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[sci.astro] Solar System (Astronomy Frequently Asked Questions) (5/9)
Section - E.13 Won't there be catastrophes when the planets align in the year 2000?

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Top Document: [sci.astro] Solar System (Astronomy Frequently Asked Questions) (5/9)
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See reader questions & answers on this topic! - Help others by sharing your knowledge
Author: Laz Marhenke <laz@leland.Stanford.EDU>, 
	Chris Marriott <chris@chrism.demon.co.uk>

Obviously there were no catastrophes in May (05-05-2000), nor were
there any in the year 1982.

For starters, the planets only "align" in a very rough fashion.  They
don't orbit the Sun in the same plane, so it's impossible to get very
many of the planets in a straight line.  Nevertheless, any time they
all get within about 90 degrees of each other, someone will claim
they're "aligned."  The last time this happened was 1982 when dire
predictions were heard about how the "Jupiter effect" would lead to
world-wide disaster.

Second, even if they *were* all aligned, the effect on the Earth would
be miniscule.  It's true that the other planets' gravity does affect
the orbit of the Earth, but the effect is small, and lining up all the
planets doesn't even come close to making it big enough for anyone to
notice.  The effect on the Earth is dominated by Jupiter and Venus
anyway (Jupiter because it's massive, Venus because it's occasionally
very close to us).  All the other planets put together only affect us
about 10% as much as those two, so the fact that they're all in the
same general direction as Jupiter and Venus doesn't make much
difference.

Third, even if all the planets could produce a strong gravitational
effect on the Earth (which they can't, unless they find a way to
increase their mass by a factor of 10--100), it wouldn't result in the
"crust spinning over the magma" or some other dire effect, since their
gravity would be pulling on every part of the Earth (almost) equally.

The "(almost)" is because the other planets do exert tidal forces on
the Earth, which means they pull on different parts of the Earth very
slightly differently.  However, tidal forces decrease *rapidly* with
distance (as the third power), so these forces are very small: The
tidal force from Venus at its closest approach to Earth is only
1/17,000th as large as the Moon's, and we seem to survive the Moon's
tides well enough twice a day.  If the Moon raises tides of 1 meter
(three feet) where you live, Venus at its closest will raise tides of
1/20th of a millimeter, or about the thickness of a hair.  The other
planets have even smaller tidal effects on the Earth than Venus does.

Finally, it's worth remembering that the Earth is about 4.5 billion
years old.  Whilst these "alignments" may be rare in terms of a human
lifetime (occurring once every few decades), they've occurred numerous
times during the time that life has existed on this planet, and many,
many times in the comparatively brief time that humans have been
around.  Brian Monson found ten such "alignments" between AD 1000 and
AD 2000, <URL:http://drumright.ossm.edu/astronomy/conjunctions.html>.
Thus, over the history of this planet there have been about 45 million
such "alignments."  The fact that we're still here to talk about it is
proof enough that nothing *too* terrible happens!

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Top Document: [sci.astro] Solar System (Astronomy Frequently Asked Questions) (5/9)
Previous Document: E.12.2 What about a planet inside Mercury's orbit?
Next Document: E.14 Earth-Moon system

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