Sun-Compass

Yesterday’s topic logically brings me onto today’s, and another ancient but useful navigation device.

If you have even just glanced through a survival manual you will most likely have seen the shadow-stick method. I have previously described this in the context of navigating by the moon. The moon and stars also rise in the east and set in the west. The more usual context for the shadow-stick is using the sun.

 

The method is simple. Place a stick or similar in the ground so that it casts a shadow. Mark the end of the shadow. Wait for at least twenty minutes. The tip of the shadow will have moved. Mark this new position. The first marker will be west of the second. Draw a line between the two markers, then run a line perpendicular to this and back towards the base of your stick (gnomon). This second line will be true north-south. The greater the distance/longer the time between marker placement, the more accurate will be your determination of north-south.

Logically, we will get a more accurate estimate if we take several hours and place a number of markers. If we do this we will observe that the shadow is longest in the morning and evening, and shortest when the sun is directly overhead. The arc plotted on the ground will be flattened rather than constant, unlike some illustrations of this method! When the shadow is at its shortest, it is on the north-south line, and the time will be local midday. The shadow will be shortest at local apparent noon (LAN), which is midway between sun-up and sun-down, so may differ from 1200 hrs. As well as determining distance, you have also made yourself a crude sundial. This can be useful in determining true local time. Some countries on the same longitude use different times. China spans several time zones but uses one official time for the whole country!

The principle is simple enough, but it can get confusing which end of the north-south line is north. In the northern hemisphere the sun (or moon) rises in the east and travels west, passing through the south. In the southern hemisphere it goes through the north.

• In the northern hemisphere the shadow always points in a northerly direction. At midday the shadow will point due north.
  
• In the southern hemisphere the shadow always points in a southerly direction. At midday the shadow will point due south.

Memorize that and solar navigation becomes much less confusing.

In the movies air-crash survivors usually undertake an epic journey back to safety. In the real world your prudent strategy is to stay near the crash site if practical. Setting up a shadow stick is a practical way to spend the time, and establishing the bearings of visible landmarks may be useful later on.

Suppose, for whatever reason, you need to travel. This decision should never be made lightly. Thanks to your shadow-stick you know what bearing you are heading out on, and that of some of the local features. But we cannot take our compass/ sundial with us! With a few modifications, we can make a portable variant.

If you have ever read about the early days of the SAS or of the Long Range Desert Group (LRDG) you will most probably have encountered to references to sun-compasses on their vehicles. In those days trying to use a magnetic compass while riding in a large lump of steel was problematic. The solution was the sun-compass.

Descriptions of how the sun-compass was used used to be hard to find. Thankfully, this is changing. The sun-compass is an ancient device, and was used by the Vikings, among others. A version was also used by some Apollo missions on the moon.

 

As you can see, some sun-compasses are very complicated or sophisticated, so not really something you can improvise. I am going to describe a less accurate variant that can be easily constructed.

In the previous blog I described how a circle of 57 mm radius had a circumference of c.360 mm. (The person who taught me this trick had 5 mm and 57 mm marked on the zipper of her jacket. I notched my penknife handle). Create such a circle on a piece of paper, back of a notebook, etc. An alternate name for a sun-compass is “shadow board”, which reminds us it can be made with a piece of plank or other flat material. A folded piece of paper will help us mark off the 45, 90 and even 22.5 degree points. Each 1.5 cm of the circumference is 15 degrees. The radius of a circle can be measured twelve times along its edge using a drawing compass, or an improvisation of one. By folding the marked points together the circle can be divided further into 24 parts, or the compass can be used further. A nail or pin can be used as the gnomon, but a sliver of wood is more likely. In fact, you do not have to mount the gnomon, just place a shadow-casting object in the centre of the circle whenever you take a reading.

