outdoor living » the 10 bushcraft books » time & direction (pt. 1)

.introduction

The measurement of time, and the obtaining of accurate direction (from North) are not primitive skills. Of the two, direction is the more recent development, although to the Polynesians it is older than their awareness of time.

Obtaining time and direction without equipment is practical, and in general can be more accurate than the average person's watch or compass.

Both words, "time" and "direction", are inter-related because if one has accurate time, accurate direction is obtained in a matter of seconds, or if one has accurate direction (from North) then accurate time is immediately practical without a watch.

The methods given in this book have been proved in jungle and desert and are applicable anywhere on the earth's surface.

The subject of navigation has been surrounded by many technical words, necessary to the science, but in this work the author has attempted to simplify the whole subject, and endeavoured to avoid words which would have no meaning to the average reader.

Although a compass is the accepted method of obtaining direction, it is not always reliable, nor is it of very great value in dense bush, or areas where deposits of iron affect its needle. A watch is the accepted means of measuring time, but the watch may be out of action, and therefore it is necessary to have other methods to obtain both time and direction.

.definitions

'Time' is our method of measuring the intervals between events. The most regular event in our daily lives is the movement of the sun, and therefore for everyday purposes time is measured by the sun's movement. The stars provide a more accurate method of measurement and are used by navigators and astronomers. 'Direction' is the line or course to be taken, and in this case can be considered as from North or one of the cardinal points of the compass.

.sun movement

As you know, the sun crosses the imaginary North-South line (Meridian) every day when it reaches its highest point (Zenith) above the horizon.

Therefore when the sun is at its highest point in the sky it is North or South of you, depending upon your position on the earth's surface, and the sun's position relative to the earth's equator.

For all practical purposes there are twenty-four hours between each sun crossing of your North-South line, or Meridian. During the twenty-four hours the earth will have revolved apparently 360 degrees; therefore it will move 15 degrees for each hour, or one degree in four minutes. This is very convenient to know, because if you know the North or South accurately, you can easily measure off the number of degrees the sun is from the North-South line, and this will give you the number of hours and minutes before, or after noon. These measurements must be made along the curved path of the sun, and not on a horizontal or flat plane.

.time from the sun with compass

A means of measuring degrees - arms must be fully extended.

These vary slightly like your personal dimensions and for accuracy should be accurately checked by each individual with a compass.

By this means, if you have a compass, time can be easily read from the sun's position. This should be possible to within four or five minutes. Decide from your compass your true North-South line and remember to make allowance for the magnetic variation from True North. Measure the number of degrees the sun is from this imaginary line, and multiply the number of degrees by four to obtain the number of minutes.

For Example:

Here the sun is 34 degrees from the North-South line. It is morning, because the sun is on the eastern side of the North-South line, 34 x 4 = 136 minutes before noon; therefore it is sixteen minutes to ten in the morning local sun time.

This does not mean that it will be 16 minutes to ten by the local clock, because there are two corrections to be made before local standard (or clock) time can be determined. These two corrections are dealt with under the headings of Equation of Time, and Longitude Corrections.

It is sufficient for the moment that you can measure time accurately from the sun.

.accurate direction from sun with a watch

The method of obtaining direction from a watch by pointing the hour hand (or 'twelve o'clock' depending upon which hemisphere you live in) is not accurate, but only approximate.

The accurate method, knowing the time, is to calculate the number of degrees changed to minutes in time, before or after noon, and then to measure from the sun's position along the curved path of the sun through the sky. Even if you make no allowance for the two corrections (Equation of Time and Longitude Corrections), you will be accurate within five to eight degrees and if you make the two corrections for time you will be accurate to less than one degree.

For Example:

It is 2.16 p.m. by your watch, therefore the sun is to the west of the North-South line. 2.16 p.m. means that the sun has travelled 136 minutes of time past the North-South line. It travels one degree along its curved path in the sky every four minutes of time, so that it is 34 degrees along its path past noon. Measure this back along the sun's path and you will have true North. (For Northern Hemisphere read South for North and reverse all other cardinal points.)

