Re: axis shift

From: Leo Barhorst (
Date: Sun Jan 02 2005 - 06:34:43 EST

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    Just came across an interresting article explaning the small axis shift
    of the earth caused by the asian earthquake.
    What's Really Happened to the Terrestrial Axis?
    A short explanation about the effects caused by the earthquake in the Indian
    Ocean to our planet's rotational axis
    by Alfonso Mantero
    Earth's South Pole
    Earth is similar, more or less, to a ball which is suspended in space. This
    ball is not still, rather it performs a good number of motions. The sum of
    all of those motions is called global motion. The rotation around the Sun is
    one of those motions: our planet follows an elliptical orbit while rotating
    around it. This orbit lies on a plane (orbital plane or ecliptic plane)
    thanks to that law of physics known as conservation of angular momentum.
    Earth rotates around the Sun and while doing that it also rotates upon
    itself, that is, along an axis passing through its baricentre. The direction
    this axis has in space, that is tilting with respect to orbital plane is
    dependent on forces external to our planet, eg. the gravitational attraction
    coming from the Sun and the Moon. These phenomena are real, they have been
    measured in the past and have always shown a progressive activity. They have
    been measured with highly-precision tools over years. But humans may
    appreciate them over extremely long periods, such as millennia. Equinox
    precessions (one of the reasons invalidating astrology) are an example of
    phenomena changing over long periods. No phenomena, excepted an impact with
    a celestial body of huge dimensions, such as an asteroid, are able to
    suddenly vary the tilting of the terrestrial axis with
    respect to orbital plane.
    Now, one may ask what do geologists from the Centro di Geodesia Spaziale of
    the Italian Space Agency (ASI) mean when they say the quake in the Indian
    Ocean caused the terrestrial axis to shift. Surely none is arguing with the
    laws of physics. The experts explained that what is changed is the tilting
    of the terrestrial rotation with respect to Earth, not to the orbit. Let us
    see how this can happen.
    At the beginning of this article, we said that the Earth is similar to a
    ball which is suspended in space and rotating along its axis (please bear in
    mind that this example is meaningful only over a short period of time). A
    train travelling at a certain speed has what is commonly known as "a certain
    inertia" (physicists call it quantity of motion); in the same way, a
    rotating body has its "rotational inertia", which is the angular momentum.
    The angular momentum is there each time a body rotates in any way.
    A body which has an angular momentum is difficult to move, and this is
    because the angular momentum is preserved. An example for that is the
    direction of a wheel's axis, which is fixed. This is why a bike which is not
    moving needs a support, while you can safely remove the small wheels when
    you are more or less able at cycling.
    In order to modify the direction of a body's rotational axis, its angular
    momentum has to be modified and, to reach this result, external forces are
    necessary. During the earthquake occurred on December 26, great forces came
    to action. But they were internal to our Earth, that is they were generated
    by the motion of some masses and they affected other masses on our planet.
    Now, those kind of forces cannot modify the angular momentum of a rotating
    body (Earth is an example of rotating body).
    When a body is rotating freely, it rotates along what physicists call
    principal axis: each body has three of them, along which the body may
    rotate. In the Earth, its three axes are dependent on the way masses are
    distributed inside our planet. An earthquake is a sudden motion of
    terrestrial masses. That motion may vary the distribution of those masses.
    Hence, the principal axes may shift a little.
    Having said that, the final step is to understand what has happened to our
    planet. On one hand we have the angular momentum which is preserved. On the
    other hand, our Earth now needs to rotate along a main axis which is
    different from the main axis along which our planet has been rotating a few
    seconds ago. So, the solution is to align the new main axis with the
    rotational axis, that is possible through a rotation that will bring the two
    axes in line. We will see this as a variation of two thousandth arcsecond:
    now the "North Pole" sign has to be moved of about 6 centimetres toward
    Greenwich. In other words, our coordinates have slightly changed: our
    latitude has increased, toward North. From an astronomical point of view,
    the apparent locations of stars have changed. In particular, the Pole star
    is now 2 thousandth arcsecond higher, and subsequently all other stars.
    So the experts said. We would like to underline that the motion was
    extremely small: during a year, the pole moves of more than 6 cm. In the
    previous years, it has moved several metres. While moving, it follows a
    spiral-pattern which is of uttermost interest to scientists. The anomaly in
    our case, is that the 6-cm motion happened all of a sudden. This motion will
    not bring any effect on weather and it will be noticeable and measurable
    only through extremely sophisticated tools, used by the experts.
    Translation: Claudia Gioia
    Greetings and clear, dark skies
    Leo Barhorst, Medemblik NL
    Cospar 4252 52.76350 N 5.09114 E 2 m ASL
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