tag:theconversation.com,2011:/uk/topics/magnetic-compass-15365/articlesMagnetic compass – The Conversation2020-11-16T13:21:46Ztag:theconversation.com,2011:article/1492252020-11-16T13:21:46Z2020-11-16T13:21:46ZHow do geese know how to fly south for the winter?<figure><img src="https://images.theconversation.com/files/369377/original/file-20201113-13-7fvj1e.jpg?ixlib=rb-1.1.0&rect=1218%2C0%2C3624%2C2488&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Geese fly day or night, depending on when conditions are best.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/migrating-canada-geese-royalty-free-image/108309781">sharply_done/E+ via Getty Images</a></span></figcaption></figure><figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=293&fit=crop&dpr=1 600w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=293&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=293&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=368&fit=crop&dpr=1 754w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=368&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=368&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<p><strong>How do geese know how to fly south for the winter? – Oscar V., age 9, Huntington, New York</strong></p>
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<p>To be ready to migrate in the fall, geese start preparing in midsummer. Babies born in the spring are mostly grown up by then. Adult geese <a href="https://doi.org/10.2307/1369611">grow a new set of plumage</a> after shedding their old feathers – a <a href="https://doi.org/10.1007/978-1-4613-0425-8_6">process called molting</a>.</p>
<p>They need flight and body feathers to be in good condition for the long flight ahead, and to insulate their bodies from the winter cold. For a few weeks during this process, geese can’t fly at all, and they stay out on the water to avoid predators.</p>
<p>Geese have a clock in their brain that <a href="https://www.americanscientist.org/article/avian-migration-the-ultimate-red-eye-flight">measures how much sunlight there is each day</a>. The days grow shorter during the late summer and early fall, and that’s how geese know it’s time to get ready for the journey south. Families join together in larger flocks. Geese gorge on grains and grasses to fatten up in preparation for their journey.</p>
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<a href="https://images.theconversation.com/files/369378/original/file-20201113-19-z06ost.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="two geese tails emerge from water as they look for food" src="https://images.theconversation.com/files/369378/original/file-20201113-19-z06ost.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/369378/original/file-20201113-19-z06ost.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=442&fit=crop&dpr=1 600w, https://images.theconversation.com/files/369378/original/file-20201113-19-z06ost.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=442&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/369378/original/file-20201113-19-z06ost.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=442&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/369378/original/file-20201113-19-z06ost.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=555&fit=crop&dpr=1 754w, https://images.theconversation.com/files/369378/original/file-20201113-19-z06ost.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=555&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/369378/original/file-20201113-19-z06ost.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=555&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Geese fattening up by eating some underwater foods.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/two-canada-geese-searching-for-food-royalty-free-image/1282657623">Jennifer Yakey-Ault/iStock via Getty Images</a></span>
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<h2>When it’s time to go</h2>
<p>There are two different types of bird migration. For most bird species that migrate from temperate climates to the tropics in winter, <a href="https://doi.org/10.1016/j.tree.2011.07.009">migration is instinctual</a>. These birds, such as swallows, orioles and warblers, leave their northern breeding place before weather turns harsh and food becomes scarce.</p>
<p>Most migrate at night, individually rather than in flocks, and they know where to go and how to get there without guidance from parents or other birds. They migrate continuously, except for short stopovers to fuel up on insects, fruit, or seeds before continuing on their way. </p>
<p>Canada geese and other migratory geese species are different. They usually remain in their summer range until the weather is cold, water starts to freeze, and food gets hard to come by. <a href="https://doi.org/10.1111/oik.03121">Once conditions become so tough</a> that they can’t find enough to eat, geese migrate.</p>
<p>Maybe you’ve observed flock members signaling they’re ready to go: <a href="https://doi.org/10.2307/4083454">They honk loudly and point their bills toward the sky</a>. Single families of geese, or flocks of several families together, take off and head south. Flocks join with other flocks. Geese fly by day or night, depending on factors like weather conditions or brightness of the moon.</p>
<p><a href="https://www.jstor.org/stable/4161751">Geese navigate based on experience</a>, using landmarks including rivers, coastlines and mountain ranges. They may also use celestial cues such as the sun and stars. Geese have a physical compass in their head that allows them to <a href="https://doi.org/10.1641/B570207">tell north and south by detecting the Earth’s magnetic field</a>.</p>
<p>Young geese learn the migration route and landmarks by following their parents and other experienced geese. People who have raised and socially bonded with geese have even taught the birds new migration routes by leading them in an ultralight aircraft – as in the movie “<a href="https://www.imdb.com/title/tt0116329/">Fly Away Home</a>.”</p>
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<h2>On their way</h2>
<p>Geese are heavy birds, and they fly fast – over 30 miles per hour – using powerful wing beats, rather than gliding like eagles or vultures. All this flapping for a heavy bird <a href="https://doi.org/10.1111/j.0908-8857.2004.03378.x">takes a lot of energy</a>. Geese work very hard during migration flights. To reduce the effort, geese fly at night when the <a href="https://www.jstor.org/stable/4161751">air is calmer, or in the day when there’s a helpful tailwind</a>; they avoid flying into a headwind that would blow them backward.</p>
