ATLAS · REFERENCE LINES
The Lines on the Map
Some of the most important lines on Earth are ones you can never see. The Equator, the two Tropics, the Arctic and Antarctic Circles, and the Prime and 180th meridiansaren’t painted on the ground or visible from orbit — yet they decide where the seasons fall, where the Sun can stand straight overhead, and how every map coordinate is reckoned. Switch them on over any Atlas metric map to see how the world’s data lines up against the planet’s basic geography.
On the Atlas canvas, the Reference Lines switch sits just under the overlays panel. The lines draw over whatever metric (or the plain political map) you have up.
The seven lines, and what each marks
Why 23.4°, and not a round number?
Because the Sun sets the rule, not a mapmaker. The Earth spins on an axis tilted about 23.43657° off vertical relative to its orbit — the obliquity of the ecliptic. That tilt is the engine of the seasons, and it fixes the Tropics exactly: the overhead Sun can reach no further than the tilt angle from the Equator, so the Tropic of Cancer sits at 23.43657° north and Capricorn at the same angle south. The polar circles are the same number measured from the poles — 90° minus the tilt, or 66.56343° — which is why the Tropics and the polar circles are mirror images of one quantity. The lines here are pure mathematics drawn from that tilt and the agreed position of Greenwich; the value is the 2024 mean obliquity.
The Lines on the Map — Equator, Tropics, Polar Circles & Meridians Explained →About this overlay
Unlike the rest of the Atlas, these lines aren’t measured data — they’re a coordinate system humans agreed on, generated mathematically as lines of constant latitude and longitude. There are no per-place pages, because a line of latitude isn’t a place. The Tropic and polar-circle latitudes use the obliquity of the ecliptic (23.43657° / 66.56343°, 2024 mean); the Prime Meridian follows the 1884 Greenwich convention, and the 180th meridian is drawn as a clean geographic line, distinct from the deliberately crooked legal International Date Line. Basemap © CARTO.