31 COMPASS ERR & DEV. BY AZIMUTH & AMPLITUDE OBSERVATIONS

118. COMPASS ERROR AND DEVIATION BY AZIMUTH OBSERVATION OF *POLARIS

The prudent navigator never loses any reasonable opportunity of checking his compass error and hence the Dev.. The error of the compass is found by comparing the observed compass bearing of an object with that object’s true bearing at the time of the observation. In coastal navigation this is most easily achieved by observing two landmarks when they come in transit. When out of sight of land, however, the only way of checking the error of the compass is by observing the bearing of a celestial body. In merchant ships this is done at least once in every watch when on a steady course, and also shortly after the compass has steadied whenever the ship’s course has been altered. Although on small craft it may not be necessary to check the compass quite so frequently, it should nevertheless be done at least twice a day on steady courses and also whenever a marked alteration of course is made.

The azimuth or true bearing of any celestial body is, of course, found whenever a sextant observation is taken for a position line: it is the Zn used for plotting the intercept. The usual way for a vessel out of sight of land to check the error of her compass is to take a compass bearing of the Sun or Moon by day, or a star, planet, or Moon by night.  If this is performed at the time of taking sights this operation would be performed by a second person, the bearing required being that when stop is called by the observer at the instant the altitude is measured. As the azimuth has to be aken from the tables for the purpose of plotting the intercept, it can also be used to compare with the compass bearing to find the compass error and thence the Dev. for the course the vessel is steering at the time.

Suppose the compass bearing of the Sun was 110° C. at the same instant as a morning observation of the Sun for a position line, and that it was found in working up the sight that the Sun’s true bearing was 102° T. then: 

Sun’s bearing            11O° C. Compass best, error W.

Sun’s bearing            102° T. Compass least, error E.

Compass Error         008° W.

To find the Deviation of the compass you need to know the magnetic Variation for the locality. You have to remember that while crossing the oceans the value of the Variation may change daily. For example, if you are crossing the Atlantic Ocean, the Variation may change from about 5°W in the English Channel to about 20°W in mid-ocean if you are making for the West Indies (see fig. 31-1 which is a portion of the Admiralty Magnatic Variation chart of the North Atlantic Ocean). It is wise to carry such a chart of the area in which you are planning to sail.

Continuing the example above, suppose the Variation taken from the Variation chart was 5°W, then the computation would be as follows: –

The Variation Chart would then be consulted to determine the Variation of the vessel’s position, and if this happened  to be 8° W. then the Dev. must be nil as the comass error is the combined effect of variation and Dev.. If the Variation was 5° W., however, the computation would be as follows:-

Sun’s bearing            102° T.       Variation is the difference between True and

Variation                    005° W.     Magnetic

Sun’s bearing            107° M.     Dev. is the difference between Magnetic and

Sun’s bearing            110° C.       Compass

Dev.                            003° W.

This operation is not, of course, limited to the time of taking sights. An azimuth observation of a celestial body may be taken at any time the sky is clear, simply noting the G.M.T. at the same time as the compass bearing is taken. Next, calculate the L.H.A. of the body observed, enter the Sight Reduction Tables with this angle, the body’s Dec. and the D.R. Lat. and extract the body’s true bearing. Finally, apply the Variation (preferably from a Variation Chart) for the vessel’s position to convert from true to magnetic, and compare the result with the compass bearing to determine the Dev. for the direction of the vessel’s head at the time of observation.

The Compass Error is 004° E. and Dev. is 006° E. on this heading.

When using Sight Reduction Tables to find the True Bearing it must be remembered that the SRT for Air Navigation AP3270 Volumes 2 and 3 are limited to Declinations less than 30°. The SRT for Marine Navigation NP401, however, can be used for Declination of any celestial body provided that the appropriate volume covering the Latitude is used.

119. AMPLITUDE – THE THEORY

The Amplitude of a heavenly body is ‘the angle between the true EAST point of the compass and the body when rising’ and ‘between the true WEST point of the body when setting’.

This amplitude can only be observed when the body is just clear of the horizon when rising or setting and at no other time. In the case of the Sun, it is the bearing of the Sun’s centre when on the horizon and when its altitude is 0º.

When a body is at its maximum altitude, the effect of refraction is at its greatest (Norries P.160). In the case of the Sun, at 0º altitude, refraction is 33’ which is almost exactly the angle subtended by the Sun’s diameter from the Earth’s surface. To obtain the point at which the Sun’s altitude is exactly 0º, the measurement must be made when the Sun is apparently one half its diameter clear of the horizon.

