From: SFPhysics@aol.com
Date: Thu Jan 24 2002 - 23:31:25 PST
From: SFPhysics@aol.com Message-ID: <197.19be4a5.298263cd@aol.com> Date: Fri, 25 Jan 2002 02:31:25 EST Subject: South Pole Longitude & Compass Question...
<<
>By the way. Is a compass useless down there? How wide
>is a longitude degree near the pole? This is so
>typical, I don't get curious until the damn thing is
>over. Day late, dollar short!
> With firm handshake,
>Raleigh
>>
Greetings Raleigh:
The South Magnetic Pole is a fairly wide area that actually wanders just like
the North Magnetic Pole. The center of the pole field may move up to 12 km
in a day and over the last century it has migrated up through Canada nearly
90 km in a northerly move. It may even stay centered on one spot for some
time and a solar magnetic storm can push the field on the ground surface and
shift the field center several Km in a very short time. As you approach both
of the magnetic poles the compass needle actually wants to point downward and
this is called "dip" angle. Many Physics labs have a "dip" demonstrator
sitting on the shelves and most teachers have no idea what it is. You can
even measure dip in San Francisco! In fact, the magnetic equator is about
the only place the needle is truly horizontal. Centered in either magnetic
pole the needle can point in any direction as the bearing friction of most
needles is more than the force applied by the weak spread out field. Away
from the center the needle will give all kinds of erroneous readings because
the field is weak and cannot apply enough force for a dependable reading.
BTW - Do not confuse "dip" with declination. Declination is the angle in
degrees a compass points to the North Magnetic Pole and away from the North
axis Pole. The South Pole is the same with a declination factor for
longitude in the southern hemisphere.
Your degree question is one that would require spherical trigonometry and I
am not up to that tonight. Let's see if we can figure something out that
will be in the ballpark. The earth turns 360 degrees in one theoretical
solar day (we are neglecting the path around the sun affecting a true solar
day which is not a perfect 360). The equator is *about* 24,000 miles (yes
miles, I'm old). There are 24 hours in a day so each hour must mean the
earth at the equator travels roughly 1000 miles eastward. 360 divided by 24
gives us a figure of 15 degrees. 15 degrees divided into 1000 means every
degree at the equator is 66.67 miles long. Grabbing a calculator to find the
sin of 89 degrees for a rounded figure of 0.01745 gives me what a degree of
longitude might be at 89 degrees south latitude. That would be 1.16 mile. I
don't know if this is reasonable or correct. Now I am going to do to you
what I did with my students. For 100 bonus points earned over the weekend,
prove me right or prove me wrong. If wrong, where is the flaw in my
thinking? If right, is there a better way to calculate this?
Al Sefl
Physics Phun Phans Phantastic Phutures Phor Phabulous Philosophers...
PS: I hope everyone in the San Francisco area caught the absolutely gorgeous
circle of light around the moon tonight. Amazing what can be done with
photons and high altitude ice crystals... God has to be an artist!
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