How fast does the Earth rotate? In 24 hours, obviously, for that is how we define our day. That is how fast the surface of the Earth, where we live, rotates. What about other parts of the earth. That seems silly question. Because if, say, from 100 kilometers below the surface, the inside of the earth rotated at a different speed, the friction between that and the outside would split the crust apart. So the whole Earth has to rotate at the same speed. But that question is not silly after all. Have you experimented with a rotating egg? Below the crust of the Earth lies the mantle, and below that lies core. The core of the earth is believed to be iron. It is liquid up to some depth, but the innermost core is solid iron. It must be something worth seeing, huge iron crystal 2400 kilometers in diameter, as massive as the Moon. That gives the Earth the freedom it needs.
The solid core, separated core, separated from the outer hulk of the Earth by the liquid in between, can rotate at a different speed. This was pointed out by two American scientists, Gary Glatzmeier and Paul Roberts. They did some calculation and suggested that the inner core should be rotating slightly faster than 24 hours. Geologist Xiaodong Song and seismologist Paul Richards have found that Glatzmeier and Roberts were absolutely right. The inner core goes around in 1 second less than a day. To put it more dramatically, the inner core that was below the western tip of Africa in the year 1900 is under our feet today. How did they find this out? The seismic waves of earthquake (Jantar Mantar, Nov- Dec ’93) travel right through the Earth. When a tremor takes place in Antarctica and is recorded in Alaska, the seismic wave has traveled through the Earth’s core. Because this wave is going eastwards, it should be carried slightly faster through the core. Another wave, going westwards, for instance when an earthquake in New Zealand is recorded in Norway, will be slower. Checking such earthquake data, Song and Richards found that the westward waves were indeed a fraction of a second slower than the eastward ones, and from that they calculated the speed of the core’s rotation. Several questions arise. As is known to physicists, such a difference in speed gives rise to a current (in this case, of billions of amperes) flowing between the solid inner core and the liquid outer core. Such a current must be causing a magnetic field as well. In this linked to the magnetic field of the Earth?
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