3. Azimuths

A certain officer of the Navy once recounted his youthful relief when he found that the azimuth of the sun was nothing more than its bearing. Another meaning of the Arabic alsumut is direction. Either word solves the mystery and finding the azimuth of. any ob­served body is most simply. Rather, the student blunders in the use of the figures.

Azimuths, defined. Consideration of several definitions is helpful to a complete understanding of the azimuth angle. It may be defined in various ways:

(1) From the ship: Bearing of body observed.

2. On earth: Angle at ship between meridian arid line to geo­graphic position (G. P.) of body.

3. On celestial sphere: Angle at zenith between celestial meridian and arc from zenith to body.

4. Per Bowditch: "...angle at the zenith between the meridian of the observer and the vertical circle passing through the point."

5. Mathematically: The angle of the astronomical triangle at the corner marked by the ship on earth or by the zenith on the ce­lestial sphere.

How found and used. Consider the case of the sun. Its bearing will vary with the time of day, its declination, and the ob­server's latitude. With these factors the azimuth may be computed.

However, since the publication of Burdwood's Tables (1866), these and other tables and azimuth diagrams have made special computa­tions for azimuth unnecessary.

Azimuth tables are now used principally as a source of true bear­ings of the sun for use when swinging ship for compass errors. They are also useful for indicating when the sun's bearing will have changed sufficiently to give a good running fix, or otherwise give a needed line of position.

Computed azimuths are used for plotting the lines of position of celestial navigation. The required azimuths result from the solution of the sights or are taken from tables of computed solutions.

Azimuths are measured by the arc of the horizon from north or from south to the point on the horizon directly below the body. This point is where a vertical circle from the zenith through the body meets the horizon.

The azimuth of an observed body may be expressed in various ways. However, all azimuth tables in common use and the different methods of computing the azimuth when working sights give the azi­muth as from 0 to 180°, and it is assumed that the navigator knows how to use the figures. This simple question perplexes many students.

The beginner should avoid the confusing older method of expres­sing the azimuth only up to 90°, as with the quadrant card, although it is required with certain azimuth diagrams and with Lecky's famous A.B.C. tables.

Naming the azimuth. Tabulated or computed azimuth values must be named to indicate how they are measured. Examine Fig. 36 and observe that:

(1) Azimuths are measured from N to S, 180° through E or W.

2. In the northern hemisphere, azimuths are measured from N through E when the body observed lies east of the meridian: from N through W when the body is west of the meridian. In the drawing, the 110° azimuth of the sun is measured from N through E, and named N 110° E. That of the moon, west of observer's meridian, is named N 110° W.

3. When in the southern hemisphere, azimuths are measured from S through E when the body is east of the meridian and from S through W when the body lies west of the meridian. Thus the 120° azimuth of the pictured star is named S 120° E, and Jupiter's azi­muth is named S 110° W.

. How to name azimuth values of the 0° to 180° variety is sum­marized in this rule: Put the name of the latitude (NorS) in front of the value and the name of the meridian angle (EorW) after it.

The azimuth value named as above (Z), may be used without further modification for -plotting summer lines. Navy teaching is to convert Z to Zn, which is the azimuth measured from the north point clockwise to 360°, as most convenient for plotting lines of po­

н

sition and for compass work. No name need be given to the value of Zn because Zn means azimuth measured from north.

Converting Z into Zn. The 0° point of what may be called the azimuth rose of the northern hemisphere is true north, 0° on the 360° rose. In the southern hemisphere, however, 0°for azimuth is true south or 180° on the 360° rose. Study Fig. 36 and determine for yourself how N to E, N to W, S to E, or S to W azi­muths must be handled to find Zn. Or if this be difficult, observe the following rule for converting Z to Zn.

When N to E, no change in value N to W, subtract from 360° S to E, subtract from 180° S to W. add to 180°.

It is difficult to understand the ever-present classroom difficul­ties with naming as azimuth and converting it to Zn. The azimuth of a body is its bearing. At sea a man knows where the body was that he observed. Try to think in the same way, and avoid mistakes when naming, converting, and plotting azimuths.