When we wish to know the mass of an object, we weigh it in a balance. The size of the balance varies with the size of the object to be weighed. The Earth is a giant body and to think of a balance for weighing it, is almost impossible. Then, what is the way of measuring the mass of the Earth? Scientists use Newton’s law of gravitation to determine the Earth’s mass. According to this law, there exists a force of attraction between any two bodies in the Universe and this is dependent on the masses of the two bodies as well as on the distance between them. The force of attraction is directly proportional to the product of the masses of the two bodies, and inversely proportional to the square of the distance between them. An experiment is performed to determine the Earth’s mass with the help of the above-mentioned law. In this experiment, a small metallic ball is suspended with the help of a thin thread. The position of this ball is accurately determined. Now a big lead ball weighing a ton is brought near this ball. The small ball is attracted towards the big ball due to the gravitational pull and as such it is slightly displaced from its initial position with respect to the big ball. The change in the position of the small ball is even less than one-tenth of an inch. This displacement is very accurately measured with the help of precision instruments. Using the value of displacement in a formula of physics, the mass of the Earth is calculated. The mass of the Earth has been found to be 598×1019 (5980,000,000,000,000,000,000) tonnes.