Great circles are straight lines that go all the way around the center of the earth. The equator is a great circle. Meridians of longitude that cross over the north and south poles are also great circles. For every location on a great circle, it’s antipodal location is also on the circle. Other than the equator itself, any great circle crosses the equator at two antipodal locations, 180° apart. Other than the equator and meridians of longitude that run due north and south, any great circle reaches it’s maximum latitudes at two locations that are 90° of longitude east and west of the two locations where the great circle crosses the equator.

All great circles have two antipodal axis points. The two axis points for the equator are the north and south poles. Every point along the equator is equally distant at one quarter of the circumference of the earth from the north and south poles. Every point along any great circle is equally distant at one quarter of the circumference of the earth from it’s two axis points. For any great circle other than the equator, the longitude of the axis points are 90° east and west of the two points where the great circle crosses the equator, and the two axis points are 90° of latitude away from the two points where the great circle reaches it’s maximum latitudes.

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