From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.science
Subject: Re: the earth is flat/round
Date: Tue, 17 Mar 1998 22:38:00 GMT

In article <6emgo6$j6q$1@eagle1.cc.GaSoU.edu>,
BENJAMIN_H. ZELLNER <ZELLNER@GSVMS2.CC.GASOU.EDU> wrote:
> > Here is how the ancient Greeks concluded that the planet is round...
>
>Correct.  You could also add:  Accurate surveying will show that the
>earth is a sphere.  However it was difficult to survey that accurately
>without modern instruments.

They don't have to be all *that* modern.  18th-century surveying methods
were good enough to not only assess the shape of the Earth in considerable
detail, but also to yield strong hints about its underlying structure.

(The great British survey of India -- which, among other things,
discovered that Mount Everest is the world's highest mountain -- carefully
calculated the proper corrections for the gravitational effect of the
Himalayas on plumb bobs.  The analysts were quite surprised to discover
that the necessary corrections, as determined from the highly-redundant
survey data, were in fact considerably smaller than the calculated values.
What they didn't know is that the weight of the Himalayas is supported on
Earth's semi-fluid mantle by a big bulge of light crustal rock sticking
*down* into the dense mantle.  So while the Himalayas are much denser than
the air surrounding them, and deflect a plumb bob toward them, the deep
rocks underlying them are lighter than the surrounding deep rocks, and
that reduces the effect.)
--
Being the last man on the Moon                  |     Henry Spencer
is a very dubious honor. -- Gene Cernan         | henry@zoo.toronto.edu