120. PRACTICAL USE

The observer afloat can make use of the Sun’s Amplitude at Sunrise and Sunset to check the error of a vessel compass.

The normal Amplitude Tables (Norries, MacMillans etc) are entered with the observers latitude and the Sun’s declination at sunrise or sunset (GMT). The Amplitude extracted may be a true bearing (360º notation) or may be given in the following way:

For Sunrise the amplitude is ‘Degrees NORTH or SOUTH of EAST’

For Sunset the Amplitude is ‘Degrees NORTH or SOUTH of WEST’

The NORTH or SOUTH element is established by the Sun’s Declination (N or S) and the final angle can be converted to the 360º notation which can then be compared with a bearing of the Sun taken by the vessel compass and corrected for Variation (see fig. 31-1).

121. COMPASS ERROR AND DEV. BY AMPLITUDE OBSERVATION

Unless a small vessel’s steering compass is fitted with an azimuth ring it will not be possible to take accurate compass bearings of celestial bodies which have any appreciable altitude. In the case of the Sun, it is much easier, and far more accurate to take the compass bearing when the Sun is on the horizon at sunrise or sunset. It will be evident that any celestial body of N. Dec. will rise N of E and set N of W and any body of S. Dec. will rise S of E and set S of W. Accordingly, its azimuth at rising and setting will differ from 90° and 270° (90°W.). The difference between 90° (E or W. as the case may be) and the azimuth of a body at rising or setting are called the Bearing Amplitude, and because the azimuth is changing very slowly at rising or setting, the Sun’s azimuth at sunrise or sunset forms a very convenient basis for a check on the compass. It is also extremely simple to calculate the Sun’s true bearing at this moment.

The true bearing of the Sun at sunrise and sunset is given in Burton’s or Norie’s Nautical Tables under True Amplitudes and in Reed’s N.A. under Sun-True Bearing at Sunrise and Sunset. All three of these tables require entry with only the D.R. Lat. and the Sun’s Dec. to the nearest ½°. Entered with these arguments the tables give either the Sun’s True Bearing Amplitude (Burton and Norie), or the Sun’s true bearing (Reed’s).

The only difference is that the Amplitude is the number of degrees between due E. (090°) on rising, or due W. (270°) on setting, and the direction of the Sun, measured towards N. when the Sun’s Dec. is N. or towards S. when the Sun’s Dec. is S. The bearing given in Reed’s N.A. is the true bearing measured from N. if the Sun’s Dec. is N, or from S. if the Sun’s Dec. is S, towards E. on rising or towards W. on setting (see fig. 31-3).

When observing the amplitude of the Sun or the Moon it is important that the body should be on the celestial horizon at the time of observation, that is when the body’s True. Alt. is 0°.  This occurs when the centre of the Sun or Moon’s disc is in a horizontal plane through the observer. Unfortunately, this does not occur when the centre of the body’s disc is on the sea horizon as in fig. 31-4 (a) – this is called the apparent amplitude.

Owing to the effects of refraction and parallax the true amplitude (as recorded in all amplitude tables) should be observed when the body’s lower limb is above the sea horizon by a distance approximately equal to its own radius, as shown in fig. 31-4 (b). In moderate Latitudes, it is sufficient to judge this moment and observe accordingly, but in latitudes above 55°, where the diurnal circles of celestial bodies cut the horizon obliquely, greater precision is required. In the case of these higher latitudes, amplitude observations should either be timed or the apparent amplitude (the body appearing as in fig. 31-4 (a)) observed and corrected by means of the supplementary table in Burton or Norie.

Compass Error 002° W. and Dev. 003° W. for Vessel’s Heading at Sunset

122. COMPASS ERROR AND DEV. BY AZIMUTH OBSERVATION OF *POLARIS

Another method used to determine the compass error and Deviation during the hours of darkness on a clear night in the northern hemisphere is by taking the compass bearing of *Polaris. Inspection of the Pole Star Tables in the Nautical Almanac (Page 16 of the AN Pamphlet) shows that the altitude corrections are followed (at the bottom of the page) by a table giving the azimuth of *Polaris. This is entered with the L.H.A. Aries (to the nearest degree) at the top fo the page, and the true bearing of *Polaris extracted opposite the Latitude.