E. +2.2., Ht. of eye. 9 ft.