A compass Credits: Wikimedia


With the pandemic, I've been staying indoor most of the time browsing the Internet.  While looking around Google Map for ideas on future trips, I noticed something very interesting about the Forbidden City.


Do you also see it?  The Forbidden City is oriented at a small angle counter-clockwise from the true North-South line!

I dug a little online, and some have suggested that this deviation is due to the use of compasses in fengshui.  What that means is, the Forbidden City was likely planned along the "magnetic North-magnetic South" line instead of the "true North-true South" line.  The phenomenon that the magnetic North differs from the true North is called magnetic declination or magnetic variation.

The exact cause of magnetic declination is complicated, but the short version is that the magnetic materials in the earth's crust are not perfectly aligned with the true North and true South.  Imagine suspending a bar magnet in liquid in a revolving globe, but off axis.  Now imagine someone spinning this globe, which causes the magnet to move around.  Similarly, the Earth's magnetic declination also changes over time.  The National Center for Environmental Information, a part of the National Oceanic and Atmospheric Administration, publishes the historic magnetic declination experienced around the world since 1600 AC.

This declination map is very important for navigation purposes: when using map and compass, one should orient oneself by compensating for the local magnetic declination.  Interestingly, due to naming conventions, the magnetic declination impacts not just the airplanes in the sky, but also the airports on the ground!  The airport runways are named by their magnetic orientation, so with the magnetic declination changing over time, occasionally runways have to be renamed.  One recent example was when the Fairbanks International Airport renamed its runway 1L-19R to 2L-20R in 2009.

I've read an interview question before, "if you walk 1 mile to the north, then 1 mile to the east, then 1 mile to the south, and found yourself at your original location, where are you?"  I guess I will have to first ask the interviewer to specify whether he meant the true north/east/south, or the magnetic north/east/south.

Extended reading:

(NOAA) Earth's Magnetic Field: The Force That's Always with You

(Princeton University) The Backpacker's Field Manual

(ArXiv) Magnetic Compasses and Chinese Architectures


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