.cardinal points and bearings

Having found the true North, you can find any bearing from true North very easily and within five degrees of error. If the bearing you want is less than 180°, face East, and stretch out your left arm to true North. Raise your right arm along your side till there is a perfectly straight line along both arms. Your right arm is now pointing to South or 180° True. Bring the two arms together evenly, and you are pointing to East or 90° True, and you can then measure the number of degrees from these cardinal points to the bearing you require. By facing West, and pointing your right arm to the North and your left to South you can get bearings greater than 180°.

.finding north-south line without compass or watch

Knowing that the still is at its highest point in the sky at midday, and that this point is on the North-South line means that by finding where this position will be, will give you true North.

You can do this by measuring the points of shadow made by the top of a fixed stake. These points of shadow may give you a curved line either concave or convex to the stake. Continue the curve made by the points of shadow, and then draw a circle on the ground round the base of the stake. Where the curved line cuts the circle will be accurate East and West, and a right angle from these two points will be an accurate North and South line.

Here you see the stake, and points of shadow recorded over an hour in the morning. The dotted line is a continuation of the curve made by the points, and the intersection of this curved line with the circle gives you East and West. If North of the Equator the cardinal points will be reversed.

This Shadow-stick method is very accurate, if done over a period of an hour or two.

.east-west line

During Equinoctial Periods. You will find from the foregoing that it is actually easier to find the true East-West line than the North-South. The idea of always working from, or to, North is largely conventional. The top of every map is assumed, unless marked otherwise, to be North. All bearings are measured clockwise from true North, but in actual practice it is often easier to find one or other of the cardinal points, rather than concentrate on finding the North Point. An instance is the ease with which the East-West line can be discovered.

There are two days in the year when the points of shadow will form an accurate East-West line throughout the whole day. These two days are the 21st March, and the 21st September, the days when the sun is over the Equator. On these two days the sun is at right angles to the axis of the earth, and therefore directly over the Equator, and no matter where you are on the earth's surface the shadows will move true East and West on these two days. Because of this if you mark a point of shadow by putting a peg into the ground, and then, five minutes later, mark the new position of the same shadow you will have a perfect East-West line. For general purposes if less than 40° North or South latitude this method will serve you for about two or three weeks either side of the Equinoctial periods with reasonable accuracy, so that on any day between March 1st and April 14th or September 1st and October 14th you can assume that the shadow line is very nearly a true East-West line. At all other periods or when you want greater accuracy you will have to work out the curve and extend it to the edges of the circle as in the preceding section.

The points of shadow move accurately true East and West on March 21st and September 21st.

An extremely accurate method of finding true North is to work out the hour angle of the sun and transfer this hour angle to the shadow thrown onto the ground from the string of a plumb bob.

To find the hour angle, use the method given in the section on The Sun Compass and extend from the shadow of the stick, the hour angle correct for your latitude and date.

The sun compass diagram does not require to be set correctly to work out the hour angle. Any direction will serve for the imaginary North-South line.

When the triangle has been worked out, a corresponding triangle is made on the correct side of the shadow from the cord of the plumb bob.

You should work out the hour angle on the sun compass on the ground about fifteen minutes ahead of the watch time, so that when you have worked on the diagram and made the necessary time and longitude corrections, you will be able to plot the hour angle at precisely the right moment on the shadow. This method, if done accurately and corrections of time for longitude and Equation worked out, should be correct to within less than a quarter of a degree, or one minute of time.

[Get you hands on a simple Sun Compass/Clock generator here.]

.finding local time without compass

It is apparent that if you can find North-South by the method given from the shadow of the stick that you can then work out the number of degrees the sun is off the North-South line and thereby discover the correct local time, provided you know the longitude of standard time, and the longitude of your position.

.the sun's path through the sky, and to find the sun's height in the sky for any period

To be able to accurately measure the sun's path along the sky you must know how high it is at its highest point (Zenith), and to find this out, you should be able to discover the sun's position North or South of the Equator for any day of the year.

