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Chandler wobble

From Wikipedia, the free encyclopedia

The Chandler wobble or variation of latitude is a small deviation in the Earth's axis of rotation relative to the solid earth,[1] which was discovered by American astronomer Seth Carlo Chandler in 1891. It amounts to change of about 9 metres (30 ft) in the point at which the axis intersects the Earth's surface and has a period of 433 days.[2][3] This wobble, which is a nutation, combines with another wobble with a period of one year, so that the total polar motion varies with a period of about 7 years.

The Chandler wobble is an example of the kind of motion that can occur for a spinning object that is not a sphere; this is called a free nutation. Somewhat confusingly, the direction of the Earth's spin axis relative to the stars also varies with different periods, and these motions—caused by the tidal forces of the Moon and Sun—are also called nutations, except for the slowest, which are precessions of the equinoxes.

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The Earth's axis points at the North Star. And the heavens have been revolving around that star since the dawn of time - except they haven't. When the Egyptians built the pyramids, the North Star wasn't our current North Star Polaris. No, it was Thuban. Why did it change? It's because the Earth's axis is moving. This motion was first discovered by Hipparchus. He was looking at the positions of the stars and comparing them with their positions a century earlier at the same time of year. He found that the stars were moving at least one degree every century. What could be causing this motion? When you're riding in a car and the car turns, you feel an acceleration. In the same way that you experience an acceleration when a car turns, there is also an acceleration as the world turns. This acceleration works against the force of gravity, so you actually weigh less at the equator. And you weigh more when you're at the North Pole, one percent more. This also causes the earth to bulge. The earth is 26 miles wider at the equator. Because the earth is not a perfect sphere, the gravitational force of the Sun isn't perfectly balanced. One side of the earth gets tugged just a little bit more and this causes the Earth's axis to spin completing one revolution every 26,000 years. This also affects the length of the year. Now you can measure the length of a year several different ways. One way is to look at the seasons. This is called the tropical year. Another way is to look at the positions of the stars. This is called the sidereal year. In January, the Sun is in the constellation Capricorn which means children born in January have Capricorn as their astrological sign. Over the course of a year, the Sun moves through all the signs of the zodiac. Once the earth is back in its original location, we have completed one sidereal year. But the tropical year doesn't depend on the Earth's position. It depends on the Earth's axis. When the North Pole faces away from the Sun, we in the northern hemisphere experience the winter solstice, the shortest day of the year. If you measure the length of time between two winter solstices, you get a tropical year. So what's different? Well, if the Earth's axis is moving, it could line up with the Sun before the earth has returned to its original location. This is what happens, although the effect is small. The axis only moves about one-hundredth of a degree and a tropical year is only 20 minutes shorter than a sidereal year. How did this affect the development of the calendar? Let's go back to the Romans. You may remember Julius Caesar as the man who conquered the Gauls, overthrew the Roman Republic, and named the month of July after himself. But in his free time, he cleaned up the Roman calendar which was a total mess! Before Caesar, the calendar lasted between 355 days and 378 days. Caesar put it at 365 days with a leap day every four years. This was the Western calendar until Pope Gregory XIII said that's not good enough. He removed some of the leap days to better match the length of a tropical year. The Gregorian calendar wasn't adopted in Russia until 1918 and as a result the Russians showed up at the 1908 Olympics 12 days late. Why did Pope Gregory base our calendar off the tropical year? What if he had used the sidereal year? Imagine it's the year 15,000 AD and Americans are celebrating Christmas in the middle of summer and that is what a sidereal calendar would mean. Now a tropical calendar gets the seasons right, but it puts the earth of the opposite side of the Sun in the year 15,000. And so Geminis will be born in December, Cancers in January, Leos in February, Virgos in March. Everything will be backwards! And as we know from astrology, the positions of the Sun and planets at the moment you were born predicts all future events in your life. Here is a chart showing you the positions of the planets when I was born and as you can see I am destined for greatness. Don't believe me? Just look at my horoscope today. Today, you will make a YouTube video. Some people will think that you love astrology, while others will think you're been sarcastic. But everyone will love your video. For more astrological videos - I mean astronomical videos - please click to subscribe.