All we have to do now is mark off the circle. This will be a 24 hour clockface so mark off every 15 degrees with an hour. Remember that the shadow will point due north at midday in the northern hemisphere, so mark 1200 hrs as North/0 degrees (In the south, 1200 will be South/180). Fill in the rest of the face. You might get something that looks like this:

Using this simple sun-compass is simple. Hold it level and rotate it until the gnomon shadow is over the current time. If the clocks are adjusted for daylight saving/BST or similar you will have to account for this. Remember, “spring forward, fall back”, so the shadow will be on the north-south line at 1300hs, not 1200, so you will have to subtract an hour from local time to get the time to read on the dial. Once you have your sun-compass orientated use the dial to find the bearing you want. Your portable sun-compass should agree with your base-camp shadow stick. Pick out a landmark, put away your sun-compass and walk towards the landmark. 

An alternate method is to do the same as you did with a shadow-stick. Erect a little gnomon on a board or sheet of paper and plot the tip of the shadow over the course of a day. Draw a line from the base of the gnomon across the curve where it is at its closest. To use this version you do not need to know the time. Rotate it until the tip of the shadow meets the curve. The line you drew will point north (or south if you made your sun-compass in the southern hemisphere). You must use the same height gnomon for each reading, so mount this permanently. The Ottomani version is suspended from three or four points by cord to ensure that it is level.

You may have realized that if you know the bearing of something, your sun-compass can be used as a crude sundial. Related to the methods described above is using a watch directly to navigate by the sun. Remember to adjust time for BST/DST. Substitute “1 mark” for “12 mark” in the above instructions. If your watch is digital, or you are using your phone clock, use your imagination. If you become confused as to which end of the north-south (N-S) line is north, check the local shadows. Hold a blade of grass over the watch-face and see if it casts a shadow. The one direction a shadow cannot point in the north is south. Useful to recall is “the north gets up around 3 o'clock, goes to bed around 9”. In other words, the north end of the N-S line will be in the small numbers in the morning, the higher numbers in the afternoon. (In the southern hemisphere replace the word north with south and the motto still works.) Remember, sometimes shadows are still visible even when you cannot see the sun directly. 

The basic watch method is easy to remember: midpoint between “12(/1)” and the hour hand. The specifics for each hemisphere can be difficult to remember. It may help to think that “N” for north-hemisphere looks like a “H” for “hour-hand” and that this should be pointed at the sun. A cast shadow will point in the oppositie direction to the hour hand. For the southern-hemisphere the 2 in “12” looks a little like an “S” so the 12 should be pointed at the sun. A cast shadow should point towards “6”. Hopefully that will help you use this method.

If you have enjoyed this article or it has been helpful to you please feel free to show your appreciation. Thank you.

The Books

Finding Latitude.

Currently I am reading “The Time Traveller’s Guide to Medieval England”, which is very interesting. An astrolabe is listed among the possessions of at least one wealthy merchant. This got me thinking about similar devices.

Decades ago I watched a program on someone who had spend several weeks, possibly months, in a life-raft on the open sea. Using three pencils and elastic bands he improvised a sextant and used it to measure the angle of the Pole star. Since the angle of the Pole star is equal to the latitude of the observer, he was able to sail his raft into the sea lanes, where his chance of encountering another vessel and rescue were greatest.

Here is a nice image of how such a device works. Other materials such as sticks and string could be used. This device is easily improved with a plumb-line. In the southern hemisphere you can measure the angle of the Coal Sack nebula.

Similar devices date back many centuries. If you have some cord, a kamal would be easy to improvise. Note that one example below is made from a piece of cardboard. A knot on the cord is held between the teeth and the cord pulled taunt. The bottom edge of the sight is aligned with the horizon, and the top compared with the Pole star. Different knots on the cord represented different latitudes. For example, if you wanted to sail from Alexandra to Crete you would sail north, checking your kamal using the knot corresponding to 35 degrees. Perhaps you tied that knot when you sighted Polaris another time you were on Crete. When the Pole star aligns with the top of your sight you will know it is time to start sailing west.

The cross-staff worked the same way as the kamal, as the illustrations show. It was better suited to higher latitudes. There were problems with using such a device to measure the angle of the sun, so the backstaff developed, a step towards the prisms and mirrors of the modern sextant. 

A quadrant is relatively easy to make. A number of websites show the version in the first photo. The problem with this variant is that you have to remember to subtract 90 from your reading.

It is NOT a sextant!

