Ultima Thule

In ancient times the northernmost region of the habitable world - hence, any distant, unknown or mysterious land.

Tuesday, July 25, 2006

Asian Storms Push the Earth Around

By Aussiegirl

So the unchanging pole star is currently Polaris, but in 14000 AD -- if the Earth lasts that long -- the pole star will be Vega, and Polaris will have to wait until 27800 AD to become the pole star again. I'm always amazed at how much we humans can learn about our beautiful universe by force of our intellect, without even leaving Earth.
Here's the explanation of "the precession of the equinoxes" from the Wikipedia article: Precession of the equinoxes refers to the precession of Earth's axis of rotation in inertial space. It was discovered by Hipparchus that the positions of the equinoxes regress (move westward) along the ecliptic compared to the fixed stars on the celestial sphere. Currently, this annual motion is about 50.3 arcseconds per year or 1 degree in 71.6 years. The process is subtle but cumulative, and over the centuries adds up to many degrees. A complete precession cycle is a period of approximately 25700 years, (the so called great Platonic year), during which time the equinox regresses over a full circle of 360°. A consequence of the precession is a changing pole star. Currently Polaris is extremily well-suited to mark the position of the north celestial pole, as it is about a half degree away from it and it is a moderately bright star (visual magnitude is 2.1 (variable)). On the other hand Thuban in the constellation Draco, which was the pole star in 3000 BC is much less conspicious at magnitude 3.67 (one-fifth as bright as Polaris); today it is all but invisible in light-polluted urban skies. The brilliant Vega in the constellation Lyra is often touted as the best Northstar, when it fulfilled that role around 12000 BC and will do so again around the year AD 14000. In reality it never comes closer than 5° to the pole. When Polaris will be the north star again around 27800 AD, due to its proper motion it will be farther away from the pole then than it is now, while in 23600 BC it came closer to the pole.

(The illustration is from the Wikipedia article and shows the precession as a consequence of the torque exerted on Earth by differential gravitation.)

Physics news Update 786

Number 786 #3, July 25, 2006 by Phil Schewe and Ben Stein
Asian Storms Push the Earth Around

Earth's axis of rotation undergoes several gyrations, such as the precession of the equinoxes, which takes about 26,000 years. Recently two of the most important axis gyrations inadvertently cancelled each other, allowing geophysicists to measure other, subtler gyrations that would normally be difficult to detect. The two larger wobbles are the 433-day cycle Chandler Wobble (whose origin is not very well known) and the wobble caused by annual weather oscillations. Their combined effect is normally to cause the rotational axis to migrate by as much as 10 meters. But from December 2005 to February 2006 their mutual nullification reduced the axis excursion to less than 1 meter. This allowed Belgian scientists to study fainter, lesser forces whose exertions could briefly be measured. The scientists saw signs of what they believe to be an influence on Earth's wobble day by day triggered by storms over Asia and northern Europe.

Lambert et al., Geophysical Research Letters, July 2006


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