The existence of Earth's free nutation was predicted by Isaac Newton in Corollaries 20 to 22 of Proposition 66, Book 1 of the Philosophiæ Naturalis Principia Mathematica, and by Leonhard Euler in 1765 as part of his studies of the dynamics of rotating bodies. Based on the known ellipticity of the Earth, Euler predicted that it would have a period of 305 days. Several astronomers searched for motions with this period, but none was found. Chandler's contribution was to look for motions at any possible period; once the Chandler wobble was observed, the difference between its period and the one predicted by Euler was explained by Simon Newcomb as being caused by the non-rigidity of the Earth. The full explanation for the period also involves the fluid nature of the Earth's core and oceans—the wobble, in fact, produces a very small ocean tide with an amplitude of approximately 6 mm (14 in), called a "pole tide", which is the only tide not caused by an extraterrestrial body. Despite the small amplitude, the gravitational effect of the pole tide is easily detected by the superconducting gravimeter.[4]

Attempts at measurement

The International Latitude Observatories were established in 1899 to measure the wobble. These provided data on the Chandler and annual wobble for most of the 20th century, though they were eventually superseded by other methods of measurement. Monitoring of the polar motion is now done by the International Earth Rotation Service.

The wobble's amplitude has varied since its discovery, reaching its largest size in 1910 and fluctuating noticeably from one decade to another. In 2009, Malkin & Miller's analysis of International Earth Rotation and Reference Systems Service (IERS) Pole coordinates time series data from January 1946 to January 2009 showed three phase reversals of the wobble, in 1850, 1920, and 2005.[2]


While it has to be maintained by changes in the mass distribution or angular momentum of the Earth's outer core, atmosphere, oceans, or crust (from earthquakes), for a long time the actual source was unclear, since no available motions seemed to be coherent with what was driving the wobble.

One hypothesis for the source of the wobble was proposed in 2001 by Richard Gross at the Jet Propulsion Laboratory managed by the California Institute of Technology. He used angular momentum models of the atmosphere and the oceans in computer simulations to show that from 1985 to 1996, the Chandler wobble was excited by a combination of atmospheric and oceanic processes, with the dominant excitation mechanism being ocean‐bottom pressure fluctuations. Gross found that two-thirds of the "wobble" was caused by fluctuating pressure on the seabed, which, in turn, is caused by changes in the circulation of the oceans caused by variations in temperature, salinity, and wind. The remaining third is due to atmospheric fluctuations.[5]

See also


  1. ^ e.g. Mueller, I.I. (1969). Spherical and Practical Astronomy as Applied to Geodesy. Frederick Ungar Publishing, NY, pp. 80.
  2. ^ a b Zinovy Malkin and Natalia Miller (2009). "Chandler wobble: two more large phase jumps revealed". Earth, Planets and Space. 62 (12): 943–947. arXiv:0908.3732. Bibcode:2010EP&S...62..943M. doi:10.5047/eps.2010.11.002.
  3. ^ "Earth's Chandler Wobble Changed Dramatically in 2005". MIT Technology Review. 2009. Retrieved 25 July 2013.
  4. ^ See, e.g., Fig. 2.3. Virtanen, H. (2006). Studies of Earth Dynamics with the Superconducting Gravimeter (PDF) (Academic Dissertation at the University of Helsinki). Geodetiska Institutet. Retrieved September 21, 2009.
  5. ^ Gross, Richard S. (2000). "The Excitation of the Chandler Wobble". Geophysical Research Letters. 27 (15): 2329–2332. Bibcode:2000GeoRL..27.2329G. doi:10.1029/2000gl011450. Retrieved January 17, 2011. Some scientists predict the last wobble to have happened nearly 4000 to 7500 years ago during a time when there were dramatic changes in the pattern of monsoons across northern Africa. The researchers claim that it has transformed the Great Saharan Desert into somewhat greener territory. Further, they expect the wobble to happen once in nearly 20000 years, which implies that the next can be expected after a period of 16000 years; these estimates are short compared to the life of earth.

Further reading

  • Carter, B. and M. S. Carter, 2003, "Latitude, How American Astronomers Solved the Mystery of Variation," Naval Institute Press, Annapolis.
  • Gross, Richard S (2000). "The Excitation of the Chandler Wobble". Geophysical Research Letters. 27 (15): 2329–2332. Bibcode:2000GeoRL..27.2329G. doi:10.1029/2000gl011450.
  • Lambeck, Kurt, 1980, The Earth's Variable Rotation: Geophysical Causes and Consequences (Cambridge Monographs on Mechanics), Cambridge University Press, London.
  • Munk, W. H. and MacDonald, G. J. F., 1960, The Rotation of the Earth, Cambridge University Press, London.
  • Moritz, H. and I.I. Mueller, 1987, Earth Rotation: Theory and Observation, Continuum International Publishing Group, London.

External links

This page was last edited on 19 June 2018, at 06:10
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