Natural Navigation for Astronomers

NameAstronomers have some advantages when approaching the rare and ancient art of natural navigation, because both are upside down disciplines, with their foundations in the sky. In this article we will be looking at some of the ways we can navigate naturally, with no tools, tables or instruments, not even a sextant. We will look at how the sun, moon and stars can help us find our way, both directly and indirectly, but first let us touch briefly on the planets and then, sadly, discard them.

Unfortunately even the planets we can see with the naked eye are not used much in natural navigation because their orbits, although perfectly rational and predictable with study, are too complex to be of use without reference materials. There is no practical way of knowing what direction any particular planet will be at a point in the future without delving into tables, either printed or electronic, which rules them out. There are only a small number of exceptions to this, the main one is that the brightness of planets can be used to hold a course, even if it isn’t used initially to determine that course. This is a method that has been used effectively by polar explorers.

The Sun

The sun is the natural navigator’s best friend in two important ways: we can use it directly to get our bearings and, once we understand its arc across the sky, we can search our surroundings for its effects.

In the UK, and indeed all of Europe, we are further north than 23.5 degrees above the equator, which means that the sun will always pass due south of us at midday. Since the sun is due south at its highest point it follows logically that it will be there when the shadow that it casts is its shortest. In other words the shortest shadow cast on any day, by any object but a stick does the job well, forms a perfect north/south line. The geometry is kind to us here, because it follows that a line that joins two shadows of equal length must form a perfect east/west line. Using a shadow stick it is possible to refine our understanding of where we will expect to see the sun at any point in the day or year. From the UK it will always be somewhere in the large arc between northeast and northwest.

Knowing the sun’s arc, we can look for evidence of the sun’s impact all around us. Plants, not least the trees, need the sun for the sustenance and so it is no surprise that they reflect its range in the sky – they will often grow more densely on their southern side. Tree branches also tend to grow marginally more horizontally on the southern side in the UK and more vertically on the northern side, which can yield what I call the ‘tick effect’ when viewed from the west or a backwards tick from the east. Mosses and lichens are very sensitive to light and moisture and tell their own tales, although it is a little more complex than the old theory of moss only growing on the north side of things; the wind, proximity to the ground and plenty of other factors need to be taken into account.

It is not just the plants either, inanimate objects from buildings to puddles reflect the aggregate effect of the sun’s heat and light. The paint on buildings may be more faded on one side of a street than another, east-west paths typically have more water on their southern side, as a small incline can shadow the southern side and allow puddles to form.

Moving to even more indirect effects, the sun is behind nearly all our weather phenomena and its regular patterns lead to trends in our weather that can also be read. The prime example in the UK is that we have a prevailing wind, from the southwest. Exposed trees often have upper extremes combed over from southwest to northeast by the southwesterlies that blow in. Together these details start to build a jigsaw and the probability of reading direction accurately starts to look more favourable. Nature does not always behave in the way we want or expect it to, but it has little time for wasted effort either so the signs can usually be found. This is where the art lies.

The Stars

Most astronomers are very comfortable with finding the north and south celestial poles so we will not dwell there. Astronomers do have a key advantage when it comes to finding south from the northern hemisphere as a strong understanding of the night sky gives an ability to gauge where the invisible south celestial pole is and then work up to due south on the horizon from ‘underground’. One example might be by using the line between Castor and Pollux, running down through, and well beyond, Procyon. The North Star is worth a brief mention as it has another use for navigators as the angle that Polaris is above the horizon equates almost exactly to the observer’s latitude. An outstretched fist’s width approximates to 10 degrees for most of us, and so a natural navigation party trick is to gauge both north and latitude in under a minute using no tools and only one hand.

The subtler methods concern the celestial equator. Since all objects on the celestial equator rise due east and set due west, it is worth the navigator’s time to understand how to find the equator in the night sky. The quickest win in this area is the star, Mintaka, in Orion’s belt. It is wonderfully easy to find and as close as you’re ever likely to need to the equator. From there, like so much star-gazing, it is a case of building signposts. There are simple ways using Aquarius, Aquila and Virgo, more obscure methods too: if you move from Gomeisa to Procyon and then go double the distance at right angles it is possible to find [delta] Monoceros, which sits very close to the equator also. This methodology will not be new to most astronomers, but the difference in navigating can lie in the use of time. Astronomy is concerned with understanding when things will be visible, navigation is more interested in the near-invisible, or to be more specific, the horizon. The star-gazer who spies Mintaka above the eastern horizon will likely think about its future passage higher in the sky, but the navigator must work backwards to understand where on the horizon it would have emerged from. Both will use a similar tactic of understanding that the angle that a star rises from the horizon is approximately the colatitude, 90 degrees minus the observer’s latitude. It is just the time arrow that is switched.