Easier is to use half of a typical semi-circular protractor, mount the protractor with the straight edge vertical, or mark out a piece of card or paper. When improvising navigational devices a useful thing to remember is that a circle of 57mm radius has a circumference of just under 360mm. In other words, one millimetre equals one degree. It is no coincidence that my penknife handle has three notches, one 57mm from the first, the other 5mm from the first. Thus if I make a quadrant or compass rose I can mark it in 5 degree increments. By folding paper you can accurately mark 90, 45 and 22.5 degree angles. There are several ways to draw a good circle or quarter circle with improvised means.

Or just draw around something round.

Such navigation devices can be used on the land as well as at sea. They have a long tradition of being used in deserts, exploiting the clear night skies. There are also situations where you might wish to determine the angle of a slope.

I will close with two trigonometry-based tricks:

The first can be used to determine the width of a river or road. This uses the principle of similar triangles.

Face a landmark on the far bank of the river. Turn 90 degrees and pace out a set distance, such as five paces. Place a marker (B in the illustration) and then pace out another five paces (or whatever distance you used before). A-B should equal B-C to avoid any calculations. Turn 90 degrees and head away from C. counting your paces. Every few paces look back at your landmark. When the landmark aligns with B then C-D will equal the distance between A and the landmark.

The second technique is for applications such as measuring the height of a tree or cliff. Vertical distance will equal horizontal distance when the viewing angle is 45 degrees. You can use a quadrant or clinometer for this, but you need to remember to add the height of your eye to the horizontal distance. Alternately, you can use a stick or staff and move back until it appears to be the same height as the object. Add the length of the stick/staff to the horizontal distance. You may have noticed that wood axes often have a 45 degree cut across their end. A lumberjack would sight along this, moving back until he could see the top of the trunk. Height was horizontal distance, plus the length of the axe handle.

If you have enjoyed this article or it has been helpful to you please feel free to show your appreciation. Thank you.

The Books

Masks and Coronavirus.

English has a very large vocabulary, so it is intriguing that I can still find concepts for which there is not yet a specific term. The term I am looking for is one that means to be simultaneously both “amused” and “annoyed”.

What currently provokes this emotion? Every now and then, someone passes me in the street wearing a surgical mask. I saw the same nonsense with swine flu.

 

Surgical masks don’t work that way! A surgical mask does not protect you from airborne infectious agents. It does not protect the wearer, it is intended to protect the rest of the world from the wearer. If a surgeon is operating on a patient, he or she uses a mask. If they are operating on an infectious patient, they wear a space-helmet affair with powered air filtration. Don’t take my word for it, read this or this or do some research of your own. 

Now perhaps I am just being grumpy, and every person I see is selflessly trying to protect everyone else from their own infections. If so, staying at home might have been a more prudent move. But since I am cynical as well as grumpy, I very much doubt it. Unlike thirty years ago, information on such matters is readily available. It is both impressive and depressing that people can still be so ignorant and foolish.

Want to really avoid coronavirus? A good start is to keep your fingers out of your mouth or up your nose and follow all those other manners people were once taught but are now neglected. A radical idea would be to put your phone back in your pocket. That way you might have a chance of spotting someone coughing and sneezing, or more likely threats like traffic! As a friend of mine recently remarked: “Hoarding sanitizer is stupid. You can only sanitize your own hands so many times. If everyone else around you can't, that's going to do much more to spread the disease than you only sanitizing your hands x number of times.” People fixate on sanitizer and forget old fashioned soap is an excellent anti-microbial agent.

If you have enjoyed this article or it has been helpful to you please feel free to show your appreciation. Thank you.

The Books

If You Want To Get Ahead, Get A Hood.

A few nights ago I stayed up late, and by the time I went to bed it was very cold indeed. So cold, that as I lay waiting for my shivering to warm the bed, I considered fetching a woolly hat. I thought about the nightcaps worn by characters such as Scrooge and reflected that some of our ancestors ideas were wiser than we treat them. This got me thinking about ancient clothing styles again. For a big chunk of their history humans wore cloaks and tunics, and there is no obvious logical explanation of why this has declined. Indeed, I am old enough to remember nurses and policemen wearing capes as outdoor wear.

