23 COMPLETE SIGHTS
Students are now in a position where they can work a position line from any observation of a celestial body by the tabular method using Sight Reduction Tables N.P. 401 (H.O. 229). and can plot that position line on a plotting sheet or chart. They can also work several simultaneous or double sights and, by plotting their position lines together with any necessary runs between observations on a plotting sheet, can ascertain an observed position or Astronomical fix for any desired time. The Self Assessment at the end of §-24 provides students with an opportunity to demonstrate their ability to work complete sights from observation to plotting the final observed position using an Astro-navigation Sight Form and the Astro-navigation Plotting Sheet. An example in the working and plotting of complete sights in this way is given in the following example.
When a position line is transferred in order to obtain a fix by crossing it with an altitude of the same body after its bearing has changed sufficiently to obtain a good cut, it is important to remember that inaccuracies will arise if incorrect assessment is made of the vessel’s course made good over the ground during the run between sights.
It may well be advisable to accept the small discrepancy due to an angle of cut (say 45º), rather that to wait for the angle to ripen and incur large errors due to a long run between sights.
It should also be noted that the course made good on a run between sights, as in coastal navigation, may not necessarily be the same as the course steered if current, tidal stream or leeway are involved. On an ocean passage, the effect of surface drift or an ocean current is not likely to be known but is usually minimal and can generally be ignored, although leeway should be taken into account.
In tidal waters, such as the English Channel or North Sea, however, the effect of the tidal stream is likely to be considerable and, together with any leeway involved, must always be taken into account in plotting. The estimated leeway is applied at an angle from the true course steered away from the wind and the estimated set and drift of the tidal stream is applied at the end of the distance run before transferring the first position line.
The following example of a double Sun sight in the English Channel illustrates a transferred position line allowing for both leeway and the tidal stream. Students are recommended to work and plot this sight on their own before comparing results with those given on the following pages.
2. THE PRACTICE OF ASTRO-NAVIGATION
If any celestial body (Sun, Moon, planet or star) and the sea horizon can be seen simultaneously at any position in the world, a position line can be established within 10-15 minutes. If two or more celestial bodies and the sea horizon can be observed nearly simultaneously a fix can be established and plotted on the chart within 20-30 minutes. Taking sights is the name given to the act of securing observations of celestial bodies at sea with a sextant.
The practical side of taking a sight consists simply of measuring the altitude of a celestial body, i.e. its angle of height above the sea horizon, at the same time noting the exact time either from a chronometer or from a good clock or watch keeping time to seconds and checked by radio time signals.
This practical operation takes only a minute or so; the rest of the time in establishing the position line or fix is occupied in working up the Sight, that is, simple calculations using Nautical tables and the N.A.
Proficiency at sea demands only two main qualifications: –
Familiarity in handling the sextant as an instrument – to enable observations to be made accurately and with confidence.
Familiarity with the reference books – The N.A. and the Tables – to ensure that the necessary data can be looked out quickly and with certainty.
Both these qualifications come rapidly with experience:
86. PLOTTING SIGHTS – EX. NO. 05
A vessel in D.R. Posn. Lat. 50º 14.0 N., Longitude 01º 55.0 W. at D.W.T. 10h. 43m. 37s. on 03rd Jul 199X found the Sext. Alt. of the Sun’s L.L. to be 57º 36.5, H. of. E. 8 ft., I.E. + 1.1. Log reading 32.5., D.W.E. 26s. slow on G.M.T. At D.W.T. 14h. 10m. 22s. the Sext. Alt. of the Sun’s L.L. was 54º 09.3 when the log read 52.5.
Work and plot these sights, finding the vessel’s position at the time of the second observation if she steered 154º T. between the observations, it is estimated she made 4º leeway due to a Westerly wind, and the mean tidal stream was setting 268º at 1.7 kts.
The working of the two observations is shown on the AN Course Sight Form Ex. No. 5. and the plotting is shown on the AN Course plotting sheet Ex. No. 5. and some explanatory notes are given below: –
87. PLOTTING SHEET PROCEDURES FOR EX. NO. 05
1). Working the first Sun observation from Assumed or C.P. Lat. 50º N., Longitude 01º 59.6 W. gives an intercept of 7.0′ away from true bearing 139½º (Sight form page. column 1).
2). Working the second Sun observation from Assumed or C.P. Lat 50º N., Longitude 01º 40.4 W. gives an intercept of 9.5′ towards true bearing 232º. (Sight form page. column 2).
3). The plotting sheet for Ex. No. 5 was prepared by designating the central parallel of Lat. the chosen Lat. for the observation (Lat. 50º N.), the right-hand meridian Longitude 01º 20.0 W. the central meridian 1001º 50.0 W. and the left-hand meridian 02º 20.0 W.
4). On the plotting sheet for Ex No.5, the D.R. position at 1044, 03rd Jul was first plotted, then the first C.P. CP1.
5). From position CP1, the intercept of 7.0′ for the first observation was plotted in a 319½º direction (i.e., away from the true bearing of 139½º) and the first position line was drawn at right angles to this.
6). From the 1044 D.R. position a perpendicular was dropped onto the first position line to give position J.
7). The log at 10:44 read 32.5 and at 14:11 (the time of the second observation) it read 52.5. The vessel had therefore travelled 20.0 miles through the water between observations. The course steered was 154º T. but since the vessel was making 4º leeway in a Westerly wind, the Course with leeway would be 150º T.
8). From position J, the Course with Leeway of 150º T. was plotted for 20.0 miles to position X. The tidal stream is setting 268º at 1.7 kts. and since the time between observations (14:11 ~ 10:44) is 3.45 hours the drift in this time would be 3.45 x 1.7 = 5.9 miles. From position X. therefore, the tidal stream was plotted in 268º direction for 5.9′ miles to position L., which would be. the vessels E.P. at 14:11 hrs.
9). Through position L. (the 14:11 E.P.) the first position line was again drawn in as a transferred position line.
10). The second C.P. (CP2) was plotted, and from this the 9.5 intercept for the second observation was plotted towards the true bearing of 232º.
11). The second position line was drawn in at the end of the 9.5′ intercept at right angles to it and so as to cut the transferred position line at the 14:11 Observed Position, which was found to be:
Lat. 49º 52.8 N. Long. 01º 51.0 W.