Direction Finding

The earth’s relationship to the rest of the solar system and the position of the stars in the sky help to locate any position on its surface. Its revolution on its axis produces the changes from light to darkness and its orbit around the sun produces the seasons, for the earth is tilted at an angle to the sun, and first the north and then the south becomes nearer to it, the closest point traversing from the Tropic of Cancer (23.5 N) to the Tropic of Capricorn (23.5 S), the sun being above Cancer on 22 June and above Capricorn on 22 December. It is above the Equator on 21 March and 21 September.

The sun rises in the east and sets in the west- but not EXACTLY in the east and west. There is also some seasonal variation. In the Northern Hemisphere, when at its highest point in the sky, the sun will be due south; in the Southern Hemisphere this noonday point will mark due north. The hemisphere will be indicated by the way that shadows move; clockwise in the north, anti-clockwise in the south. Shadows can be a guide to both direction and time of day,

Shadow Stick Method #1: on a patch of flat, clear ground, place a metre- long (3 feet) stick as upright as possible. Note where its shadow falls and mark the tip with a pebble or stick (a). Wait at least 15 minutes and mark the new shadow tip (b). Join the two and you have the directions east and west; the first mark is west. North-south will be at right angles to this line. This method works at any time of the day when there is sunshine and at any latitude. Use it for spot checks as you proceed.

Astro 14

Shadow Stick Methods 1 & 2

 

 

 

 

 

 

 

 

 

 

 

Shadow Stick Method #2: another more accurate method- if you have time- is to mark the first shadow tip in the morning. Draw a clean arc at exactly this distance from the stick, using the stick as a centre point. As midday approaches the shadow will shrink and move. In the afternoon, as the shadow lengthens again, mark the EXACT spot where it touches the arc. Join the two points to give east and west-west is the morning mark.

Direction by Watch: A traditional watch with two hands can be used to find direction, provided it is set to true local time (without variation for summer daylight saving and ignoring conventional time zones which do not match real time). The nearer the Equator you are the less accurate this method will be, for with the sun almost directly overhead it is very difficult to determine its direction.

 

Direction by Watch

Improvised Compasses: a piece of ferrous metal wire- a sewing needle is ideal- stroked repeatedly IN ONE DIRECTION against silk will become magnetized and can be suspended so that it points north. The magnetism will not be strong and will need regular topping up.

Improvised Compasses.JPG

Using Electricity

Plant Pointers: even without a compass or the sun to give direction you can get an indication of north and south from plants. They tend to grow towards the sun so their flowers and most abundant growth will be to the south in the Northern Hemisphere, the north in the South. On tree trunks moss will tend to be greener and more profuse on that side too (on the other side it will be yellowish to brown). Trees with a grainy bark will also display a tighter grain on the north side of the trunk.
If trees have been felled or struck down the pattern of. Rings on the stump also indicate direction- more growth is made on the side towards the Equator so there the rings are widely spread. There are even species of plant known for their north-south orientation.

Plant Pointers

The Wind Direction: if the wind direction of the prevailing wind is known it can be used for maintaining direction- there are consistent patterns throughout the world but they are not always the same the whole year round. Where a strong wind always comes from the same direction
Plants and trees may be bent in one direction, clear evidence of the wind’s orientation. But plants are not the only indication of wind direction: birds and insects will build their nests in the lee of any cover and spiders cannot spin their webs in the wind. Snow and sand dunes are also blown into distinctive patterns by a prevailing wind which blows from the outside of the high central ridges.

Making use of the Moon

Making Use of the Moon.JPG

The moon has no light of its own; it reflects that of the sun. As it orbits the earth over 28 days the shape of the light reflected varies according to its position. When the moon is on the same side of the earth as the sun no light is visible- that is the ‘new moon’ (a) – then it reflects light from its apparent right- hand side, from a gradually increasing area as it ‘waxes’. At the full moon it is on opposite side of the earth from the sun (b) and then it ‘wanes’, the reflecting area gradually reducing to a narrow sliver on the apparent left- hand side. This can be used to identify direction.
If the moon rises BEFORE the sun has set the illuminated side will be on the west. If the moon rises AFTER midnight, the illuminated side will be in the east. This may seem a little obvious, but it does mean you have the moon as a rough east-west reference during the night.

