The Earth’s circumference – the distance around the equator – is 40,075 kilometers around. That’s sounded nice and simple, but the question is actually more complicated than that. The circumference changes depending on where you measure it. The Earth’s meridional circumference is 40,008 km, and its average circumference is 40,041 km.
Why are there different numbers for the Earth’s circumference? It happens because the Earth is spinning. Think about what happens when you spin around holding a ball on a string. Your rotation creates a force that holds the ball out on the end of the string. And if the string broke, the ball would fly away. Even though the Earth is a solid ball of rock and metal, its rotation causes it to flatten out slightly, bulging at the equator.
That bulge isn’t very much, but when you subtract the meridional circumference (the equator when you pass through both poles), and the equatorial circumference, you see that it’s a difference of 67 km. In other words, if you drove your car around the equator of the Earth, you would drive an extra 67 km than you would if you drove from pole to pole to pole.
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And that’s why the average circumference of Earth is 40,041 km. Which answer is correct? It depends on how accurate you want to be with your calculation.
We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.