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The seven great geographic lines

The seven great lines

Five of these lines are set by astronomy and two were decided in committee. The equator comes from the spin of the planet. The tropics and the polar circles come from its tilt, which is why they sit at 23.4394° and 66.5606° rather than anywhere tidy, and why they drift a few metres every year. Zero longitude was voted on in Washington in 1884, and the date line was never agreed by anyone at all.

Each page below draws its line on a map you can drag, zoom and print, and lists every country the line crosses with the coordinates where it enters and leaves. Those tables were computed by intersecting the line with national borders rather than copied off a list, which is why they occasionally disagree with the encyclopedias in interesting ways.

All seven compared

Line up the numbers and the odd ones jump out. The Arctic Circle is less than half the length of the equator and crosses almost as many countries, because the far north is mostly land and the equator is mostly ocean. The Antarctic Circle crosses nothing at all.

Length, countries crossed and land fraction of each of the seven lines
LinePositionLengthCountriesOver land
The Equator0°40,075 km1021%
The Tropic of Cancer23.4394° N36,788 km1736%
The Tropic of Capricorn23.4394° S36,788 km1024%
The Arctic Circle66.5606° N15,986 km778%
The Antarctic Circle66.5606° S15,986 km08%
The Prime Meridian0° longitude20,004 km834%
The International Date Lineroughly 180°, but it bends20,004 km26%

The two meridians are measured pole to pole, 20,003.93 km, which is why they’re half the length of the equator. Country counts come from the border data and exclude Antarctica, which the prime meridian and the date line both run across.

How the country lists were worked out

Each line is intersected with the national borders in Natural Earth’s 1:110 million dataset. For a parallel the intersection returns pairs of longitudes where the line enters and leaves a country; for a meridian it returns pairs of latitudes. Distances are then measured on the WGS84 ellipsoid, so a crossing high in the Arctic isn’t inflated by the map projection the way it would be if you measured it on screen.

The whole job runs once, when the site is built, and the results ship inside the page. That means the tables are in the HTML before any JavaScript runs, which matters if you’re reading on a slow connection or printing.

1:110 million is coarse. Ten thousand points for the whole world, so coastlines are smoothed and small islands vanish entirely. Where that loses a crossing that really happens, the page says so by name instead of pretending otherwise: the Bahamas on the Tropic of Cancer, Grímsey on the Arctic Circle, Rolas on the equator.

FAQ
Why do these pages disagree with Wikipedia on country counts?
Usually because of islands, and occasionally because of politics. The border dataset drops anything smaller than a few hundred square kilometres, so island crossings go missing, and each page lists the ones it knows about separately. The political case is the Tropic of Cancer, where Natural Earth splits Western Sahara along the sand berm and most published lists don't.
Which line crosses the most countries?
The Tropic of Cancer. The computed table returns 17, and the usual published answer is also 17 with a different cast: it includes the Bahamas, which these borders are too coarse to see, and it files the Moroccan-controlled stretch of Western Sahara under Western Sahara rather than splitting it in two. The equator gets quoted as 13, though only 11 of those have land on the line.
Do the lines actually move?
Four of them do. The tropics and the polar circles are defined against the tilt of the Earth's axis, which is currently shrinking, so the tropics creep towards the equator and the polar circles creep towards the poles at roughly 15 metres a year. The equator, the prime meridian and the date line stay put, though the date line gets redrawn by politicians every so often instead.
Can I print these maps for a class?
That's a large part of why they exist. Every page prints from the browser with no account, no watermark and no sign-up, and the country table prints with it. The useful exercise is to print the map without the table, have students work out the countries, then show them the answer.
Does the site track what I look at?
The map tiles come from OpenFreeMap and that's the only request leaving your browser. Nothing about which line you looked at or where you dragged the map goes anywhere.

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