Main Page

Welcome to the explain xkcd wiki! We already have 6 comic explanations!

(But there are still 3201 to go. Come and add yours!)

Latest comic

Go to this comic

Bad Map Projection: Zero Declination

|< < Prev #3207 (February 13, 2026)

Title text: '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.

|< < Prev Random Explanation

Explanation

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 declination 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° declination by searching through previous WMM maps. He appears to have not found the answer, so explainxkcd requests its readers 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.

Transcript

Is this out of date? Clicking here will fix that.

New here?

You can read a brief introduction about this wiki at explain xkcd. Feel free to sign up for an account and contribute to the wiki! We need explanations for comics, characters, themes, memes and everything in between. If it is referenced in an xkcd web comic, it should be here.

• If you're new to wikis like this, take a look at these help pages describing how to navigate the wiki, and how to edit pages.

• Discussion about various parts of the wiki is going on at Explain XKCD:Community portal. Share your 2¢!

• List of all comics contains a complete table of all xkcd comics so far and the corresponding explanations. The red links (like this) are missing explanations. Feel free to help out by creating them!

Rules

Don't be a jerk. There are a lot of comics that don't have set in stone explanations, feel free to put multiple interpretations in the wiki page for each comic.

If you want to talk about a specific comic, use its discussion page.

Please only submit material directly related to—and helping everyone better understand—xkcd... and of course only submit material that can legally be posted (and freely edited.) Off-topic or other inappropriate content is subject to removal or modification at admin discretion, and users posting such are at risk of being blocked.

If you need assistance from an admin, feel free to leave a message on their personal discussion page. The list of admins is here.

Logo

Explain xkcd logo courtesy of User:Alek2407.