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<a href="https://images.theconversation.com/files/369379/original/file-20201113-15-xo4vqf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="geese fly in a V against a clear sky" src="https://images.theconversation.com/files/369379/original/file-20201113-15-xo4vqf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/369379/original/file-20201113-15-xo4vqf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/369379/original/file-20201113-15-xo4vqf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/369379/original/file-20201113-15-xo4vqf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/369379/original/file-20201113-15-xo4vqf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/369379/original/file-20201113-15-xo4vqf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/369379/original/file-20201113-15-xo4vqf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">The different wing positions of these greylag geese show their flapping motion, with the individual at the tip of the V working the hardest.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/flock-of-greylag-geese-in-the-sky-royalty-free-image/461948687">Anagramm/iStock via Getty Images</a></span>
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<p>In addition, they have another energy-saving trick. To reduce drag and to receive a little extra lift, geese fly close behind and about one wing length to the side of the one immediately in front. When all flock members do this, the familiar V shape appears.</p>
<p>This <a href="https://doi.org/10.1016/0022-5193(83)90110-8">form of drafting, also called vortex surfing</a>, saves a lot of energy. Following another bird at the right distance blocks any headwind. The flapping of the bird ahead creates a forward movement of air called a slipstream, which helps pull the trailing bird forward. And little pockets of spinning air, called vortices, produce lift that helps keep a trailing bird aloft. The same physics explains why fighter jets fly in V formation to conserve fuel.</p>
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<iframe width="440" height="260" src="https://www.youtube.com/embed/-bkxG28OUZw?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">This video explains some of the physics of how the V formation helps keep geese up in the sky for less energy.</span></figcaption>
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<p>The bird at the point of the V, in the front of the flock, gets no advantage from drafting. It is working much harder than the others. When it gets too tired, it drops back and another takes the lead. Recently, ornithologists have discovered that when families migrate together as a flock, the <a href="https://doi.org/10.1111/jav.02392">parents take turns at the tip of the V</a>. The younger geese, which are not as strong, line up along the V behind the lead parent.</p>
<p>Most geese that breed in a particular region will migrate along similar routes, <a href="https://www.ducks.org/conservation/waterfowl-research-science/understanding-waterfowl-the-flyways">called flyways</a>. For example, geese that pass by my house in Northern New York follow the Atlantic flyway. They’ll end up on the Atlantic Coast and migrate south following the shoreline.</p>
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<a href="https://images.theconversation.com/files/369380/original/file-20201113-15-1k8iqp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="three Canada geese fly over sand dunes" src="https://images.theconversation.com/files/369380/original/file-20201113-15-1k8iqp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/369380/original/file-20201113-15-1k8iqp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=414&fit=crop&dpr=1 600w, https://images.theconversation.com/files/369380/original/file-20201113-15-1k8iqp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=414&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/369380/original/file-20201113-15-1k8iqp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=414&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/369380/original/file-20201113-15-1k8iqp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=520&fit=crop&dpr=1 754w, https://images.theconversation.com/files/369380/original/file-20201113-15-1k8iqp.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=520&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/369380/original/file-20201113-15-1k8iqp.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=520&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Many geese head to the coast and then navigate south along the shoreline.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/canada-geese-over-sand-dunes-at-jones-beach-long-royalty-free-image/639801946">Vicki Jauron, Babylon and Beyond Photography/Moment via Getty Images</a></span>
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<p>Rather than migrate nonstop to their wintering grounds, many geese travel in stages, pausing at traditional stopover sites to rest and regain lost fat. Geese from the <a href="https://www.allaboutbirds.org/guide/Canada_Goose/lifehistory">most northern populations travel to the farthest south</a>. More southerly breeding populations don’t migrate as far. This is called leapfrog migration, since the northern geese literally fly over the more southern birds. <a href="https://doi.org/10.2307/3676124">Why this happens is a bit of a mystery</a>, but it’s possible the northern breeders continue further south to avoid competing for food with southern geese that have already found good wintering conditions closer to their summer homes.</p>
<p>Because geese learn migratory routes, they can flexibly adjust where <a href="https://doi.org/10.1111/gcb.14061">they go as conditions change</a>. Goose migration stopover sites and wintering grounds have shifted, for example, because of changes in farming practices, availability of lawns and golf courses, and other changes in land use. Migratory geese are now adjusting when and where they migrate <a href="https://doi.org/10.3389/fevo.2019.00502">as a consequence of global climate change</a>. And some groups of Canada geese have decided <a href="https://doi.org/10.1675/063.034.0403">to just stay put and skip the migration altogether</a>.</p>
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<p class="fine-print"><em><span>Tom Langen does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Geese honk loudly and point their bills toward the sky when they’re ready to start the migration. Here’s how they know it’s time, how they navigate and how they conserve energy on the grueling trip.Tom Langen, Professor of Biology, Clarkson UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1232652019-09-16T20:35:31Z2019-09-16T20:35:31ZExplainer: what happens when magnetic north and true north align?<figure><img src="https://images.theconversation.com/files/292338/original/file-20190913-190002-1sm4kgi.jpg?ixlib=rb-1.1.0&rect=42%2C16%2C5615%2C3638&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Very rarely, depending on where you are in the world, your compass can actually point to true north.