Direction by the stars: the stars stay in the same relation to one another and pass over the same places on the earth night after night. Their passage over the horizon starts four minutes earlier each night–a two-hour difference in time over a month. If you study a star at a certain position at a certain time one evening and then check its position the next evening at the same time you will find that it has moved one degree of arc anti-clockwise in the Northern Hemisphere, clockwise in the Southern. Rising in the east, stars attain a zenith and set on a western horizon at the same distance from the zenith as they rose. In the Northern Hemisphere there are groups of stars that remain visible throughout the night, wheeling around the only star that does not appear to move- the Pole Star ( a valuable navigation aid, for it is located almost above Polar North). In the Southern Hemisphere the Pole Star is not visible and there is no comparable bright and stable southern star but direction finding in the southern hemisphere makes use of a constellation called the Southern Cross.

Astro 7.JPG

The Northern Sky

The northern sky: the main constellations to learn are the Plough, also known as the Big Dipper (a), Cassiopeia (b) and Orion (c) , all of which, like all stars in the northern sky, apparently circle the pole star (d), but the first two are recognizable groups that do not set. These constellations come up at different times according to latitude and Orion is most useful if you are near the Equator. Each can be used in some way to check the position of the pole star, but once you have learned to recognize it you probably will not need to check each time.
A line can be drawn connecting Cassiopeia and the Plough (the Big Dipper), through the Pole Star. You will notice that the two lowest stars of the Great Bear (as shown here) point almost to the Pole Star. It will help you to find these constellations if you look along the Milky Way, which stretches right across the sky, appearing as a hazy band of millions of stars.

 

 

The Plough

The Plough (Big Dipper)

a. The Plough (the Big Dipper) is the central feature of a very large constellation, the Great Bear (Ursa Major). It wheels around the Pole Star. The two stars Dubhe (x) and Merak (y) point, beyond Dubhe exactly to the Pole Star about four times further away than the distance between them.

b. Cassiopeia is shaped like a W and also wheels around the North Star. It is on opposite side of the Pole Star and about the same apparent distance away as the Plough (the Big Dipper). On clear, dark nights this constellation may be observed overlaying the Milky Way. It is useful to find this constellation as a guide to the location of the Pole Star, if the Plough is obscured for some reason. The centre star points almost directly at it.

Cassiopeia

Cassiopeia

c. Orion rises above the Equator and can be seen in both hemispheres. It rises on its side, due east, irrespective of the observer’s latitude and sets due west. Mintaka (a) is directly above the Equator. Orion appears further away from the Pole Star than the previous constellations. He is easy to spot by the three stars making his belt and the lesser stars forming his sword.

 

 

Orion

Orion

Other Stars that rise and set can be used to determine direction. Set two stakes in the ground, one shorter than the other, so that you can sight along them (or use the sights of a rifle propped in a steady position). Looking along them at any star- except the Pole Star- it will appear to move. From the star’s apparent movement you can deduce the direction in which you are facing.

Apparently rising = facing east
Apparently falling = facing west
Looping flatly to the right = facing south
Looping flatly to the left = facing north

These are only approximate directions but you will find them adequate for navigation. The directions will be reversed in the Southern Hemisphere.

Other Stars.JPG

Other Stars

Reading the Southern Sky
There is no star near the South Celestial Pole bright enough to be easily recognized. Instead a prominent constellation is used as a signpost to south: the Southern Cross (Crux), a constellation of five stars which can be distinguished from two other cross- shaped groups by its size- it is smaller-and its two pointer stars.

 

Astro 4.JPG

Southern Cross

Finding the Southern Cross: One way to find the Southern Cross is to look along the Milky Way, the band of millions of distant stars that can be seen running across the sky on a clear night. In the middle of it there is a dark patch where a cloud of dust blocks out the bright star background known as the Coal Sack. On one side of it is the Southern Cross, on the other the two bright pointer stars.

 

Finding South.JPG

Finding South

Finding south: to locate south project an imaginary line along the cross and four and half times longer and then drop it vertically down to the horizon. Fix, if you can, a prominent landmark on the horizon- or drive two sticks into the ground to enable you to remember the position by day.
Courtesy of : The SAS Survival Handbook by John Wiseman, Harper Collins London 1986

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