Orbital compass

On Sun, 7 Nov 2004 05:26:37 GMT, (Henry Spencer)
wrote:

A further complication is that because Earth's magnetic center is not at
its geometric center, the places where a three-axis magnetometer reports
that the field is vertical -- which are the places usually reported as the
magnetic poles -- are *not* in fact the places where the axis of Earth's
magnetic dipole intersects the surface.

To say nothing of the fact that the field is not an exact dipole... and
is changing with time in poorly-understood ways.

The declination, or correction factor, to convert magnetic north to
true north is printed on good maps. It changes slowly because the
magnetic poles are moving. In aviation, most directions are magnetic
(although the use of GPS seems to be changing this, much to the
confusion of some pilots), particularly runway headings.

The magnetic poles have moved enough that many runway designations,
which are just magnetic headings divided by ten, are off one.
However, the aviation world decided not to renumber, mostly because
the expense would be inordinate. Every chart would have to be redrawn
and reissued, which makes everyone involved flinch whenever it's
mentioned.

Mary

--
Mary Shafer Retired aerospace research engineer

Mary Shafer wrote:
On Sun, 7 Nov 2004 05:26:37 GMT, (Henry Spencer)
wrote:


A further complication is that because Earth's magnetic center is not at
its geometric center, the places where a three-axis magnetometer reports
that the field is vertical -- which are the places usually reported as the
magnetic poles -- are *not* in fact the places where the axis of Earth's
magnetic dipole intersects the surface.

To say nothing of the fact that the field is not an exact dipole... and
is changing with time in poorly-understood ways.


The declination, or correction factor, to convert magnetic north to
true north is printed on good maps. It changes slowly because the
magnetic poles are moving. In aviation, most directions are magnetic
(although the use of GPS seems to be changing this, much to the
confusion of some pilots), particularly runway headings.

The magnetic poles have moved enough that many runway designations,
which are just magnetic headings divided by ten, are off one.
However, the aviation world decided not to renumber, mostly because
the expense would be inordinate. Every chart would have to be redrawn
and reissued, which makes everyone involved flinch whenever it's
mentioned.

Mary



We changed our magnetic variation in 1986, IIRC when I was
stationed at RAF Mildenhall. In reality, the dipole had only moved
from 5.2 degrees off to 4.8 degrees off, so the real change was
very minimal.

"Greg D. Moore (Strider)" wrote in message
...

"Henry Spencer" wrote in message
...

To say nothing of the fact that the field is not an exact dipole... and
is changing with time in poorly-understood ways.

And apparently changing more rapidly lately.

Better hang on! Wear a tether!