'The zero line in WMM2025 passes through a lot of population centers; I wonder what year the largest share of the population lived in a zone of less than 5° of declination,' he thought, derailing all other tasks for the rest of the day.
This is the tenth comic in the Bad Map Projections series, displaying Bad Map Projection #216: Zero Declination.
While the Earth's magnetic field is broadly aligned North-South, the actual alignment of the magnetic field varies over time and position. The difference between True North (the axis of Earth's rotation) and Magnetic North (the direction a compass will point) will vary depending on your position, and is known as the Magnetic Declination of that point.
The comic shows a map that has been distorted based on the Magnetic Declination such that Magnetic North for every point is pointed toward the top of the map. If this were reality, then Magnetic North would always be aligned with True North, or in other words, there would be Zero Declination at all points.
The red arrows indicate the distortions from the starting map required to make Magnetic North be at the top.
In the title text, "WMM2025" refers to the 2025 version of the World Magnetic Model, a representation of the Earth's magnetic field. You can see it here. The "zero line" is in green, which shows where in the world magnetic declension is 0°. Randall has presumably wasted a day trying to figure out what year has had the most population living in an area of less than 5° declension by searching through previous WMM maps. He appears to have not found the answer, so explainxkcd requests its readers to finish the job. Judging by magnetic declination historical data lines with 0 degree declination were near India and China at some points recently, so maybe it is reasonable to assume it happened. Unfortunately declination can easily go up to 15 degrees or even higher, so when one high population density area has declination close to 0, some other one often has an extremely high declination.