</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/download/confirm/1393250468?src=vKUFR7i2pguMpY8BD0TNHg-1-62&size=huge_jpg">https://www.shutterstock.com</a></span></figcaption></figure><p>At some point in recent weeks, a once-in-a-lifetime event happened for people at Greenwich in the United Kingdom.</p>
<p>Magnetic compasses at the historic London area, known as the <a href="https://www.rmg.co.uk/discover/explore/prime-meridian-greenwich">home of the Prime Meridian</a>, were said to have pointed directly at the north geographic pole for the <a href="https://www.sciencealert.com/compasses-are-about-to-do-something-that-hasn-t-happened-in-over-300-years">first time in 360 years</a>. </p>
<p>This means that, for someone at Greenwich, magnetic north (the direction in which a compass needle points) would have been in exact alignment with geographic north. </p>
<p>Geographic north (also called “true north”) is the direction towards the fixed point we call the North Pole. </p>
<p>Magnetic north is the direction towards the north magnetic pole, which is a wandering point where the Earth’s magnetic field goes vertically down into the planet. </p>
<p>The north magnetic pole is currently about 400km south of the north geographic pole, but can move to about 1,000km away.</p>
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<a href="https://images.theconversation.com/files/292137/original/file-20190912-190065-q685ai.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/292137/original/file-20190912-190065-q685ai.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/292137/original/file-20190912-190065-q685ai.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=515&fit=crop&dpr=1 600w, https://images.theconversation.com/files/292137/original/file-20190912-190065-q685ai.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=515&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/292137/original/file-20190912-190065-q685ai.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=515&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/292137/original/file-20190912-190065-q685ai.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=648&fit=crop&dpr=1 754w, https://images.theconversation.com/files/292137/original/file-20190912-190065-q685ai.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=648&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/292137/original/file-20190912-190065-q685ai.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=648&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">The lines of the Earth’s magnetic field come vertically out of the Earth at the south magnetic pole and go vertically down into the Earth at the north magnetic pole.</span>
<span class="attribution"><span class="source">Nasky/Shutterstock</span></span>
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<h2>How do the norths align?</h2>
<p>Magnetic north and geographic north align when the so-called “angle of declination”, the difference between the two norths at a particular location, is 0°. </p>
<p>Declination is the angle in the horizontal plane between magnetic north and geographic north. It changes with time and geographic location.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/292139/original/file-20190912-190021-lr3y1f.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/292139/original/file-20190912-190021-lr3y1f.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/292139/original/file-20190912-190021-lr3y1f.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=402&fit=crop&dpr=1 600w, https://images.theconversation.com/files/292139/original/file-20190912-190021-lr3y1f.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=402&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/292139/original/file-20190912-190021-lr3y1f.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=402&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/292139/original/file-20190912-190021-lr3y1f.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=505&fit=crop&dpr=1 754w, https://images.theconversation.com/files/292139/original/file-20190912-190021-lr3y1f.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=505&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/292139/original/file-20190912-190021-lr3y1f.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=505&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">The declination angle varies between -90° and +90°.</span>
<span class="attribution"><span class="source">Author provided</span></span>
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<p>On a map of the Earth, lines along which there is zero declination are called agonic lines. Agonic lines follow variable paths depending on time variation in the Earth’s magnetic field.</p>
<p>Currently, zero declination is occurring in some parts of Western Australia, and will likely move westward in coming years.</p>
<p>That said, it’s hard to predict exactly when an area will have zero declination. This is because the rate of change is slow and current models of the Earth’s magnetic field only cover a few years, and are updated at roughly five-year intervals. </p>
<p>At some locations, alignment between magnetic north and geographic north is very unlikely at any time, based on predictions.</p>
<h2>The ever-changing magnetic poles</h2>
<p>Most compasses point towards Earth’s north magnetic pole, which is usually in a different place to the north geographic pole. The location of the magnetic poles is constantly changing.</p>
<p>Earth’s magnetic poles exist because of its magnetic field, which is produced by electric currents in the liquid part of its core. This magnetic field is defined by intensity and two angles, inclination and declination.</p>
<p>The relationship between geographic location and declination is something people using magnetic compasses have to consider. Declination is the reason a compass reading for north in one location is different to a reading for north in another, especially if there is considerable distance between both locations.</p>
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Read more:
<a href="https://theconversation.com/new-evidence-for-a-human-magnetic-sense-that-lets-your-brain-detect-the-earths-magnetic-field-113536">New evidence for a human magnetic sense that lets your brain detect the Earth's magnetic field</a>
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<p>Bush walkers have to be mindful of declination. In Perth, declination is currently close to 0° but in eastern Australia it can be up to 12°. This difference can be significant. If a bush walker following a magnetic compass disregards the local value of declination, they may walk in the wrong direction.</p>
