|MadSci Network: Physics|
Earth’s Irregular Rotation The rotation of the Earth is undergoing constant change. The speed at which it turns varies from day-to-day, and the orientation of the axis around which it turns varies from day-to-day. The Earth is like a ball spinning around an imaginary stick. The stick slowly wobbles a little, and this changes the way the Earth rotates. Things are also happening to the ball that change the speed at which it rotates [ref. 1]. Not all of the things that effect the way the Earth rotates are well understood. One of the things which effects the way the Earth rotates is construction. The effect of construction, however, is small compared to the other things that change the Earth’s rotation. Two of the organizations that monitor the changes in the Earth’s rotation are the NEOS (National Earth Orientation Service) and the IERS (International Earth Rotation Service). To understand the way the Earth’s rotation changes, it is important to know a little about the structure of the Earth. A simple model of the Earth has four layers. The outer layer is called the crust. It is made up of relatively solid rock and has very liquid oceans pooled in some of its low points. The next layer is the mantle. It is made up of relatively solid rock on the top and partly molten rock on the bottom. The next layer is the liquid outer core. It is made up of liquid metal. The last layer is the solid inner core. It is made up of solid metal. It is believed that the metals are nickel and iron. Surrounding all this, of course, is the Earth’s atmosphere—the air we breathe. These layers are undergoing constant changes in the way they move. This leads to constant changes in the distribution of mass on the planet. Anytime mass on Earth moves, the rotation of the Earth changes. Some movements of mass that effect the Earth’s rotation are seasonal wind patters and ocean currents, the tides in the oceans, large earthquakes, pumping of ground water, construction of reservoirs, and the freezing and melting of glaciers. Some of the bigger contributors to these mass movements in the Earth are the gravitational effects of the Sun, Moon, and planets and seismic activity in the Earth [refs 1, 2, &3]. The impact of all the above events (and others not listed) on the rotation of the Earth is not perfectly understood. The time it takes for the Earth to undergo one complete rotation (one day) is slowing down because of tidal friction. The gravitational pull of the moon causes the water in the oceans to slosh around. This sloshing is slowing down the Earth. The average length of a day is increasing from 0.0015 seconds to 0.0020 seconds each century because of tidal friction [ref. 1]. The length of today will probably not be the same as the length of yesterday. The day-to-day variation in the length of a day can be as much as 0.00205 seconds because of changes in the direction and speed of air currents and the gravitational effects of the Sun and Moon on the Earth’s liquid core [ref. 1]. These changes are too small for a person to notice by observing day-to-day changes with a wristwatch or by comparing the time of sunrise and sunset to our daily schedules. But these are some of the biggest changes in the Earth’s rotation. The construction of cities certainly can change the way the Earth rotates. However, the amount of mass moved for the construction of even a very large city is miniscule compared to the amount of mass moved by air currents and ocean tides. The changes in the Earth’s rotation caused by changes in large masses like the atmosphere and oceans cannot be perceived without very expensive and sensitive equipment. So we cannot expect to perceive the changes in the Earth’s rotation caused by construction, but we do know that small changes are caused by construction. With careful observation and study, we may even someday be able to predict the impact on Earth’s rotation of building a skyscraper. References Ref. 1 http://maia.usno.navy.mi l/eo/whatiseop.html Ref. 2 http://geology.about.com/science/geology/library/weekly/aa090797.htm Ref. 3 http://ilrs.gsfc.nasa. gov/earth_rotation.html
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