Antipode finder

Worked example: Honolulu

Honolulu's stored capital point is 21.307° N, 157.858° W. Negate the latitude and add 180° to the longitude, wrapping into the −180 to 180 range: 21.307° S, 22.142° E.

Testing that point against the Natural Earth 1:50m country outlines puts it inside Botswana. The nearest city in the Natural Earth list is Grootfontein, Namibia, 287 miles from the antipode.

How the antipode is computed

On a sphere the opposite point has the negated latitude and a longitude 180 degrees away. A place at 40° N, 105° W has its antipode at 40° S, 75° E. The rule is exact on a sphere and differs from the ellipsoidal antipode by far less than the resolution of the country outlines, so it does not change which country is named.

The surface distance between a point and its antipode is half the circumference of the sphere, about 12,437 miles on the model used here. Straight through the center it is the diameter.

Land or ocean

Most of the Earth is ocean, and the antipodes of most of the contiguous United States fall in the southern Indian Ocean. The tool checks the antipode against Natural Earth 1:50m country polygons in your browser and reports the country or open ocean. Small islands below that scale can be missed, and coastlines are generalized, so a point within a few miles of a coast may be reported on the wrong side.

The nearest listed city comes from the Natural Earth populated places file of 1,172 cities, so it is the nearest well-known city rather than the nearest settlement.

Reading the two globes

The left globe is centered on your place and the right globe on its antipode. Each is an orthographic view, the way the Earth looks from far away, so each shows exactly one hemisphere. The two views together cover the whole planet, with your place and its opposite each at the center of its own disk. Land near the rim of a disk is foreshortened because it curves away from the viewer, so judge shapes near the center of each globe. Both globes redraw as soon as you pick a new starting point.

Half the world away, measured

On the sphere every antipode is exactly 12,437 miles away over the surface. On the WGS84 ellipsoid the figure depends on direction. Half the equator is 12,451 miles, while pole to pole along a meridian is 12,430 miles, because the Earth is flattened at the poles.

Straight through the Earth, the distance is the diameter. The model sphere gives 7,918 miles. The ellipsoid's equatorial diameter is 7,926 miles and its polar diameter 7,900 miles.