<p>The polarity of Earth’s magnetic poles has also changed over time and has undergone <a href="https://www.nasa.gov/topics/earth/features/2012-poleReversal.html">pole reversals</a>. This was significant as we learnt more about plate tectonics in the 1960s, because it <a href="https://divediscover.whoi.edu/mid-ocean-ridges/magnetics-polarity/">linked the idea</a> of seafloor spreading from mid-ocean ridges to magnetic pole reversals. </p>
<h2>Geographic north</h2>
<p>Geographic north, perhaps the more straightforward of the two, is the direction that points straight at the North Pole from any location on Earth. </p>
<p>When flying an aircraft from A to B, we use directions based on geographic north. This is because we have accurate geographic locations for places and need to follow precise routes between them, usually trying to minimise fuel use by taking the shortest route. All GPS navigation uses geographic location.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/five-maps-that-will-change-how-you-see-the-world-74967">Five maps that will change how you see the world</a>
</strong>
</em>
</p>
<hr>
<p>Geographic coordinates, latitude and longitude, are defined relative to Earth’s <a href="https://www.scientificamerican.com/article/earth-is-not-round/">spheroidal</a> shape. The geographic poles are at latitudes of 90°N (North Pole) and 90°S (South Pole), whereas the Equator is at 0°.</p>
<h2>An alignment at Greenwich</h2>
<p>For hundreds of years, declination at Greenwich was negative, meaning compass needles were pointing west of true north.</p>
<p>At the time of writing this article I used an <a href="https://ngdc.noaa.gov/geomag/calculators/magcalc.shtml#declination">online calculator</a> to discover that, at the Greenwich Observatory, the Earth’s magnetic field currently has a declination just above zero, about +0.011°. </p>
<p>The average rate of change in the area is about 0.19° per year, which at Greenwich’s latitude represents about 20km per year. This means next year, locations about 20km west of Greenwich will have zero declination.</p>
<p>It’s impossible to say how long compasses at Greenwich will now point east of true north. </p>
<p>Regardless, an alignment after 360 years at the home of the Prime Meridian is undoubtedly a once-in-a-lifetime occurrence.</p><img src="https://counter.theconversation.com/content/123265/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Paul Wilkes does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Recently, magnetic compasses at Greenwich pointed directly at true north for the first time in 360 years. This is currently happening in Western Australia too. But what does it mean?Paul Wilkes, Senior Research Geophysicist, CSIROLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1128272019-03-12T10:45:18Z2019-03-12T10:45:18ZOld stone walls record the changing location of magnetic north<figure><img src="https://images.theconversation.com/files/263220/original/file-20190311-86686-98h77f.jpg?ixlib=rb-1.1.0&rect=617%2C12%2C3503%2C2305&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The orientations of the stone walls that crisscross the Northeastern U.S. can tell a geomagnetic tale as well as a historical one.</span> <span class="attribution"><span class="source">John Delano</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span></figcaption></figure><p>When I was a kid living in southern New Hampshire, my family home was on the site of an abandoned farmstead consisting of massive stone foundations of quarried granite where dwellings once stood. Stone walls snaked throughout the forest. As I explored the deep woods of tall oaks and maples, I wondered about who had built these walls, and why. What stories did these walls contain?</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/263249/original/file-20190311-86699-45fjwd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/263249/original/file-20190311-86699-45fjwd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/263249/original/file-20190311-86699-45fjwd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=799&fit=crop&dpr=1 600w, https://images.theconversation.com/files/263249/original/file-20190311-86699-45fjwd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=799&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/263249/original/file-20190311-86699-45fjwd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=799&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/263249/original/file-20190311-86699-45fjwd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1004&fit=crop&dpr=1 754w, https://images.theconversation.com/files/263249/original/file-20190311-86699-45fjwd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1004&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/263249/original/file-20190311-86699-45fjwd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1004&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">As nature reclaimed farmland, stone walls continued to mark historical boundaries.</span>
<span class="attribution"><span class="source">John Delano</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>Decades later, while living in a rural setting in upstate New York and approaching retirement <a href="https://scholar.google.com/citations?user=eUpUocsAAAAJ&hl=en&oi=ao">as a geologist</a>, my long dormant interest was rekindled by treks through the neighboring woods. By now I knew that stone walls in New England and New York are iconic vestiges from a time when farmers, in order to plant crops and graze livestock, needed to clear the land of stones. Tons and tons of granite had been <a href="https://www.earthmagazine.org/article/history-science-and-poetry-new-englands-stone-walls">deposited throughout the region during the last glaciation</a> that ended about 10,000 years ago.</p>
<p>By the late 1800s, <a href="https://books.wwnorton.com/books/detail.aspx?id=4294967847&LangType=1033">nearly 170,000 subsistence farming families</a> had built an <a href="https://www.worldcat.org/title/stone-industries-dimension-stone-crushed-stone-geology-technology-distribution-utilization/oclc/551991">estimated 246,000 miles of stone walls</a> across the Northeast. But by then, the Industrial Revolution had already started to contribute to the widespread abandonment of these farms in the northeastern United States. They <a href="https://www.cambridge.org/us/academic/subjects/history/early-republic-and-antebellum-history/those-who-stayed-behind-rural-society-nineteenth-century-new-england-2nd-edition?format=PB">were overgrown by forests</a> within a few decades.</p>
