Niv asks: Why does the compass point north if the dynamo that flows under the surface of the ground, under the entire earth, is the one that creates the magnetism?
Let's start with the fact that the compass does not point exactly north. The compass does not point to the North Pole, but to a magnetic pole that is located in the far north, but at a distance of about 400 kilometers from the "true" pole, that is, from the axis of rotation of the earth around itself. A compass needle at this point will point straight down, it is also possible to draw a magnetic equator that does not coincide with the geographic equator and where the compass needle will be perfectly balanced.
A magnet created from currents of liquid metal in the earth's core creates the magnetosphere around us: a magnetic field that surrounds the sphere and deflects from it electrically charged particles originating from the sun. If the magnetosphere did not protect it we would be subjected to a constant energetic bombardment that would make life on earth impossible. Particles that still manage to penetrate the magnetosphere drain into the "funnel" which is the magnetic pole. The result of the meeting between the particles from the sun and the atmosphere is the northern lights. The magnetic pole is, therefore, also the center of an ellipse from which this spectacular light display can be observed.
This magnetic pole not only does not coincide with the geographic north but also, as expected from a system formed by the flow of a liquid, changes its position. In recent years, the magnetic pole has been moving towards the northwest at a speed of about 55 kilometers per year. The result of this migration is that the optimal viewing area for the northern lights gradually moves from Canada and Scandinavia to Siberia. Ancient evidence of the appearance of a glow in the sky south of the current border allows us to estimate that the magnetic pole was once closer to us. Thus we find reports of northern lights in the writings of Seneca and Pliny the Elder from Rome and in the writings of astronomers in China. The magnetic north shift affects animals that navigate using magnetic sensing. Sea turtles return to the beach where they hatched to lay their eggs. They find the same spot after wandering for years across the seas according to the imprint of the magnetic field to which they were exposed as embryos in the egg. Tracking of laying turtles in Florida revealed that the magnetic pole shift is shifting the turtles to more geographically northern beaches where the current magnetic field is equal to that of the home coast.
North and down
But Niv is right: the compass needle points in the direction of the dynamo flowing deep below us and therefore the compass points not only north but also in the vertical direction - down. The first compasses developed in China were based on a magnetic needle floating in a water tank and under these conditions it is not possible to notice the force pulling it down. Since then humans began to use a compass based on a needle suspended in the air was diagnosed Magnetic bow That is, a deviation of the north end of the needle downwards (in the northern hemisphere). Compass manufacturers are aware of this bias and balance the compass needle so that it remains parallel to the ground in the part of the earth where it is supposed to be. But in animals the magnetic tilt is not a problem but an important element in navigation. The most primitive navigators are bacteria living in the depths of the sea. In these bacteria the navigation is all vertical, the force of gravity acting on a cell in sea water is zero and the bacteria needs a means to show it the direction up or down. Chains of magnetite crystals (Fe3O4) tiny ones wrapped in a membrane react to the magnetic field and direct the cell to a depth where the correct concentration of oxygen will be found.
Migratory birds require more sophisticated navigation but they too find their way around the globe using the Earth's magnetic field. Just as human navigation does not have enough of a compass that shows the direction in which you are going, but also needs a map that tells you where you stand. So the bird needs information that a compass needle simply does not provide: its position in relation to the destination. This information is provided by the magnetic field.
The magnetic sense of birds
Magnetic sensors based on the same iron oxides are found in nerve cells in the upper source and a bird (for example, a pigeon) that knows the house can know its position on the north-south axis depending on the strength of the force in the vertical direction: a stronger magnetic field in the downward direction means "I am north of your home" and a weaker field means "I am south of your return", a bird in the southern hemisphere will translate the message in reverse. Navigation based on magnetic tilt and the strength of the field also allows young eels migrating from Sargasso waters in the middle of the Atlantic Ocean to Europe or North America to find their way to the ocean currents that lead them to the rivers. It seems that the eel hatching in the Sargasso Sea undergoes imprinting, meaning that it quickly learns to know the angle of the magnetic tilt and its strength in its birthplace and will navigate there many years later as an adult. This mechanism of magnetic acquaintance with the motherland may explain the splitting of the species into genetically distinct populations that reproduce each in its own part of the sea.
The avian equivalent of our compass, that is, for horizontal orientation in space, is a different mechanism based on an organic compound. Every substance reacts to a magnetic field, but most of the compounds that make up living things are diamagnetic, that is, those in which all the electrons are arranged in pairs and therefore will experience a weak repulsion from a magnetic field. This is how researchers succeeded Make the frog hover over an extremely strong magnet. The compass of the winged animals is based on the creation of a paramagnetic molecule, meaning that it has single electrons. To achieve this, a chemical bond in a light-sensitive molecule needs to be broken so that for a few millionths of a second a compass needle with 2 electrons sensitive to a magnetic field is formed. A bird will not be able to navigate as long as light with sufficient energy has not reached its eyes, meaning a wavelength of about 560 nanometers (green). Therefore, the magnetic tilt, i.e. the identification of the latitude line based on the strength of the vertical field using magnetite crystals whose magnetism does not depend on light, plays an important role in the navigation ability of migrating birds at night.
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More of the topic in Hayadan:
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The article would have been more useful and interesting if there had been a reference to additional reading sources about the facts narrated in it.
The reason is a tilt angle shift, the magnet has not yet had time to align itself
A self-rotating axis moves...or was moved
In a flock of migratory birds there are several generations that pass on migration information, here the learning ability of the birds is reflected, as well as the ability to preserve the memory of the migration.