NavList:

   

Reply

Lisa,

Compass needles line up with the earth's magnetic field. This means that
near the north magnetic pole the north tip of the needle points down from
the horizontal to be in alignement with the field, and up, near the south
magnetic pole. Near the "magnetic equator", so to speak, the needle sits
more or less horizontal. Manufacturers of compasses can weight the needle to
counterbalance the pointing down or up effect, so that the needle sits
horizontally...when it is used in that part of the earth's magnetic field it
is designed for. Compass manufacturers have settled on dividing the earth up
into 5 magnetic zones, and manufacture compasses to work in one or more of
those zones. Alternatively, the manufacturers may sit the needle on a
special bearing, such that it sits horizontally when used all over the
earth....called a global needle.

See the Silva or Suunto sites for further discussion, and charts of the 5
zones. Or do a Google on compass zones. Note that there is quite a
convergence of zones immediately north of Australia, while the US and most
of Europe is covered by one zone.

It sounds like you will find the needle or card on your handbearing compass
"sticking" once you move north of Australia. This will be because the
bearing the needle or card sits on is binding due to uncompensated tilt.

Some compasses provide for compensation by adjustment.Check yours. I am very
unclear on binnacle mounted compasses. The adjustment devices on them are
clearly designed to make compensatory adjustments in the horizontal plane, I
am not so sure about the vertical plane. I suspect it varies by compass
make. Maybe others might like to comment on this point...I have always
wondered

Lee Martin


----- Original Message -----
From: "Lisa Fiene" 
To: 
Sent: Thursday, October 21, 2004 8:30 AM
Subject: Compass - southern hemisphere/northern hemisphere


> Our hand bearing compass is one designed for the southern hemisphere, as
> we're in Australia. I remember being taught that it's important to have
> a southern hemisphere compass in the southern latitudes and a northern
> hemisphere compass in the northern latitudes.
>
> What's the principle involved here, and how did the early navigators (eg
> going from Europe to rounding Cape Horn etc.) compensate?
>
> Thanks, Lisa
>

   

Reply