To know how much the Earth weighs, you have to take gravity into account and read this note to the end.

And how is gravity measured? The intensity of the force that keeps the Moon orbiting around the planet and us with our feet on the ground is determined by what physics calls «G», the universal gravitational constant.

«Newton’s law of gravity described the motion of the planets and moons with amazing precision, and what we call Newton’s constant of gravitation is what determines the scale, the magnitude of gravity,» Terry Quinn explained to the BBC, emeritus director of the International Bureau of Weights and Measures.

«Small-scale gravity is very weak, large-scale gravity is very, very large,» said the expert. Therefore, calculating a constant value was a challenge.

«The first person to measure it was a British scientist, Henry Cavendish, in the late 18th century.»

According to the scientists, Cavendish’s calculation had a margin of error of 1%.

In the ensuing 200 years, Quinn said, this measurement has become more and more precise.

«At the end of the 20th century we thought we were doing quite well,» the scientist told the BBC, because the margin of error for the new estimates of the value of G was 0.001%.

The German 1%

But then the official German physics institution came up with a result that was 1% different, and that upset the scientists, who scrutinized the calculations and found no errors.

«So a lot of people around the world started making measurements,» the expert explained, including the international body for weights and measures.

«These measurements take time. After 4 or 5 years the results started coming in and we didn’t confirm the German results, but we didn’t agree with each other either,» Quinn said.

For this reason, new measurements and calculations are underway and experts in the field dedicate an entire issue of the scientific journal Philosophical Transactions of the Royal Society to Newton’s constant of gravitation.

«It’s one of the fundamental constants in physics, and if we can’t measure it accurately, that indicates we’re not taking the measurements correctly, and if that’s wrong, other things may be wrong,» Quinn said.

However, despite doubts about the value of G, an estimated weight of the Earth is accepted by the International Bureau of Weights and Measures.

If you’re still interested in knowing, it’s around 7,000,000,000,000,000,000,000,000 kilograms.