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The Bushwalker's Guide to the Galaxy Home Page
An Introduction to the Bushwalker's Guide to the Galaxy
Improvised Angle Measuring Techniques
Finding Reference Directions from your Surroundings
by Alan Sheehan B.E.
The sun is very important to our "sense of direction", but that doesn't mean it is a good guide for navigation. Accurate direction finding from the sun is complex and depends on the time of day, the time of year (seasons) and the observer's latitude. To illustrate the problem consider these scenarios:
You are located on the equator. It is sunrise. It is the middle of the northern hemisphere summer (southern winter). The sun is rising in the East (from a "sense of direction" point of view) but the bearing to the sun will be about 23 degrees North of East (ie 67 degrees). This error, 23 degrees is due to seasons. At the same time an observer above the Arctic Circle didn't even see the sun set to see it rise! This effect is a result of the observerís latitude. The effect produces an additional error in the direction to the sun, which increases with increasing latitude.
Popularised methods of direction finding using the sun often fail to mention these sources of error, leaving people to think they only have their personal accuracy to worry about.
The method goes as follows for the southern hemisphere temperate zone:
Align the 12 on the watch face with the sun.
Mentally halve the angle between the hour hand and the 12. This direction is "North".
In the northern hemisphere temperate zone the method is:
Align the hour hand with the sun.
Mentally halve the angle between the hour hand and the 12. This direction is "South".
The following points are relevant to this method.
Firstly, the watch must be working, and set to standard time not daylight saving time. Daylight saving time will simply result in a 15 degree error to the east.
Secondly, a digital watch can be used provided you have adequate knowledge of how an analogue one works and can draw the watch face in the dirt or somewhere. A bit less accurate again, but...
This method is really quite rough: errors up to 25 or 30 degrees or more are possible unless you understand the movements of the sun and can compensate for the errors. It is most accurate during the middle of the day. This method should be abandoned for the previous method during early morning and late afternoon.
This method also loses accuracy and reliability as your latitude approaches the tropics, and this method is not recommended for use there. Knowledge is also required of what times of the year the sun is south during the middle of the day. This requires knowledge of your latitude and what time of year the sun moves directly overhead.
South of the tropics, the sun will be true North (plus or minus 7.5 degrees) at midday (12:00pm standard time). Vice versa for observers North of the tropics. The reason for the error of plus or minus 7.5 degrees is because a time zone is 15 degrees wide on the face of the earth. So civilian "midday" can be up to half an hour either side of true, or celestial, midday.
In addition to the error due to the observer's position within the time zone there is also the slight matter of "the Equation of Time". The Equation of Time is essentially a measure of the difference between solar time (measured from the position of the sun) and standard time which we use for clocks and most time keeping purposes. The equation of time error is due to the eccentricity of the earths orbit - solar time varies a bit between different times of the year, but standard time is constant. We won't discuss the equation of time further - it is more involved than we need to get apart from noting that it is an additional source of error when trying to find direction from the sun using this technique.
In the tropics the sun could be north, south or directly overhead, which is not all that helpful for year round rules on improvised navigation.
Another method popularised in the survival manuals, is to place a stick in the ground before mid day and mark the position and length of its shadow. Periodically plot the length of the end of the shadow till it is at a minimum, then continue for a similar amount of time afterwards. The shadow is at its minimum length at midday. The line through the tip of the shadow, sometime before midday, and the tip of the shadow the same time after midday will point West-East. The same time each side of midday is important because the tip of the shadow will describe a curve on the ground - not a straight East-West line.
Both these methods are accurate, but are limited in that they can only be used once per day! The shadow stick method will work anywhere in the world but takes time to do. These methods are most useful if applied at a location where good visibility of the horizon is possible so that a prominent landmark can be used for guidance once the sun is no longer usable.
Death before Disorientation!