Noon Sight

Q. Have you ever taken a sight?
Ans. Yes, I have been taking Sun’s morning and noon sights.

Essex Brass® 66096 Brass Sextant

Q. What is the best time to take a morning Sun’s sight?
Ans. Since the basic aim is to run fix the position line with the E-W position line obtained from meridian passage of Sun, it would be best if the AM position line is N-S or nearly so, which means the sun should be on the observer’s prime vertical or near it. This however, is not always possible because, for the Sun to be on the Observer’s Prime Vertical, the Lat and Dec must be of same name with D < L.

The two conditions which are important for the AM sight then are:

  1. The Sun should not be less than 150 above Observer’s Rational Horizon. This is because, when the body’s altitude is less than 150, the refraction correction is high and may not be accurately known from the tables.
  2. Azimuth should be as close to 900 as possible, so that the angle of cut with the parallel of latitude at noon is nearly 90o.

Q. How do you ensure that the time and altitude are accurately taken?
Ans. To start with, one always must ensure that errors are corrected or are very small and are known correctly. When the reflected Sun is seen on the horizon normally with lower limb touching the visible horizon and plane of sextant vertical, the sextant is rocked about horizontal axis so that lower limb of Sun swings along an arc tangent to horizon. When the body just touches the horizon, the officer shouts, ‘Stop’. Just before shouting ‘stop’, the person taking the sight would caution the cadet reading the chronometer shouting ‘standby’. At the time ‘stop’ is shouted, the cadet at the chronometer reads and notes down the seconds of time.

Q. What details regarding the meridian passage may be found now?
Ans. Let us say the morning sun’s sight is taken at 0900 ship’s time. Using DR latitude & calculated longitude with best estimated course and speed that will be made good, position at 1200 ship’s time is found. Longitude at 1200 ST is used to find longitude in time (LIT) that will be applied to the LMT meridian passage, read from the almanac to give GMT mer-pass. GMT meridian passage with ship’s time difference wrt GMT will give ship’s time of meridian passage.

Q. Meridian passage time may be different than 1200 ST. Will you re-calculate using a new LIT then?
Ans. No, an error of 15 min in longitude, used to find LIT will give an error of only one minute in time. The error in time found is not likely to be more than 1 min. Second officer usually would have ensured that mer-pass time by ship’s clock is within about 20 minutes of 1200. In any case, the second officer must be ready with sextant and pre computed altitude at least about 10 minutes earlier than the expected time of meridian passage.

Q. How is the sextant meridian altitude calculated?
Ans. Suppose, the ship’s time of meridian passage is 1215. DR latitude of 1215 ST is found. Using this latitude and declination of Sun at meridian passage time, with the help of observer’s rational horizon diagram, MZD and hence meridian altitude can be found. Corrections to altitude are now applied in reverse way to get computed sextant altitude or CSA

Q. How do you actually take lat by meridian altitude? In which direction will you find the Sun?
Ans. For an estimated mer-pass time of say 1215ST, the 2nd officer should be ready on the bridge wings with the sextant set to CSA at say 1205 ST itself. In a particular case, say Lat N and smaller N’ly dec, as shown in the fig, observer will find the Sun bearing 180o at mer-pass.

The azimuth at mer-pass can be easily found by drawing a diagram of Celestial Sphere in plane of the Observer’s Rational Horizon. The lower limb is set on horizon & altitude is taken and then after say a minute or so altitude is taken again (Stage 1 & 2). Observer finds increase in altitude as the lower limb leaves the horizon. The altitude is again set with the Sun’s LL touching the horizon (3). After about a minute or so the LL again leaves horizon (4). This process is repeated (5-6 & 7-8). The rate of change of altitude reduces & gradually approaches zero (8) and then the Sun appears to become steady on the horizon (9).

One must witness the setting in altitude of the Sun (10). The steadying of the Sun may occur, say at 1216 ST. The mer-pass is said to occur at 1216 ST. This time is simply noted down from a wrist watch or from the clock in the navigating bridge. Accurate chronometer time or GMT is not required as only the declination of Sun is required and not the GHA. Change of the dec of Sun is negligible in say 1 or 2 minutes. From the ORH diagram above, observed lat at 1216 is found using MZD & Dec. This lat is worked backwards for 16 minutes to get observed lat at 1200 ST. Thus, in effect 0900ST position line is run forward till 1200ST. Intersection of the two transferred PLs (AM & Noon) gives the fix at 1200ST.

\dfrac {d'lat}{d'long\cos lat}=\tan Az
\therefore d'long=\dfrac {d'lat}{\tan Az\times\cos lat}=d'lat\times C

Q: Can you find the noon longitude without plotting?
Ans: Yes, on ships, the formula used is, long correction = latitude error × ‘C’ correction” (AM sight)”. The longitude correction (d’long) is applied to DR long at 1200ST. Let us assume that, the azimuth of Sun was S800 E in morning. In a particular example, if the observed lat at 1200ST is found to the north of 1200 DR lat, latitude error is northerly. We can see in the figure that, for a northerly error of latitude, the longitude error is easterly.

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