34 THE ART OF TAKING SIGHTS

128. GENERAL REMARKS ON MAKING OBSERVATIONS.

The art of taking sights cannot be learned from books or texts such as this; only actual practice at sea in fair weather and foul, in daylight and in darkness, will give the experience necessary to make an efficient Astro-navigator. Consideration of the following points, however, should enable the budding navigator to understand thoroughly how to use his most treasured possession at sea – his sextant – to the best advantage.

It is not intended here to repeat the basic methods of observing the various celestial bodies with a sextant which were described in detail in § 9-13 and which the beginner must practise repeatedly in non-critical circumstances until he is thoroughly familiar with them.

Here we are concerned more with the techniques to be adopted at sea when sights are being taken in earnest to establish a position line or the vessel’s observed position In taking an observation the navigator should chose a position which is well protected from spray as he can find and in which he can stand comfortably, with his feet and legs firmly braced and the upper part of his body free to bend and sway. In the average small yacht two such positions are; the forward end of the cockpit close against the cabin, or on deck supported by the mast or shrouds. In a seaway, a high position is preferable since from such a position, the effect of waves breaking the horizon or giving a false horizon, will be less. For the same reason it is best to take a sight when the vessel is on the crest of a wave, but in this case the navigator should add half of the estimated height of the wave to his still-water H. of. E. when later reducing the sight. When the sea is relatively calm, a lower position is preferable especially in gloomy weather, because then the horizon is usually more clearly defined. It is a waste of time to take sights against an obviously woolly or hazy horizon from a small craft.

When angles are measured with a sextant, it is important that the body should be observed in the centre of the field of the telescope in order that the ray from the body may be parallel to the plane of the instrument. Having brought the celestial body into initial contact with the horizon, always swing the sextant a few degrees either side of the vertical plane to make the body appear to describe the arc of a circle (see fig. 00 in § 00 Lesson 3). This arc should then be raised or lowered by means of the tangent screw or micrometer drum until it just kisses the horizon. If the Sextant Altitude is measured at this point, there will be no doubt that the vertical angle has been obtained. It is distinctly preferable to allow contact to be made by the motion of the celestial body itself if this is possible, and when this method is used, knowledge of whether the body is rising or setting is important (if the body is W. of the observer’s meridian it will be setting, if E. of the meridian it will be rising). If the body is setting, move the tangent screw or micrometer drum of the sextant until the body is slightly above the horizon; then, leaving the sextant set, swing the instrument gently from side to side until the body (or the limb in the case of the Sun or Moon) just kisses the horizon, when the time and measurement of the observation should be recorded. If the body is rising, move the tangent screw or micrometer drum until the body or limb is slightly below the horizon, then carry out the same procedure.

For star observations it is well to remember that by adjusting the rising piece of the sextant a greater or lesser amount of light enters the telescope from the horizon or from the observed body respectively. During the end of evening or the beginning of morning twilight, when the horizon is dim but the observed star bright, it is advisable to adjust the rising piece to ensure the maximum amount of light is entering the telescope through the un-silvered portion of the horizon glass. Conversely, during the beginning of evening twilight, or the end of morning twilight, when the horizon is bright but the observed star dim, the rising-piece should be adjusted to allow only a small amount of light to enter the telescope through the un-silvered part of the horizon glass, so that the contrast between the brilliancy of the observed star and that of the horizon is sharp.

129. THE TIMING OF OBSERVATIONS

Except in the case of the meridian altitude of a body (when the time is not required), a sextant observation must be timed by a chronometer or deck watch. Although in small vessels a single observer may have to do the whole operation himself, it is infinitely preferable for the observer to be assisted by another crew member (who should, of course, be properly instructed in advance and tested for competence, since his function is of supreme importance in the successful navigation of the vessel).

When the navigator is about to go on deck to take his sights, he should ask his timekeeper to prepare to take his times, i.e. the time of each observation. This means that the time-keeper should station himself at the open chronometer or deck watch with the Sight Record Book open and pen or pencil ready; he should also, for example, ensure the cabin hatch is open and generally make the best arrangement to make certain he will hear the navigators call.

When the navigator has the celestial body close to the horizon in his sextant, he should call out stand-by, which should be answered by his timekeeper (this is important) who should then stand with his eyes fixed on the chronometer face and count round with the seconds. As the navigator makes exact contact between the horizon and the celestial body and feels he has a good sight, he should shout stop! On hearing this, the time-keeper should record the exact second of hearing this call, then the minute and finally the hour.

He should then immediately check this, especially the minute, to make sure this has not been read in error (when the second-hand points to about 50 sec, it is extremely easy to read the next minute by mistake,

and as one minute of time means an error of about 15 miles, great care must be taken to avoid this). The time-keeper should also record the clock time (Zone Time) and the log reading in the Sight Record Book.

The navigator should have by this time read his sextant, and will call out the name of the body observed and the altitude. Before moving the index bar of the sextant, the navigator should again check the altitude, which the time-keeper should repeat as a check, after he has recorded it in the Sight Record Book. The next call should be to stand-by again – when the same operation should be repeated as many times as necessary.

