How long is a trip once around the Earth along the equator, in kilometres?
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Answer
About 40,000 km
This round number is no accident: in the 1790s the metre was defined as one ten-millionth of the distance from the North Pole to the equator.
How to estimate it
Use time zones. London and New York are 5 hours apart and about 5,600 km from each other, so one hour of time difference is worth 1,120 km at that latitude. A full turn is 24 hours: 1,120 × 24 = 27,000 km. Circles at 45 degrees north are only 0.7 times the equator, so 27,000 / 0.7 = 38,000 km. The real figure is about 40,000 km.
Source: NASA; IUGG
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