Observations
Polar motion is observed routinely by Very Long Base Line Interferometry, by Lunar Laser Ranging, and by Satellite Laser Ranging. The annual component is rather constant in amplitude, and its frequency varies by not more than 1 to 2%. The amplitude of the Chandler wobble, however, varies by a factor of three, and its frequency by up to 7%. Its maximum amplitude during the last 100 years never exceeded 230 mas.
The Chandler wobble is usually considered a resonance phenomenon, a free nutation, which is excited by some source and than dying away with a time constant τD of the order of 100 years. This term is a measure of the elastic reaction of the Earth. This is also the explanation for the deviation of the Chandler period from the Euler period. However, rather than dying away, the Chandler wobble, continuously observed for more than 100 years, varies in amplitude and shows a sometimes rapid frequency shift within a few years. This reciprocal behavior between amplitude and frequency has been described by the empirical formula:
(2) m = 3.7/(ν - 0.816) (for 0.83 < ν < 0.9)
with m the observed amplitude (in units of mas), and ν the frequency (in units of reciprocal sidereal years) of the Chandler wobble. In order to generate the Chandler wobble, recurring excitation is necessary. Seismic activity, groundwater movement, snow load, or atmospheric interannual dynamics have been suggested as such recurring forces, e.g. Atmospheric excitation seems to be the most likely candidate. Others propose a combination of atmospheric and oceanic processes, with the dominant excitation mechanism being ocean‐bottom pressure fluctuations.
Read more about this topic: Polar Motion
Famous quotes containing the word observations:
“Science is a dynamic undertaking directed to lowering the degree of the empiricism involved in solving problems; or, if you prefer, science is a process of fabricating a web of interconnected concepts and conceptual schemes arising from experiments and observations and fruitful of further experiments and observations.”
—James Conant (1893–1978)
“I have never yet seen any plan which has not been mended by the observations of those who were much inferior in understanding to the person who took the lead in the business.”
—Edmund Burke (1729–1797)
“The natural historian is not a fisherman who prays for cloudy days and good luck merely; but as fishing has been styled “a contemplative man’s recreation,” introducing him profitably to woods and water, so the fruit of the naturalist’s observations is not in new genera or species, but in new contemplations still, and science is only a more contemplative man’s recreation.”
—Henry David Thoreau (1817–1862)