<p>During my more recent walks through the woods, on a whim I used a hand-held GPS unit to map several miles of stone walls. And that was how I realized that in addition to being part of an American legacy, their locations record a centuries-long <a href="https://doi.org/10.1029/2018JB016655">history of the Earth’s wandering magnetic field</a>.</p>
<h2>Connecting the walls with historical maps</h2>
<p>The complex array of walls that emerged from my GPS readings made no sense to me until I found an old map of my town’s property boundaries at the local historical society. Suddenly I saw that some of the stone walls on my map lay along property lines from 1790. They marked boundaries.</p>
<p>My subsequent searches of church records and decades of the federal census revealed the names of these farm families and details of their lives, including annual yields from their harvests. I started to feel like the stone walls were letting me connect with the long-gone folks who had worked this land.</p>
<p>Now the wheels in my scientist’s mind really started spinning. Did the original land surveys from the 18th and 19th centuries in this part of town still exist? What were the magnetic compass-bearings of those boundaries on the original surveys? </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/263252/original/file-20190311-86707-1aapxwy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/263252/original/file-20190311-86707-1aapxwy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/263252/original/file-20190311-86707-1aapxwy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=363&fit=crop&dpr=1 600w, https://images.theconversation.com/files/263252/original/file-20190311-86707-1aapxwy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=363&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/263252/original/file-20190311-86707-1aapxwy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=363&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/263252/original/file-20190311-86707-1aapxwy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=456&fit=crop&dpr=1 754w, https://images.theconversation.com/files/263252/original/file-20190311-86707-1aapxwy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=456&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/263252/original/file-20190311-86707-1aapxwy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=456&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Historical maps and surveys underscore the orderly way plots were divvied up from the landscape in a grid.</span>
<span class="attribution"><a class="source" href="https://www.stoddardnh.org/about-us/pages/charles-peirce-maps-stoddard">Charles Peirce/Stoddard, New Hampshire</a></span>
</figcaption>
</figure>
<p>I knew that the location of <a href="https://www.sciencealert.com/navigation-systems-finally-caught-up-with-the-mysteriously-north-pole-shift">magnetic north drifts over time</a> due to <a href="http://www.physics.org/article-questions.asp?id=64">changes in the Earth’s core</a>. Could I determine its drift using stone walls and the old land surveys? My preliminary map of stone walls and a few historical surveys showed that the approach had potential.</p>
<p>To have any scientific value, though, this work had to encompass much larger areas. I needed a different strategy for finding and mapping stone walls. Luckily I found two troves of useful information. First, the New York State Archives had hundreds of the original land surveys from the 18th and 19th centuries. And secondly, airborne LiDAR (light detection and ranging) images were <a href="https://orthos.dhses.ny.gov">publicly available that could reveal</a> <a href="http://www.granit.unh.edu/resourcelibrary/specialtopics/stonewalls/">stone walls hidden beneath the forest canopy</a> over much larger areas than I could cover on my own by foot.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/263259/original/file-20190311-86686-1cp2owv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/263259/original/file-20190311-86686-1cp2owv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/263259/original/file-20190311-86686-1cp2owv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=426&fit=crop&dpr=1 600w, https://images.theconversation.com/files/263259/original/file-20190311-86686-1cp2owv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=426&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/263259/original/file-20190311-86686-1cp2owv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=426&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/263259/original/file-20190311-86686-1cp2owv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=535&fit=crop&dpr=1 754w, https://images.theconversation.com/files/263259/original/file-20190311-86686-1cp2owv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=535&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/263259/original/file-20190311-86686-1cp2owv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=535&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Magnetic north and geographic north aren’t the same – and their difference changes over time.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-vector/earth-magnetic-field-geomagnetic-diagram-vector-1177065301">Siberian Art/Shutterstock.com</a></span>
</figcaption>
</figure>
<h2>Tracking magnetic north’s drift over time</h2>
<p>The Earth rotates on its axis once every 24 hours. The location of that spin axis in the Northern Hemisphere is called true north, and <a href="https://doi.org/10.1146/annurev.ea.16.050188.001311">wanders very slowly</a>. The location of true north can be considered stationary, though, on a timescale of a few centuries.</p>
<p>But that’s not where a compass aims when it points north. The location of the north magnetic pole is not only at a different location from true north, but also changes rapidly – currently about one degree per 10 years in New England.</p>
<p>The difference in direction between true north and magnetic north (at a specific time and location on the Earth) is known as the <a href="https://www.ngdc.noaa.gov/geomag/calculators/magcalc.shtml#declination">magnetic declination</a>. Global information about historic variations in magnetic declination is currently based on thousands of <a href="https://doi.org/10.1098/rsta.2000.0569">magnetic compass-bearings recorded in ships’ navigational logs</a> from 1590 onwards. </p>