Having established direction using the stars, the navigator will typically then look down from the sky for clues on the ground, near their horizon, to hold course. If it is too dark for that then other natural clues like feeling the wind direction and using that as a crude compass can be used.

Natural navigators must also understand the relationship between star declination, latitude and zenith, although this was a method used long before any of these terms existed. In its simplest form: if you hold your boat on a course in the Pacific that keeps Arcturus passing overhead then you will at some point bump into Hawaii!

The Moon

The moon is the navigator’s ficklest friend. It promises so much but then is slippery in execution. The only exact natural method is the shadow stick one, as with the sun the shortest shadow will form a true north-south line, but this is not very practical in a time or light sense for most evenings (in theory the same is true of Venus, which can just cast a shadow).

Two other methods rely on the fact that any bright light visible from the moon is a clue to two things: the direction of the sun and the moon’s phase relationship with the sun.

Let us take the example of a five-day-old moon. We can deduce that the sun is roughly east of the moon from the phase relationship, which means that the crescent will be aligned very roughly east/west, which in turn means that a line drawn tangentially down from the crescent horns to the horizon should give a rough indication of south.

The second method requires more experience, practice and an estimation of time. The moon regresses relative to the sun approximately 1/30th of the sky each day in its cycle, which equates to 12 degrees. A five-day-old moon will be lagging the sun’s apparent orbit around the earth by approximately 5 x 12 degrees. In other words if it the sun was setting due west at about 6pm then we would expect the moon to be on a bearing of about 210 degrees. The same theory applies and is of more practical value when the direction of the sun is harder to gauge.

Neither of these methods are 100% accurate or reliable, but perhaps somehow more intriguing for that. Like all these methods, the deeper our understanding, the better we can judge how much to read into the clues.

Natural navigation is a fascinating and satisfying art and is best approached as that. It does not rival astronomical science or modern navigational methods for accurate understanding of what is all around us and how to move through it precisely, but used in a complementary way it can offer a deep sense of connection to the natural world that both modern disciplines sometimes struggle to match.

Ancient Heritage

The navigational challenges in different parts of the world led to different methods evolving.

In the Pacific the Polynesian and Micronesian navigators faced long open boat journeys, but being near the equator they were blessed with stars that held their bearing for longer than we see in the UK. They memorized dozens of ‘star paths’ and developed a unique ability to read swell patterns to complement this, allowing them to navigate under clear skies or cloudy. Discoveries of stick charts and stone alignments have helped piece together the depth of historical understanding.

The desert people have faced similar challenges of large open spaces that could not be settled under good visibility of the skies and so some similarities exist. The Bedouin look at sand instead of water shape and have traditionally used the stars, but not just for orientation. The fact that the stars appear to rise 4 minutes earlier each evening acts as a natural calendar and was used for agricultural timing, the date harvest being closely associated with Canopus. (Sadly my short time with the Tuareg in the Libyan Sahara recently would suggest that this is a knowledge on the wane.)

One of the many high points in the relationship between astronomy and navigation came with Pytheas the Greek’s voyage over 2300 years ago. Pytheas was a bold navigator, he sailed from a near tideless Mediterranean north past Britain and Ireland into the Arctic Circle perhaps to Iceland, but he was emboldened by his advanced understanding of the relationship between sun height and latitude. He was an astronomer first, navigator second and consequently a pioneer in the use of a gnomon.

STEP BY STEP GUIDE

1) Get to know the sun’s arc by placing a stick in the ground and tracing the path of its shadow tip. Learn to interpolate the sun’s bearing between dawn, midday and dusk and then test yourself against a website like http://aa.usno.navy.mil/data/docs/AltAz.php.

2) Make sure that you can find the north celestial pole quickly and easily. The Plough, Cassiopeia, Auriga, Cygnus and Pegasus are the most common signposts.

3) Develop ways of identifying the celestial equator, starting with Mintaka in Orion and moving around the night sky. Then work forwards or backwards to the horizon to estimate west and east. You can use a compass to test how close you get.

4) Take note of how isolated and exposed trees grow. Spot denser growth and look for the ‘tick effect’ on the southern side. Also look at the extremities to see if a southwest prevailing wind can be detected.

5) Estimate the direction of the moon by phase and compare with either tables or a compass. If it is a crescent then look for where the tangent to the crescent would touch the horizon for an indication of south.

6) Really test yourself on a walk during day or night when somebody else is responsible for navigating. Use a GPS to note your position before you start then turn it off and rely entirely on nature. Estimate your position at the end of the walk and then use the GPS to see how you did.

Copyright Tristan Gooley 2009

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