Re: [Embedded troll] Easy Questions
From: Guy Macon (http://www.guymacon.com)
Date: 05/09/04
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Date: Sat, 08 May 2004 20:33:07 -0700
Google search on "precession of the equinoxes":
http://www.google.com/search?q=precession+of+the+equinoxes
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"The reason Precession occurs is that the Earth not only
rotates on its axis -- otherwise known as the Axis Munde
or World Pillar -- it also *wobbles*."
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"It takes 25,800 years for the Earth’s axis to complete
one wobble, sway, or clockwise circle. We call this
25,800-year cycle of our planet the Platonic year.
It is also referred to as the precession of the
equinoxes or the great year in a comprehensive dictionary.
The great year is one of the longest cycles that are well
known to our astronomers."
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"This motion was first noted by Hipparchus c.120 BC. ...
The precession of the equinoxes was first explained by
Isaac Newton in 1687."
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"The Earth is not a perfect sphere; it bulges slightly
around the equator. The gravitational attraction of
both the Sun and the Moon tries to pull the Earth's
equatorial bulge into the Earth's and the Moon's orbital
planes. Acting like a spinning top, the rotating Earth
resists this pull. The result, therefore, of the Sun's
and Moon's attraction and the Earth's resistance is
that the Earth's axis of rotation moves slowly westward
around the pole of the ecliptic..."
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"If you ever had a spinning top, you know that its axis
tends to stay lined up in the same direction--usually,
vertically, though in space any direction qualifies.
Give it a nudge, however, and the axis will start to gyrate
wildly around the vertical, its motion tracing a cone
(drawing). The spinning Earth moves like that, too,
though the time scale is much slower--each spin lasts a
year, and each gyration around the cone takes 26 000 years."
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"The Earth's rotation axis is not fixed in space. Like a rotating
toy top, the direction of the rotation axis executes a slow
precession with period of 26,000 years. ... This motion is
called precession and proceeds in about 25,800 years along a
cone with a half apex angle of 23.439 degrees, which causes
the vernal equinox to move along the ecliptic by 50.291
arcseconds per year. This precession of the equinoxes
affects the length of a tropical year as well as the length
of a sidereal day. Because the directions to the sun and
the moon vary and because of gravitational forces from the
planets, the true rotation axis wobbles around the precession
cone. The largest of these nutations amounts to 9.2 arcseconds
in 18.6 years and is caused by the moon. Precession was the
third-discovered motion of the Earth, after the far more
obvious daily rotation and annual revolution. Precession
is caused by the gravitational influence of the Sun and
the Moon acting on the Earth's equatorial bulge. To a much
lesser extent, the planets exert influence as well."
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-- Guy Macon, Electronics Engineer & Project Manager for hire. Remember Doc Brown from the _Back to the Future_ movies? Do you have an "impossible" engineering project that only someone like Doc Brown can solve? My resume is at http://www.guymacon.com/
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