<p>But now my work on 726 miles of stone walls <a href="https://doi.org/10.1029/2018JB016655">provides an independent check on magnetic declination</a> between 1685 and 1910. </p>
<p>Here’s the logic. When settlers were piling up those stones along the boundaries of their plots, they were using property lines that had been laid out according to the surveyors’ compass readings. Using LiDAR images, the bearings of those stone walls could be determined with respect to true north and compared with the surveyors’ magnetic bearings. The difference is the magnetic declination at the time of the original survey. </p>
<p>For example, the original surveys divided New Hampshire’s Stoddard township into hundreds of lots with boundaries with magnetic compass-bearings of N80 degrees W and N14 degrees E in 1768. As the land was cleared for farming, owners built stone walls along and within those 1768 surveyed boundaries.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/262738/original/file-20190307-82677-1bpfxz3.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/262738/original/file-20190307-82677-1bpfxz3.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/262738/original/file-20190307-82677-1bpfxz3.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=408&fit=crop&dpr=1 600w, https://images.theconversation.com/files/262738/original/file-20190307-82677-1bpfxz3.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=408&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/262738/original/file-20190307-82677-1bpfxz3.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=408&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/262738/original/file-20190307-82677-1bpfxz3.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=513&fit=crop&dpr=1 754w, https://images.theconversation.com/files/262738/original/file-20190307-82677-1bpfxz3.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=513&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/262738/original/file-20190307-82677-1bpfxz3.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=513&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Lidar reveals the stone walls hidden beneath the canopy. Comparing their orientation with true north provides the magnetic declination at this location when boundaries were surveyed in 1768.</span>
<span class="attribution"><a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>Now one can compare the bearings of those stone wall-defined boundaries relative to magnetic north and true north today. The difference shows that the magnetic declination at this location in 1768 was 7.6 ± 0.3 degrees W. That’s a good match for scientists’ <a href="https://doi.org/10.1029/2002RG000115">current geophysical model</a>. Since the <a href="https://www.ngdc.noaa.gov/geomag-web/#declination">magnetic declination at this location</a> today is 14.2 degrees W, the direction to magnetic north at this location has moved about 6.6 degrees further west since 1768.</p>
<p>Data from these stone walls strengthen the current geophysical model about the Earth’s magnetic field.</p><img src="https://counter.theconversation.com/content/112827/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Prior to my retirement in late 2016, John Delano received research funding from NASA. No current funding.
John Delano is currently a volunteer interpreter at three sites in Colonial Williamsburg, VA.</span></em></p>Scientific inspiration struck a geologist after many walks through the woods in New York and New England. These ruins hold the secret of where the compass pointed north when they were built centuries ago.John Delano, Distinguished Teaching Professor of Atmospheric and Environmental Sciences, University at Albany, State University of New YorkLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/385192015-03-11T11:08:31Z2015-03-11T11:08:31ZLeatherback sea turtles use mysterious ‘compass sense’ to migrate hundreds of miles<figure><img src="https://images.theconversation.com/files/74339/original/image-20150310-13539-4146qb.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A leatherback sea turtle pauses for air on its long migration.</span> <span class="attribution"><span class="source">Connie Merigo (NMFS Permit #1557-03)</span>, <a class="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">CC BY-NC-ND</a></span></figcaption></figure><p>Imagine yourself swimming in the Sargasso Sea in the Atlantic. The color blue dominates this part of the world - there’s nothing to see but a vast expanse of water and sky in all directions. The winds are calm. The water is warm, clear and deep. You have a destination in mind, but how do you choose your direction, and maintain it, day and night, for thousands of miles? Without a compass or GPS for guidance, this would be an impossible task for a human being. Yet many marine animals routinely achieve this feat during their yearly migrations between breeding and feeding habitats. </p>
<p>Sea turtles are known for their spectacular long-distance migrations. After many years at sea, they can pinpoint their natal nesting beaches after crossing entire ocean basins. We don’t know the distance covered during their developmental journeys but this period can last several decades, and they likely cover tens of thousands of miles. The largest, fastest and deepest-diving species of sea turtle is the <a href="http://www.nmfs.noaa.gov/pr/species/turtles/leatherback.htm">leatherback (<em>Dermochelys coriacea</em>)</a>. Leatherback sea turtles can grow to over a thousand pounds on a <a href="http://dx.doi.org/10.1371/journal.pone.0033259">diet of watery jellyfish</a>. They <a href="http://dx.doi.org/10.1098/rspb.2005.3110">travel extensively</a> between tropical and temperate habitats to gorge on seasonally abundant gelatinous prey. It’s a mystery how they maintain their headings to travel direct migratory paths over such vast distances.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/74326/original/image-20150310-13539-1p90vkx.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/74326/original/image-20150310-13539-1p90vkx.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/74326/original/image-20150310-13539-1p90vkx.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/74326/original/image-20150310-13539-1p90vkx.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/74326/original/image-20150310-13539-1p90vkx.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/74326/original/image-20150310-13539-1p90vkx.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=501&fit=crop&dpr=1 754w, https://images.theconversation.com/files/74326/original/image-20150310-13539-1p90vkx.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=501&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/74326/original/image-20150310-13539-1p90vkx.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=501&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Large Pelagics Research Center scientists collaborate with commercial fishermen to find and tag leatherback turtles at sea. Captain Mark Leach checks out a 800-pound male leatherback turtle with a GPS-linked satellite tag on its back.</span>
