Steven P. McNicoll writes:

"The magnetic variation of the earth changes at a rate of 50.27 seconds of
arc per year." That seems a bit low to me. At that rate six degrees of
change would take 430 years.

It changes at a variable rate, and once it approaches a degree, charts and
navaids have to change. It's a tremendous amount of extra work, an additional
source of error, and an imprecise and fickle basis for navigation.

--
Transpose mxsmanic and gmail to reach me by e-mail.