This position of the sun is called 'Declination'. As you know, the sun is farthest North on June 21st, crosses the Equator September 21st, farthest South December 21st and recrosses the Equator on its way North on March 21st. This is caused by the inclined angle of the axis of the earth in relation to its path round the sun.

.to find the sun's position north or south of the equator

The degree, or slope of the inclined path is approximately 23½ degrees, so that when the sun is farthest North it is overhead 23½ degrees North of the Equator, and when farthest South it is overhead 23½ degrees South of the Equator.

It is possible to work a circle of 'Declination' showing you the path of the earth round the sun, and the reason.

You can draw this diagram on the ground. Take a straight stick and cut 23½ divisions along its length. The size of the divisions must be absolutely equal.

If you use the width of your knife blade, or some equally simple measure, it will serve. With this stick as a radius, draw a circle on the ground, and divide the circle into four quarters with straight lines that cross the centre of the circle.

Now divide each quarter of the circumference of the circle into three equal divisions. Mark these June, December, March and September as shown. Now divide each month into four smaller equal divisions. These represent the four weeks of the average month.

Draw a thick line from the start of the fourth division of June to the start of the fourth division of December, and from the start of the fourth Division of September to the start of fourth division of March.

These lines should intersect each other in the centre of the circle. The lines from June to December represent the North-South line, and the line from March to September the Equator.

For any day of the year find the approximate day on the outer circle and draw a line parallel to the Equator line to the North-South line, and then simply measure of with your stick the number of nicks from the Equator line, starting in the centre, to the date line. If the sun is on the June side of the Equator line it is North of the Equator; if on the December side it is South.

You should be accurate to within a quarter degree. This accuracy is needed for latitude work, but not necessary for the Sun Clock.

.the sun's height above the horizon

To the sun's declination you must make an allowance for your own latitude. For instance, if you are in latitude 42° North, and the date is April 21st, the sun will be 12° North, which means that at its zenith it will be 60° above the horizon. To work this out subtract your latitude from 90°, and add the sun's declination. If the sun is on the other side of the Equator, subtract the declination.

.methods of obtaining elevation of sun and stars

The elevation or height above the horizon of the sun or stars can be obtained by means of a plumb-bob quadrant - or, as Harold Gatty calls it in his 'Raft Book', a Harp. The quadrant harp is made with two pieces of cord, and a straight piece of wood. The dimensions of both cord and wood MUST be accurate. The wood should be straight and smooth, and not less than 18" [45 cm] long. Both ends should be flattened and a hole bored or burnt through the flattened ends. The holes should be exactly 18" [45.7 cm] apart on their inside edges.

Through these two holes, two lengths of cord are passed, with a thumb knot to hold them fast. Two pieces of cord are tied together at almost exactly 12¾ inches (or if 36" [91.4 cm] board - string must be 25.45" [64.6 cm]) and where they are joined a third thinner length of cord for the plumb-bob is also tied so that it swings from the joining of the two cords of the harp. This plumb-bob cord should be about 18" [45 cm] long. To the lower ends a weight such as a clasp knife or lead sinker, or a long thin stone is tied.

From the inside edge on one hole you mark off 9" [22.9 cm] on an 18" [45.7 cm] harp or 18" [45.7 cm] on a 36" [91.4 cm] harp, and again a second mark an equal distance from the other end. The two parts should meet exactly in the centre of the stick. On one side of the stick along the 9" [22.9 cm] (or 18" [45.7 cm]) side you mark the scale given in the illustration [notice that this is not a linear scale]. This scale is shown in short lengths for convenience, but for the marking on the stick you should read it as one scale. From 0° to 45° reads along one side on the stick and 45° to 90°, along the other side, or alternatively you can mark the stick continuously from end to end. To use the 'Harp,' sight upwards along the cord at the 90° end till the cord is aligned with the sun or star. The plumb-bob should be swinging almost free along the stick, and when the cord is aligned the plumb-bob string will just brush against the number of degrees of elevation of the sun or star observed. You can get a reading accurate to ½° or less with this 'Harp.'

For finer readings - make the base stick 36" [91.4 cm], the cord 25.45" [64.6 cm] and make each degree on the scale twice as long.