<span class="attribution"><span class="source">Kara Dodge (NMFS Permit #1557-03)</span>, <a class="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">CC BY-NC-ND</a></span>
</figcaption>
</figure>
<h2>Satellite tags to track turtles</h2>
<p>To understand the behavior and migratory patterns of these enigmatic turtles, we set out to locate and tag them in their northern foraging grounds in the northwest Atlantic. Productive waters off the coast of Massachusetts’ Cape Cod attract a rich diversity of marine life, including the jellyfish-eating leatherback turtle. Working with a skilled team of professionals – including commercial fishermen, a spotter pilot, veterinarians and field biologists – we placed <a href="http://dx.doi.org/10.1371/journal.pone.0091726">satellite tags on 20 leatherback sea turtles over three years</a>.</p>
<p><a href="http://www.int-res.com/articles/esr2008/theme/Tracking/TMVpp21.pdf">Satellite telemetry</a> has revolutionized scientists’ ability to track far-ranging marine animals for relatively long periods of times (months to years), often in otherwise inaccessible habitats. Virtually following animals via tracking tag has provided insight on migration timing and routes of a wide variety of ocean-dwelling species, including <a href="http://dx.doi.org/10.1016/j.cub.2009.04.019">sharks</a>, <a href="http://dx.doi.org/10.1139/F10-033">tunas</a>, <a href="http://dx.doi.org/10.1007/s00227-010-1578-2">ocean sunfish</a>, <a href="http://faculty.washington.edu/glennvb/fish475/Zerbini%20et%20al%202006%20published%20paper.pdf">whales</a>, <a href="http://dx.doi.org/10.1890/0012-9615(2000)070%5B0353:FEONES%5D2.0.CO;2">seals</a>, <a href="http://dx.doi.org/10.1073/pnas.0603715103">seabirds</a> and <a href="http://dx.doi.org/10.1007/s00227-009-1279-x">sea turtles</a>. Over the last decade, the integration of <a href="http://adsabs.harvard.edu/abs/2006AGUFMOS33C1716R">GPS</a> antennas into traditional satellite tags has greatly improved the accuracy and precision of location data, allowing us to track migrating animals with less error.</p>
<p>In our <a href="http://dx.doi.org/10.1371/journal.pone.0091726">research off Cape Cod</a>, we used satellite tags that collected location, depth and temperature information. When a turtle surfaces to breathe air, this data is transmitted from the tag to orbiting satellites. Satellites then relay the data to the satellite-based service <a href="http://www.argos-system.org">ARGOS</a> where the data is processed and then, ultimately, it’s sent to us for analysis. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/74343/original/image-20150310-13550-15vur6d.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/74343/original/image-20150310-13550-15vur6d.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/74343/original/image-20150310-13550-15vur6d.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/74343/original/image-20150310-13550-15vur6d.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/74343/original/image-20150310-13550-15vur6d.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/74343/original/image-20150310-13550-15vur6d.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=501&fit=crop&dpr=1 754w, https://images.theconversation.com/files/74343/original/image-20150310-13550-15vur6d.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=501&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/74343/original/image-20150310-13550-15vur6d.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=501&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Somehow this young leatherback knows the way.</span>
<span class="attribution"><span class="source">Kara Dodge (NMFS Permit #1557-03)</span>, <a class="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">CC BY-NC-ND</a></span>
</figcaption>
</figure>
<h2>Swimming straight ahead, but how?</h2>
<p>One goal of our study was to identify the migratory routes of male and female adult and juvenile leatherback turtles. In our recently published <a href="http://dx.doi.org/10.1098/rspb.2014.3129">paper in Proceedings of the Royal Society B</a>, we used location data from satellite tags on 15 leatherback turtles to reconstruct their tracks and analyze their migratory orientation as they traveled south to the tropics. They didn’t swim along the coast where they could use landmarks and topographic features on the seafloor to orient themselves. Instead, these turtles struck out for open ocean, swimming offshore into the subtropical gyre. The North Atlantic gyre is a large circle of ocean currents stretching from the equator to near Iceland, and from the east coast of North America to Europe and Africa. We focused our analysis on turtle movements in the middle of the gyre, in an area known as the Sargasso Sea.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/74490/original/image-20150311-24206-1q4lti.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/74490/original/image-20150311-24206-1q4lti.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/74490/original/image-20150311-24206-1q4lti.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=606&fit=crop&dpr=1 600w, https://images.theconversation.com/files/74490/original/image-20150311-24206-1q4lti.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=606&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/74490/original/image-20150311-24206-1q4lti.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=606&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/74490/original/image-20150311-24206-1q4lti.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=762&fit=crop&dpr=1 754w, https://images.theconversation.com/files/74490/original/image-20150311-24206-1q4lti.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=762&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/74490/original/image-20150311-24206-1q4lti.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=762&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Map of leatherback turtle tracks. Segments in the subtropical gyre are highlighted in red (observed) and green (corrected for the effect of currents).</span>