Should the time-keeper be experiencing difficulty in hearing the navigators calls (owing to distance or extraneous noise such as wind) the navigator should use a whistle, the most commonly used signals being two. blasts for stand-by and one blast for stop.

If the navigator does not have an experienced man reading the time, he should return to the chronometer position after each sight and glance at the chronometer himself – before reading the sextant – and note especially whether the minute has been recorded correctly.

In the above Description it will have been noted that when taking chronometer or Deck Watch times the three hands of the instrument (second, minute and hour hands) should be read in order of their rapidity of motion. For instance, the time on the chronometer in fig. 34-1 (a) would be read in the order 34 seconds., 00 min., 08 hours. and recorded in the Sight Record Book as 08.00.34, writing from right to left. Similarly, the time on the chronometer in fig. 34-1(b) would be read in the order ) 40 seconds, 57 minutes, 10 hours, and seconded in the Sight Record Book as 10.57.40, writing from right to left.

If an assistant is not available the navigator must time his own observations. The common practice is to count the seconds in your head from the instant you say stop (to yourself) till you have walked to the chronometer and noted the chronometer time, and then to reduce the chronometer time by the interval in seconds. The expert Astro-navigator should be able to count seconds accurately so that no material error results when timing his own sights, but if the beginner doubts his own ability to do this, the slightly more cumbersome method of using a stop-watch can be employed. In this, the stopwatch should either be slung round the neck or strapped to the wrist of the left hand. As soon as the altitude has been measured, the watch should be started immediately and on returning to the chronometer position, not stopped until the second hand on the chronometer or Deck Watch reaches 0. when the stopwatch should be stopped instantly. Read and record the chronometer or D.W.T. (hours and minutes) and subtract the elapsed time on the stopwatch. For example, if the stopwatch is stopped when the chronometer reads 08h.44m.00s. and the stopwatch reads 35 seconds, the correct time of observation to record in the Sight Record Book would be :- 08h.43m.25s.

No attempt should be made to correct or allow for the accumulated error of the chronometer or deck watch at the time of taking sights. The important thing at this stage is to measure altitudes and record times with the minimum of delay – the time for calculations occurs later when all the observations have been completed. Besides, students will already have noted that there is provision on the standard Sight Forms for the allowance for Deck Watch or Chronometer Error and if this is applied before recording in the Sight Record Book, there is a danger of it being applied again (with possibly disastrous results) when the sight is worked out. For the same reason the I.E. (if any) of the sextant should not be applied mentally when recording the altitude in the Sight Record Book. As stated, a series of sights must be taken and timed with the absolute minimum of delay and all that is required during this time is the Sext. Alt. and the chronometer or deck watch time as read. All allowances, errors and calculations should be left until the observations are completed and the sights are being reduced.

130. TAKING SIGHTS IN ROUGH WEATHER

In a small vessel, using a sextant is not an easy matter in rough weather, and it is strongly advisable for the observer to wear a life-harness, clipped on, since when busy taking sights he is not in a position to spot in advance the odd rogue wave which may cause the vessel to pitch or roll suddenly. In these conditions it is not advisable to take sights standing; adopt any position where the lower part of the body is well anchored but the upper torso is free to sway with the motion of the vessel, so that one can maintain a roughly vertical position. To guard against the risk of dropping the sextant (easily done when making one’s way up on deck or back). it is prudent to have a lanyard on the sextant, which can be slipped over the head.

Since in a small vessel the observer’s eye is situated only a few feet above the sea, the horizon is close and is frequently obscured by waves, care must be taken that the reflected image of a celestial body is brought down to the actual horizon and not to a false horizon or to an intervening wave. For this reason, in rough weather, sights should be taken from as high a point as practicable, and the actual sight observed just when the vessel is on the crest of a wave. Whenever possible, bring the vessel round until the celestial body being observed lies nearly ahead or astern as the vessel usually pitches less than she rolls, but if the converse is the case, then bring the body abeam. It may frequently be best to heave-to while taking sights on a small vessel.

When the sea is rough and the vessel is pitching or rolling heavily, perfect sights are impossible. It is not therefore recommended to spend too long trying to get a perfect sight. Try to establish coincidence between the body and the horizon quickly and take the time at once. Do not waste minutes striving for perfection – the result will not be good as the sight taken carefully but smartly. Whether or not it is the navigators normal practice to take a series of sights of the same body and average them (as described earlier). in rough weather this is almost essential. Three or even five sights should be taken, tested for the rate of change of altitude and any rogue sights discarded. and the mean of the altitudes and times of remaining sights used for working.

When observing meridian altitudes in small vessels in rough weather, the rise and fall due to the waves causes the sea horizon to fluctuate owing to the alteration in the observer’s H. of. E.. Since it is impossible to keep the Sun’s limb in constant contact with the horizon it is preferable to take a number of observations close to noon in quick succession, continuing to do so until it is obvious that the altitude is decreasing. Whilst generally the mean of these observations would be taken as the maximum or meridian altitude, if the mean of the highest and the two altitudes on either side is taken as the meridian altitude, the greatest accuracy will be obtained.