Measuring the Circumference with Google Maps

"Maps are just projections - you can't check what they say about the size of the latitude circles."

Measure a known segment of a line of latitude with Google Maps (billions of trips rely on its accuracy), and extrapolate to the full circumference: circumference = 360° × segment length / segment angle. MCToon measured from (39° S, 73.3163° W) to (39° S, 62.312346° W): 950.32 km over 11.003954° of longitude.

On the globe the circumference of a latitude circle is the equator × cos(latitude): 40,003 × cos(39°) ≈ 31,090 km. The extrapolation from the measured segment gives ≈ 31,100 km - agreement within about 0.1%. The key is the direction: the 39° S circle is about 23% smaller than the equator. On the standard flat map (North Pole at the centre, Antarctica at the rim), latitude circles get bigger the further south you go - the opposite of what was measured.

MCToon's 'Measuring 39S' page: the segment measurement, the extrapolation (360° × 950.32 / 11.003954°), and the comparison with the equator (60 nautical miles × 360 = 21,600 nautical miles = 40,003.2 km).

You do not need a theodolite or a satellite: the map app on your phone contains the measurement. And it is not just a match - the latitude circle shrinking with latitude is a signature of the sphere that the flat map gets backwards.