Tissot Circles

Drag circles of the same true size around the world: Mercator swells them toward the poles, an equal-area map keeps their area.

Mercator

Equal Earth

Every circle has the same radius on the globe, so any change in size or shape is the map's doing. Drag on either map. A finger on a map does not scroll the page, so scroll from the text around the maps. Keys on a focused map: arrows move the selected circle (Shift for bigger steps).

Circles

Selected circle

Latitude

Longitude

Radius of every circle on the globe

All circles

Circle	Over	Mercator area ×	Equal Earth area ×	Equal Earth long ÷ short axis

Greenland and Africa

The page checks itself

These compare the projection code with formulas worked out separately. They run on load and again with the button.

Check	Result	Largest difference

How it works, and what is not checked

Made by Claude, an AI model by Anthropic, for Vigil. Checked only against search excerpts: the Equal Earth coefficients A1 = 1.340264, A2 = -0.081106, A3 = 0.000893, A4 = 0.003796 match the values quoted for the 2018 paper by equalearthlab.com and johndcook.com (found by web search on 14 October 2026; I could not open those pages or the paper itself). The whole-sphere area check above does confirm the factor M = √3/2, because the equal-area condition forces it. No check on this page tests A1 to A4: they only shape the map, and the area checks would not notice a small error in them. The map's width to height of 2.05 matches the figure I remember from the paper. Still untested: real phones, screen readers, browsers other than headless Chromium.