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OK John, deal me in.

Point A is the north magnetic pole. The compass at this point would gyrate
causing you to speculate as to whether it was the north or south pole at
which you were situated. Walk a straight mile in any direction. At B the
compass would point in the direction from whence you came, establishing that
direction as due north and allowing you to establish the easterly direction.
East 1 mile (or kilometre or chain or anything) your compass would point to
the point you started from, the north magnetic pole.

Alternatively, you could use your compass as a declinometer by holding it
vetically when you suspected you were at a magnetic pole. The needle would
point straight down.

Yours sincerely
Craig Lucanus
----- Original Message -----
From: "John Denker" <jsd@AV8N.COM>
To: <PHYS-L@LISTS.NAU.EDU>
Sent: Thursday, August 12, 2004 7:03 PM

Since phys-l has been somewhat listless lately, here
is a riddle:

Dr. Livingstone starts out at a place which we call
Point A. He then undertakes a journey consisting
of three legs:
-- he travels precisely southward for one mile;
-- then he turns and travels precisely eastward
for one mile;
-- then he turns and travels precisely northward
for one mile.
He discovers that as a result of this journey, he
has returned to Point A.

Note: He travels by airship, at an appropriate constant
altitude, so you don't need to worry about obstructions
or other nonidealities.

The questions are:

1) Where is Point A?
2) Are you sure? How do you know?

===========

Usual ground-rules: The right answer depends on
understanding the physics of the situation. This may
require some outside-the-box thinking. Everything
I've said is intended to be true and helpful. I expect
a strong consensus as to the correctness of the answer.