The Earth is a sphere with a radius of 6,371 km, and by multiplying this value by 2, it has a diameter of 12,742 km. However, these values consider the planet a perfect sphere, ignoring the differences in topography.

The fact that the globe is slightly flattened at the poles – similar to an orange. This flatness is almost imperceptible: there is only a 40 km difference in the diameters measured in the direction of the equator and in the north-south direction.

Mankind has known these measurements for more than two thousand years, thanks to the calculations made by the mathematician Eratosthenes, who was born in Cyrene, present-day Libya. Current satellites show that the researcher was wrong with the circumference value — whose formula is twice the radius — by only 500 kilometers, which is impressive given the time constraints.

To get an idea of the planet’s size, we can compare the distance to the Earth’s center with the distances on the surface. The terrestrial radius of 6,371 km is equivalent to approximately 15 trips from São Paulo to Rio de Janeiro. In addition, the measurement is more than twice the distance between the closest points in Brazil and Africa: it is 2839 km from the continent on the other side of the Atlantic Ocean.

## What about the radii of the other planets?

Earth is already massive with a radius of just over 6,000 kilometers, but other measurements in the solar system are even larger than that. Uranus (25,362 km) has a radius four times larger, while Saturn is nine times the diameter of Earth (58,232 km). Jupiter, the largest planet in our system, has a radius of about 70,000 km – 11 times the diameter of Earth.

However, our planet is ahead of Venus, Mars and Mercury when it comes to size. Venus has a radius of only 300 km (6,052 km), but Mars is almost the same as ours (3,390 km), while Mercury is about a third of its radius (2,440 km). The dwarf planet Pluto has a radius of 1/5 that of Earth.

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