I was replaced the battery in my fancy Sunnto Vector watch which meant that I had to recalibrate some of the sensors including the compass. Calibrate a compass? We all learn in grade school that magnetic north isn’t exactly the same as true north, but it’s generally close enough that we don’t think about it. In the midwest it’s pretty close to the same, but out towards the coast the difference is significant. Here in Seattle if you follow a compass and walk north, you’ll be heading almost 17 degrees too far to the east!
Even more interestingly, the declination changes from year to year. In Seattle, it is currently decreasing by about a degree every 6 years. I strongly recommend that you click this Wikipedia link to watch an animation of how the magnetic field has changed over the last 400 years.
You probably don’t need to think about this in your day to day activities, but if you spend time outdoors, it’s good to have in the back of your mind in case you find yourself relying a compass in a survival situation.