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There are several methods to use the shadow of some kind of ‘pointer’ to find north, south, east and west. We will present three of them. Only one is really precise and works everywhere on the planet. The others, more simple to use, are more or less accurate according to the location on earth and the time of the year.
1. Equal shadows method
This is the most precise variant of the shadow/stick methods. It works anywhere on the planet at any day of the year, except for areas near the poles where there are days without any sunshine. You can even find the time of ‘solar noon’ for your location if you combine it with a watch.
1. Plant a stick or branch vertically on the ground, taking advantage of a very flat and horizontal space so that the projected shadow, which must measure at least 30 cm, can be clearly distinguished. Mark the tip of the shade with a stone, twig, etc. This should be done at least half an hour before ‘solar noon’ (this is the moment when the sun is in its highest position of the day, it can deviate more than one or two hours from 12 AM).
2. Draw a circle using the shadow as the radius and the base of the stick as the centre. This tracing can work with a string, a shoelace, or a second stick.
3. As we approach ‘solar noon’, the shadow becomes shorter. After ‘solar noon’, it lengthens until it touches the circle again. At this exact moment, mark the tip of the shadow on the ground again.
4. Draw a straight line between the two marks, this is the East-West line! By drawing a second line perpendicular to the first, you get the North-South direction, with which you will be practically oriented and able to go where you want.
This method is subject to two conditions:
a) It must be done around ‘solar noon’.
b) To carry it out, the observer has to watch the shadow patiently and conclude step 3 mentioned above (marking the second point) at the exact moment when the tip of the shadow reaches the circle.
2. Watch method
Remark: This method is accurate in spring, summer and autumn (and quite accurate in winter) for latitudes higher than 35°. The described way works in the northern hemisphere of the earth.
1. You need an analogue watch for this method. If you don’t have one, check the time on a cell phone and draw an analogue watch indicating this time on a sheet of paper.
2. Keep your watch or drawing horizontal and turn it around until the hour hand points towards the sun.
3. Find the time of ‘solar noon’ on your watch. If you don’t know it, use 12 AM (or 1 PM if you are in a period of ‘summer time’/’daylight saving time’). Imagine another ‘hand’ pointing to this time.
4. South direction is in the exact middle between the hour hand and the imaginary hand pointing to 12 AM / 1 PM (or exact ‘solar noon’).
If you live in the southern hemisphere (> 35° south), you can still use the method, but you must point the imaginary hand towards the sun instead of the hour hand. You will find north in the exact middle between the hour hand and the imaginary hand.
3. Simplified shadow method
Remark: This method is more of a ‘rough guess’. It is relatively accurate in spring and autumn for latitudes outside of the tropical belt, i.e. for latitudes higher than 23° (both North or South).
- Drive the stick or a branch into the ground, taking care to do this in a flat environment, so that a visible shadow is projected.
- With a small stone or object, mark the tip of the shadow at the moment you start the process.
- Wait about 10 to 20 minutes for the tip of the shadow to move a few centimetres. The longer the stick, the "faster" your shadow will move.
- After waiting, mark the new position of the tip of the shadow with another stone or small object.
- Draw a line between the two marks for an approximate East-West direction. You can use string or chalk. The first point will always indicate approximately the West, and the second approximately the East, at any time of the day and in any part of the Earth.
- By drawing a second line perpendicular to the first, you will get the approximate North-South direction, with which you will be practically oriented and able to go where you want.
NOTE: Tilting the stick to achieve a more convenient shadow due to its size or direction does not influence the accuracy of this method. Thus, whoever walks on slopes or areas with much vegetation does not need to look for smooth terrain. All required to mark the two points of the shadow is a small flattened space no more significant than the palm. It is not even essential to use a bar or a branch for this operation; the same result is obtained with any fixed object.
Evaluation
- Identify all cardinal points. Compare your result with the indication of a (magnetic) compass. Discuss the reasons why the two readings differ.
- What is the difference between magnetic north and geographic north?
THEORY
The sun rises in the East and reaches its maximum height when it indicates the North (southern hemisphere) or South (northern hemisphere) direction, at the moment we call ‘solar noon’; Finally, it descends until it sets, at which point it marks the West direction. But only twice a year – at the days we call ‘equinox’ - it goes exactly from East (sunrise) to West (sunset). The ‘declination’ at the other days of the year, that is, the angle it forms with each of the two cardinal points, varies according to the year's seasons and the hemisphere where we live.
Did you know…
Magnetic north is changing and different for various locations on earth, while geographic north is fixed. The difference can be up to 5°. Your solar compass points – more or less accurately - to the geographic north.
There are more precise models of solar compasses. They are a kind of a reversed sundial where must know the solar time and your latitude and the shadow of the pointer indicates North or South. They have been used by sailors for centuries.
Additional activities
You can use the first method to determine ‘solar noon’. Note the times of a watch or cell phone when you mark the first point and the second point when the shadow reaches the circle again. ‘Solar noon’ (indicated in ‘local time’, i.e. the time your watch or phone shows) is exactly in the middle between the two time readings. (An example: If your first reading is 11:44 am and your second reading is 2:16 pm, that is a difference of 2 hours and 32 minutes. ‘Solar noon’ is 11:44 am plus 1 hour and 16 minutes, i.e. 1:00 pm at this day and location).
Relevant information if you are facilitating
Please remember that the accuracy of the presented models is influenced by factors like geographic location and time of the year. If you will make the compass with youth or adults and plan to go into more details, it can be good idea to look up the time of ‘solar noon’ for your location in the internet when you plan and prepare your activity.
