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> Here at latitude 51.5° and given an Earth circumference of roughly 40,000 km — one degree of longitude is 69 km

The math is wrong here. If the earth circumference is 40,000 km, then divide by 360 to get one degree equals 110 km. I notice dividing by 1.6 gives 69 miles, so perhaps there's a units error here.

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One degree is 40,000 km / 360 = 111 km at the equator. But Greenwich is at 51 degrees latitude so you apply a cos (51.5°) = 0.622 factor. That's why 1 degree of longitude at latitude 51 is 69 km. Does that help clear it up?

Note also if you're walking in a circle 57 meters from the monument at the South pole, then each 1 m step you take crosses one degree of longitude. Google for: south pole ceremonial sphere

Thanks, I didn't think about it too hard.

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