<span class="attribution"><a class="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">CC BY-NC-ND</a></span>
</figcaption>
</figure>
<p>In the deep blue realm of the Sargasso Sea, these turtles were able to maintain remarkably consistent compass headings for over 600 miles (1000 km). Individual turtles followed widely-spaced parallel paths within the gyre. It looked as if the turtles shared the same directional orientation despite being in different parts of the gyre at different times. These consistent headings suggest that leatherback turtles migrating within the gyre use a common compass sense. It remains a mystery just what that compass sense could be.</p>
<p>Within the gyre interior, leatherback turtles have access to limited known sensory information. The seafloor is inaccessible at such depths. Weak ocean currents and lack of stationary reference points make hydrodynamic cues improbable. Wind- or current-borne cues such as odor plumes disperse rapidly over long distances. And sea turtles’ <a href="http://dx.doi.org/10.1242/jeb.015792">poor eyesight above water</a> likely reduces the utility of celestial cues such as stars. They lack all these bathymetric, hydrodynamic, celestial and chemosensory modes of guidance.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/74337/original/image-20150310-13543-12pt2ld.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/74337/original/image-20150310-13543-12pt2ld.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/74337/original/image-20150310-13543-12pt2ld.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/74337/original/image-20150310-13543-12pt2ld.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/74337/original/image-20150310-13543-12pt2ld.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/74337/original/image-20150310-13543-12pt2ld.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/74337/original/image-20150310-13543-12pt2ld.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/74337/original/image-20150310-13543-12pt2ld.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Stay on target, stay on target….</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/myfwc/6948027070">Florida Fish and Wildlife Conservation Commission, NOAA Research Permit #15488</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">CC BY-NC-ND</a></span>
</figcaption>
</figure>
<h2>Invisibly orienting by solar or magnetic compass</h2>
<p>We hypothesize that leatherbacks migrating through the subtropical gyre may orient to some aspect of the earth’s geomagnetic field and/or the position of the sun on the horizon. Leatherbacks could have a time-compensated sun compass, similar to what has been proposed for <a href="http://dx.doi.org/10.1242/jeb.00657">loggerhead turtles</a> and exists in some <a href="http://dx.doi.org/10.1016/S0022-5193(05)80452-7">birds</a>, <a href="http://dx.doi.org/10.1073/pnas.152137299">butterflies</a> and other animals. These animals orient themselves using the time of day from their circadian clocks and the position, or azimuth, of the sun. Solar and magnetic features are ubiquitous and vary in a predictable way from north to south in this region, making them potentially useful for compass orientation.</p>
<p>Magnetic orientation has been demonstrated in many long-distance migrants, including <a href="http://dx.doi.org/10.1038/ncomms5164">monarch butterflies</a>, <a href="http://dx.doi.org/10.1007/BF00610853">yellowfin tuna</a>, <a href="http://jeb.biologists.org/content/199/1/29.short">birds</a>, <a href="http://dx.doi.org/10.1007/BF00657119">sockeye salmon</a> and <a href="http://jeb.biologists.org/content/199/1/73.abstract">sea turtles</a>. In laboratory experiments where leatherback hatchlings were exposed to reversed magnetic field conditions in a darkened room, the turtles oriented in approximately the opposite direction, suggesting they possess a <a href="http://www.biolbull.org/content/185/1/149.full.pdf">light-independent magnetic compass</a>. If leatherbacks retain this compass into adulthood, it could explain their ability to orient consistently day and night in the gyre. Evidence for a <a href="http://dx.doi.org/10.1242/jeb.00657">solar compass</a> has also been found in other sea turtles, and leatherback turtles may be able to interchangeably use magnetic and visual (solar) compasses during migration.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/74347/original/image-20150310-13539-1y8oxed.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/74347/original/image-20150310-13539-1y8oxed.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/74347/original/image-20150310-13539-1y8oxed.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/74347/original/image-20150310-13539-1y8oxed.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/74347/original/image-20150310-13539-1y8oxed.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/74347/original/image-20150310-13539-1y8oxed.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/74347/original/image-20150310-13539-1y8oxed.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/74347/original/image-20150310-13539-1y8oxed.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Starting out on the journey, baby leatherbacks in Trinidad.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/quintenquestel/14767799655">Quinten Questel</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">CC BY-NC-ND</a></span>
</figcaption>
</figure>
<p>At this stage, we can only speculate on the importance of different compass senses during leatherback migration. But there’s no doubt that adult and juvenile leatherback turtles are capable of remarkable compass orientation in the seemingly featureless expanse of the Sargasso Sea. How they actually accomplish these feats remains a mystery, but our study provides some tantalizing clues. Future work should focus on understanding the sensory systems that allow leatherbacks, and other ocean navigators, to find their way across the open sea.</p><img src="https://counter.theconversation.com/content/38519/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Kara Dodge does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>How do these massive sea turtles stay on target as they migrate hundreds of miles through featureless open ocean?Kara Dodge, Postdoctor Investigator in Biology, Woods Hole Oceanographic InstitutionLicensed as Creative Commons – attribution, no derivatives.