Most modern people spend much of their time indoors. Features such as central heating, air-conditioning and insulation make this a very comfortable environment. Most of us wear just a shirt or tee-shirt, our modern underwear barely qualifying as a layer. In some places a jacket and tie is required, and cooling is turned up to a higher level, wasting energy. (Remember, the cloth used to make one ugly neck tie could be used to make two Brazilian bikinis!). When we leave this protective environment we need more insulation, and this usually takes the form of a thick outer garment we can easily remove in one piece when we reach the indoors again.

For our ancestors, things were different. Uniformly heating a large structure was problematic and resource intensive. Ironically, the peasant in his little hut would be better off in this respect than the lord in his vast castle or manor. I imagine certain select chambers of a castle had fires or braziers while the corridors and other chambers might be cold enough to fog the breath.

In general there would be less discrepancy between interior and exterior temperatures. Hence the typical clothing ensemble was orientated towards being comfortable outdoors. Travellers or soldiers might have to spend several days at a time out of doors. For the Celt, Roman, Viking, Saxon or Norman the answer to this was the cloak and tunic. These were made of wool, which has the merit of retaining some warmth if soaked by the rain. Some woollen garments had the lanolin left in or added, and might have a nap that shed rain and snow. Other fabrics such as linen and hemp saw some use, particularly in drier climates, but generally the cloak and the outermost tunic would be woollen. The early Roman tunic lacked true sleeves, or was short sleeved. Many Romans went bare-legged, or wore short breeches. In latter centuries the Roman soldier wore long sleeves and trousers, much like his earlier Germanic foes. (By this time many legionaries were of Germanic ancestry).

Tunic, trousers and cloak were not the whole story. Except in hot conditions, some form of short or undertunic may have been used. These might have been of easily laundered cloth such as hemp and linen. Underpants were rare or unknown in some cultures or classes. A noble may wear braies, while the poorer medieval man tucked the long tails of his shirt between his legs. In an era where many people could not afford large quantities of clothing such a shirt might also serve as a night shirt. Roman attire included a long scarf, worn under the tunic. For the soldier this prevent armour chaffing his neck, but the scarf is recorded as being a general item of wear. In addition to its other functions, such a scarf may have served like a detachable collar, protecting the tunic and other garments from soiling. Many tunics show trimming of the neck, cuffs and hem. This is often contrasting or decorated. Easily replaced, this may have allowed the practical life of a garment to be extended, an important consideration when many individuals owned only a few garments. 

If we look at medieval peasants and other commoners a very common item of clothing is the hood. A modern hood is usually part of a larger garment, and hooded cloaks date back thousands of years. What is distinctive about these medieval hoods is they are separate garments. Initially I thought this might be some prudent frugality. If a tunic wore out you could still make use of a perfectly good hood. Also, one only need buy or make a single hood that could be used with any of your garments. Some of this may have been the case, but the following video provided further enlightenment:

The hood was as much a shoulder cape as it was a head covering. The hood was usually lined, so it provided an extra couple of layers of insulation over the part of the body heat would be rising from.

Further on the video explains how in warmer conditions the hood can be configured into a chaperon-style hat. 

Elsewhere in this blog I have described how a shoulder covering can contribute to personal camouflage. A garment based on the medieval hood could have “garnish” applied as described here. A shaped hem adds further disruption. Camouflage cloth could be used, but this will work with any suitably neutral-coloured monotone. Being a lined garment it could be turned inside out to hide the camouflage if necessary. If made with camouflage cloth at least one side will need to cotton, so perhaps it should be made with a layer of fleece sandwiched between two layers. Some of the hoods sold for re-enactors are reasonably priced, but choice in good tactical colours is limited. Do not overlook the potential of modern versions, such as a section of poncho-liner, possibly combined with the M-51 hood attachment.

Worn as a chaperon, the hood is suitably irregular to still work as camouflage. At a distance, it looks a bit like a turban, which may be useful. One is reminded of the Mitznefet used by the Israelis.

Much to my surprise, the medieval hood has considerable potential for modern use by anyone who ventures out in cold weather.

If you have enjoyed this article or it has been helpful to you please feel free to show your appreciation. Thank you.

The Books