As it rotates, the Earth wobbles as it spins on its axis. And as it slows down, the planet develops a host of different wobbles, ranging in time period from a few minutes to billions of years.
The mystery of why the position of the Earth's poles wobbles by 7 meters has been solved by a NASA scientist. The wobble, which has a 433-day cycle, was discovered by the astronomer Seth Chandler in 1891 but no one could explain it. This is known, quite rightly as the Chandler wobble.
It would appear that changes in the ocean pressure on the Earth's crust are causing the wobble.
The earth vibrates at certain resonant frequencies. Small shifts in ocean circulation, caused by changes in winds and sea salinity, affect the water pressure on the crust. This pressure change in turn alters one of the Earth's natural vibration frequencies, causing the wobble.
There is also an annual wobble caused by the pull of the Sun and Moon as the earth rotates around the Sun through the year, tilted at an angle of 23.5 degrees on the earth's axis of rotation.
The daily rotation of the planet creates a bulge at the equator, and the gravity of the Sun and Moon tends to pull this bulge back toward the orbital plane.
However the Earth tends to resist this pull as it hurtles around it's orbit. Consequently the axis moves in a cone-shaped pattern, called a precession, with the celestial North Pole completing a full circle every 26,000 years roughly. As we speak the north celestial pole is pointing towards Polaris, the North Star, but it used to point to Vega, and in 14,000 years it will point at Vega once again. During this time the earth's tilt remains at 23.5 degrees.
These two wobbles together can tilt the Earth's axis up to 30 feet from its normal axis.
However, smaller variations, lasting a week or so, have proved more difficult to study, partly because they're cloaked by the more conspicuous wobbles. Now scientists say changes in the weather cause small wobbles in the entire planet's spin. From November 2005 until February 2006, the Chandler and annual wobbles pretty much cancelled each other out. This allowed scientists to study the minor variations. Using GPS data to establish the exact location of the poles, these clever boffins determined that world weather patterns play a significant role in the small wobbles.
The location of cyclonic and anti-cyclonic centres and the relation of these high and low pressure systems to each other played a measurable role in producing small, short-term wobbles. These systems caused the pole positions to swing in small loops ranging from a couple of centimetres to several inches. The study also proved that ocean pressure variations also coincided with these small polar loops.
So it would appear that day to day changes in the world's weather has a small, but definite and measurable, effect on the earth's rotation.
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