When a compass is not available, different techniques may be used to determine the four cardinal directions.
Shadow-Tip Method. This simple and accurate method of finding
direction by the sun consists of four basic steps.
Determining Directions and Time by shadow
Step 1. Place a stick or branch into the ground at a level spot where a
distinctive shadow will be cast. Mark the shadow tip with a stone, twig, or
other means. This first shadow mark is always the west direction.
Step 2. Wait 10 to 15 minutes until the shadow tip moves a few inches. Mark
the new position of the shadow tip in the same way as the first.
Step 3. Draw a straight line through the two marks to obtain an approximate
eastwest line.
Step 4. Standing with the first mark (west) to your left, the other
directions are simple; north is to the front, east is to the right, and south is
behind you.
(1) A line drawn perpendicular to the east-west line at any point is the
approximate north-south line. If you are uncertain which direction is east and
which is west, observe this simple rule-the first shadow-tip mark is always in
the west direction, everywhere on earth.
(2) The shadow-tip method can also be used as a shadow clock to find the
approximate time of day.
(a) To find the time of day, move the stick to the intersection of the
east-west line and the north-south line, and set it vertically in the
ground. The west part of the east-west line indicates 0600 hours, and the
east part is 1800 hours, anywhere on earth, because the basic rule always
applies.
(b) The north-south line now becomes the noon line. The shadow of the
stick is an hour hand in the shadow clock, and with it you can estimate the
time using the noon line and the 6 o’clock line as your guides. Depending on
your location and the season, the shadow may move either clockwise or
counterclockwise, but this does not alter your manner of reading the shadow
clock.
(c) The shadow clock is not a timepiece in the ordinary sense. It makes
every day 12 unequal hours long, and always reads 0600 hours at sunrise and
1800 hours at sunset. The shadow clock time is closest to conventional clock
time at midday, but the spacing of the other hours compared to conventional
time varies somewhat with the locality and the date. However, it does
provide a satisfactory means of telling time in the absence of properly set
watches.
(d) The shadow-tip system is not intended for use in polar regions, which
the Department of Defense defines as being above 60 degrees latitude in
either hemisphere. Distressed persons in these areas are advised to stay in
one place so that search/rescue teams can easily find them. The presence and
location of all aircraft and ground parties in polar regions are reported to
and checked regularly by governmental or other agencies, and any need for
help becomes quickly known.
Watch Method. A watch can be used to determine the approximate true
north and true south.
(1) In the north temperate zone only, the hour hand is pointed toward the
sun. A south line can be found midway between the hour hand and 1200 hours,
standard time. If on daylight savings time, the north-south line is found
between the hour hand and 1300 hours. If there is any doubt as to which end of
the line is north, remember that the sun is in the east before noon and in the
west after noon.
(2) The watch may also be used to determine direction in the south temperate
zone; however, the method is different. The 1200-hour dial is pointed toward the
sun, and halfway between 1200 hours and the hour hand will be a north line. If
on daylight savings time, the north line lies midway between the hour hand and
1300 hours
Determining Direction using a Watch.
(3) The watch method can be in error, especially in the lower latitudes, and
may cause circling. To avoid this, make a shadow clock and set your watch to the
time indicated. After traveling for an hour, take another shadow-clock reading.
Reset your watch if necessary.
Star Method. Less than 60 of about 5,000 stars visible to the eye are
used by navigators. The stars seen as we look up at the sky at night are not
evenly scattered across the whole sky. Instead they are in groups called
constellations.
(1) The constellations that we see depends partly on where we are located on
the earth, the time of the year, and the time of the night. The night changes
with the seasons because of the journey of the earth around the sun, and it also
changes from hour to hour because the turning of the earth makes some
constellations seem to travel in a circle. But there is one star that is in
almost exactly the same place in the sky all night long every night. It is the
North Star, also known as the Polar Star or Polaris.
(a) The North Star is less than 1 degree off true north and does not move
from its place because the axis of the earth is pointed toward it. The North
Star is in the group of stars called the Little Dipper. It is the last star in
the handle of the dipper. There are two stars in the Big Dipper, which are a
big help when trying to find the North Star. They are called the Pointers, and
an imaginary line drawn through them five times their distance points to the
North Star.
(b) Many stars are brighter than the North Star, but none is more important
because of its location. However, the North Star can only be seen in the
northern hemisphere so it cannot serve as a guide south of the equator. The
farther one goes north, the higher the North Star is in the sky, and above
latitude 70 degrees, it is too high in the sky to be useful.
Determining Direction by the North Star and Southern Cross
(2) Depending on the star selected for navigation, azimuth checks are
necessary. A star near the north horizon serves for about half an hour. When
moving south, azimuth checks should be made every 15 minutes. When traveling
east or west, the difficulty of staying on azimuth is caused more by the
likelihood of the star climbing too high in the sky or losing itself behind the
western horizon than it is by the star changing direction angle. When this
happens, it is necessary to change to another guide star. The Southern Cross is
the main constellation used as a guide south of the equator, and the general
directions for using north and south stars are reversed. When navigating using
the stars as guides, the user must know the different constellation shapes and
their locations throughout the world.
No comments:
Post a Comment