In the preceding chapters, we have so far explained the theory of circles of equal altitudes position circles, intercepts, and the Marcq St. Hilaire method of obtaining an Astronomical position line. We have also shown how the calculated altitude for an assumed position can be obtained quickly and easily with the minimum calculation from Sight Reduction Tables for Marine Navigation N.P. 401 (H.O. 229). This knowledge, together with a knowledge of how to locate the G.P. of a celestial body for the instant of G.M.T. from the N.A. and how to correct an observed Sext. Alt. to the True. Alt. is all a navigator requires to be able to work out a complete sight for a position line to plot on his chart. It only remains for us to explain how all the threads are pulled together.

The procedure for taking and wording a complete Position Line Sight by the Tabular method is as follows: –

  • Measure the altitude of the chosen celestial body above the sea horizon with a sextant noting the precise D.W.T. (Deck watch or Chronometer Time) for the instant of observation.
  •  From the local date and Zone Time determine the correct Greenwich Date with which to enter the N.A.
  • Apply the D.W.E. (Deck Watch (or Chronometer) Error) to the D.W.T. of the observation to obtain the G.M.T. of observation.
  • Obtain the G.H.A. of the observed body for the instant of observation from the N.A., choosing an assumed Longitude, nearest the D.R. Longitude, which will make the L.H.A. a whole number of degrees.
  • Choose assumed Lat. nearest in whole degrees to the D.R. Lat.
  • Note the Dec. of the observed body for the instant of observation from the N.A.
  • Enter the Sight Reduction Tables with the L.H.A. found in (4) above, the assumed Lat., and the nearest whole degree of Dec. below the actual Dec.
  • Extract the Tab. Alt. (Hc), tabulated azimuth angle (Z), the d value and its sign (+ or -). If (and only if) the d value is printed in italics and followed by a dot, also note-the Double Second Difference (D.S.D.).
  • Enter the Interpolation Table with the Dec. increment (Dec. Inc.) and the d value and extract the First Difference Correction. If the D.S.D. has been noted in (8), also extract the Second Difference Correction.
  •  Apply the altitude corrections) found in (9) to the Tab. Alt. (Hc) found in (8) above, using the sign of d for the First Difference Correction but always adding the Second Difference Correction. The result is the Corrected Tab. Alt. (Corr. Tab. Alt.).
  •  Interpolate the azimuth angle (Z) for the Dec. increment mentally, and convert to a true bearing (Zn) using the instructions printed on every page of the Reduction Tables.
  • Correct the observed Sext. Alt. for I.E. and dip to obtain the apparent altitude. then apply the corrections to apparent altitude from the tables in the N.A. to obtain the True. Alt.
  • The difference between the True. Alt. and the corrected Tab. Alt. found in (10) above is the intercept which should be named Towards if the True. Alt. is greater (GOAT) or Away if the Tab. Alt. is greater.
  •  The intercept should be plotted from the Assumed Position either towards or away from the observed celestial body which lies in the direction of the true bearing found from the azimuth in (11) above.
  •  The required position line is drawn in at right angles to the line of bearing at the end of the measured intercept.

Since the above procedure is standard for all position line sights apart from only minor variations depending on whether the celestial body observed is the Sun, the Moon, a planet or a star, there is no need for the navigator to have to remember which step comes next, and all sights can be worked on a standard form. (fig. 20-1)

The navigator now has to make up his own book of Sight Forms for use with NP 401 S.R.T. (circa 2010) as the Admiralty Hydrographic Office no longer publishes them.

Due to their extreme simplicity It is very much recommended that students of Astro-navigation continue with this form layout and make up their own copies of the template for use.

Students should now compare the standard sight form with the procedure for working a position line sight as outlined in (1) to (15) above. They will see how the procedure is closely followed by the standard form, which leaves spaces for the insertion of the various figures relevant to the particular sight being worked.

The remainder of this chapter is devoted to demonstrating actual worked sights as far as calculating the intercept (i.e., steps (1) to (13) of the procedure outlined above.

The plotting of intercepts and position lines is a subject unto itself and will be dealt with in the next chapters. To be able to work sights satisfactorily for themselves, students should study the following worked examples very carefully step by step, checking all the extractions from the N.A. and N.P. 401 as well as the actual additions and subtractions.

Once the procedure and principle has been grasped, the examples should be reworked independently from the original observation data, the results then being checked against the worked examples. Any discrepancies should be carefully investigated until the student fully understands his original error and can guard against a repetition of this error.

Using the Intercept Method described in these chapters, a position line sight can be worked from any observable celestial body at any time of the day including morning and evening twilight.

Observations for navigational purposes during the hours of darkness are not normally taken because the horizon is not clearly enough defined to enable an accurate altitude to be measured with a sextant. The following examples include observations of the Sun. planets and stars.

The Moon, being somewhat of a special case, is covered in more detail in § 29-31 on Special Observations. Stellar Observations are also covered in more detail in § 25-28. Ex. No. 14/16 are worked on Sight Form (B). Fig.20-2.

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