Earth mass

The mass of our planet has been worked out. It is near to 5.9722×10²⁴ kg. This was published by the International Astronomical Union in 2009.

For short, people write it as $M_{\mathrm {E} }$ or $M_{\oplus }$. This symbol is a unit of mass for planets, moons, and other large things in space.

Newton's law of universal gravitation was the first step to measuring the mass of Earth. Scientists had the idea to hang a pendulum near a mountain. The Earth's gravity would pull the pendulum down, but the mountain's gravity would pull it towards the mountain. They could use the angle of the pendulum to measure the pull of the mountain, compared with the pull of Earth. After they estimated the mass of the mountain, they could estimate the mass of Earth.

A group of British scientists did this experiment in 1774. They went to Schiehallion, a mountain in Scotland. They estimated that the density of Earth was about ${\tfrac {9}{5}}$ that of Schiehallion mountain. Their estimate for mass was about 20% less than the "correct" Earth mass as understood today.

Henry Cavendish did another experiment, which he completed in 1798. He built an apparatus that could measure the force of gravity between lead balls. This also found the ratio between two forces of gravity. He could then find the density of Earth, and then the mass. The results were only about 1% different from the "correct" Earth mass as estimated at present.