The circumference of the Earth
The earth has a circumference of approximately 24,900 miles.
More precisely the circumference of the                  earth
at the equator is 24,902 mi / 40,076 km.
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How fast would you have to travel to see                  the sun set twice,
or outrun the shadow of the setting sun?
Since the sun "travels" once around                  the earth every 24 hours,
we can get the speed by dividing the circumference of the earth                  by 24:
miles divided by hours