When Santa Claus finishes delivering presents on Christmas Eve, he has to return home to the North Pole, even though it is snowing so hard that the reindeer cannot see the road.
He could use a compass, but then he has a challenge: he has to be able to find the right North Pole.
There actually are two North Poles– the geographic north pole that you see on maps and the magnetic north pole that the compass relies on. They are not the same.
The two North Poles
The geographic North PoleAlso called true north, it is the point at one end of the Earth’s rotation axis.
Try taking a tennis ball in your right hand, place your thumb on the bottom and your middle finger on the top, and spin the ball with the fingers of your left hand. The place where the thumb and middle finger of your right hand contact the tennis ball as it spins determines its axis of rotation. The axis extends from the south pole to the north pole as it passes through the center of the ball.
Earth’s magnetic north pole is different.
More than a thousand years ago, explorers discovered started using compassesusually made with a floating cork or piece of wood with a magnetized needle in it, to find their way. The Earth has a magnetic field that acts like a giant magnet, and the compass needle is aligned with it.
The magnetic North Pole is used by devices such as smartphones for navigation – and it moves over time.
Why the magnetic north pole moves
The movement of the magnetic North Pole is the result of the Earth has an active core. The inner core, which starts about 2,000 miles (3,200 kilometers) beneath your feet, is solid and… under such enormous pressure that it cannot melt. But the outer core is molten and consists of molten iron and nickel.
Heat from the inner core causes the molten iron and nickel in the outer core to move, much like soup in a pot on a hot stove. The movement of the iron-rich fluid induces a magnetic field that covers the entire earth.
As the molten iron moves in the outer core, the magnetic North Pole walks.
For most of the past 600 years, the Arctic has roamed northern Canada. Until about 1990 it moved relatively slowly, about 6 to 9 miles per year, when the speed increased dramaticallyup to 34 miles per year.
About a century ago it began moving in the general direction of the geographic North Pole. Earth scientists can’t say exactly why, except that it reflects a change in the flow within the outer core.
Bringing Santa Claus home
So if Santa’s house is the geographic North Pole (which, by the way, is in the ice-covered center of the Arctic Ocean) how does he correct his compass heading if the two North Poles are in different locations?
Whichever device he uses (compass or smartphone), both rely on magnetic north as a reference to determine the direction in which he should move.
While modern GPS systems can tell you exactly where you are if you’re heading to grandma’s house, they can’t accurately tell you which direction to go without your device knowing the direction of magnetic north.
If Santa uses an old-fashioned compass, he must adjust it to the difference between true north and magnetic north. To do that, he must know the knowledge declination at its location (the angle between true north and magnetic north) and make the correction on its compass. The National Oceanic and Atmospheric Administration has a online calculator that can help.
If you use a smartphone, your phone has a built-in magnetometer that does the work for you. It measures the Earth’s magnetic field at your location and then uses the World magnetic model correct for accurate navigation.
Whatever method Santa uses, he may be relying on magnetic north to find his way to your house and back again. Or maybe the reindeer just know the way.
Scott Brame is a research assistant professor of earth sciences at Clemson University.
This article is republished from The conversation under a Creative Commons license. Read the original article.
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