Keep going due east from Seoul and you arrive at Seoul
Seoul sits at 37.5665° N. Pick out the cities on the same latitude from our 524 and there are ten foreign ones within 0.6 degrees — about 67km.
| City | Country | Latitude | From Seoul |
|---|---|---|---|
| Mykonos | Greece | 37.45 | 8,440km |
| Seville | Spain | 37.39 | 10,384km |
| San Francisco | USA | 37.77 | 9,029km |
| Córdoba | Spain | 37.89 | 10,276km |
| Pamukkale | Türkiye | 37.92 | 8,139km |
| Ashgabat | Turkmenistan | 37.95 | 5,883km |
| Granada | Spain | 37.18 | 10,278km |
| Athens | Greece | 37.98 | 8,516km |
| Faro | Portugal | 37.02 | 10,516km |
| Palermo | Italy | 38.12 | 9,199km |
The Mediterranean, the Iberian peninsula, and California. Same latitude means the sun climbs to the same height, which makes it a plausible-looking list — but the interesting question is a different one. These cities are at the same height as Seoul, so can you reach them by going sideways?
The parallel is 8,300km shorter than the equator
The circle that runs around the Earth along Seoul's parallel — 37.5665° N — is 31,730km long. That is 8,300km shorter than the equator's 40,030km. One degree of longitude is 111.2km at the equator and 88.1km on Seoul's parallel. Obvious enough, since the circle shrinks as you go up, but it is hard to see on a map.
Follow that circle due east and you pass through this sequence.
South Korea → Japan → Pacific Ocean, 8,504km → United States, 4,077km → Atlantic Ocean, 5,898km → Portugal → Spain → Italy → Greece → Türkiye → Iran → Turkmenistan → Uzbekistan / Tajikistan / Afghanistan (the borders interlock, so it alternates) → China → South Korea
Fourteen countries where it touches land. Of the 31,730km, 42.4% is land and the remaining 18,280km is water. The longest stretch of sea is 8,504km from the east coast of Japan to California; the longest stretch of land is 5,736km from Turkmenistan to the east coast of China. Fourteen countries and two oceans is all it takes to circle the planet.
Except this service does not draw that line
Connect Seoul and San Francisco in CouchColumbus and the line on screen does not run sideways. It bulges north, grazes the Aleutians and comes back down. Because that is shorter than following the parallel.
| Destination | Great circle | Along the parallel | Difference | Peak latitude of the great circle |
|---|---|---|---|---|
| Ashgabat | 5,883km | 6,046km | +2.8% | 43.2° |
| Pamukkale | 8,139km | 8,625km | +6.0% | 49.7° |
| Athens | 8,516km | 9,100km | +6.9% | 51.3° |
| San Francisco | 9,029km | 9,748km | +8.0% | 53.6° |
| Seville | 10,384km | 11,719km | +12.9% | 62.5° |
| Faro | 10,516km | 11,891km | +13.1% | 63.3° |
San Francisco saves 719km, Faro 1,375km. The price of that saving is a route that climbs north. The Faro route reaches 63° N — the same latitude as the sea south of Iceland — before coming back down. Origin and destination are both at 37° N, and yet 65% of the route runs above 50° N. Even the San Francisco route spends 70% of its length above 45° N.
Why going sideways costs you
The shortest path between two points on Earth is a great circle: the circle where a plane through the centre of the planet meets the surface. The equator and every meridian are great circles, but a parallel other than the equator is not. The 37° N line is a circle cut by a plane that passes above the centre of the Earth rather than through it, so following it means going the long way around.
The bigger the difference in longitude, the worse the penalty. Ashgabat is 69 degrees away, hence 2.8%; Faro is 135 degrees away, hence 13.1%. As the gap approaches 180 degrees the great circle goes over the pole while the parallel has to run all the way around to the other side, and the difference widens sharply.
What we are not certain about
- Distances are calculated treating the Earth as a sphere of radius 6,371km. The real Earth is flattened at the poles, so its equatorial circumference is closer to 40,075km. This service does all of its maths on the spherical model, and we kept the same basis here.
- The countries the parallel crosses, and the land percentage, were counted by sampling the low-resolution shapes used for the map (Natural Earth 1:110m) at 0.02-degree intervals. The resolution is coarse, so small islands and narrow straits drop out.
- City coordinates are a single point downtown. A large city wobbles by about 0.1 degrees of latitude.
Being on the same latitude means the sun rises to the same height. It does not mean you can get there by going sideways. Spread a map out and it keeps reading as the second thing — which is why the line on screen keeps bending north.