The Earth is the third planet in the Solar System, formed about 4.6 billion years ago and is the only planet known to support life. But how do we measure the size of the Earth?
Our planet contains everything from rocks to minerals and millions of species of living organisms, covered by countless structures created by nature and humanity.
So, how much do all these things weigh? In fact, there is no specific answer to this question. Just as a person’s weight on the Moon is lighter than on Earth, our planet does not have a single weight. The weight of the Earth depends on the gravitational force acting upon it, meaning it could weigh trillions of kilograms or not weigh anything at all.
Currently, there are two main methods to calculate the mass of the Earth. The simplest way is to measure the weight of an object on the Earth’s surface and infer its mass using Isaac Newton’s famous formula from 1687 regarding the law of universal gravitation, which connects the distance and mass of two objects with the gravitational force they exert.
Isaac Newton Jr. (1643 – 1727) was a distinguished English physicist, astronomer, philosopher, mathematician, theologian, and alchemist, often regarded as one of the greatest scientists with an influence surpassing even Einstein.
Using Newton’s method, the mass of the Earth is approximately 6.102 x 10^24kg.
In his treatise *Philosophiæ Naturalis Principia Mathematica* (Mathematical Principles of Natural Philosophy) published in 1687, he described the law of universal gravitation and presented Newton’s three laws, which are considered the foundation of classical mechanics and dominated the concepts of physics and science for the next three centuries.
Using Newton’s method, it was shown that the mass of the Earth is approximately 6.102 x 10^24kg. However, this measurement is only an approximation because the Earth is not perfectly spherical, leading to an inconsistent radius. Therefore, this mass figure currently serves only as a reference for researchers.
The second method is more complex, based on Kepler’s third law from the 17th century. Johannes Kepler (1571 – 1630) was a German mathematician, astronomer, and astrologer. He is one of the key figures of the scientific revolution of the 17th century, best known for the laws of planetary motion that bear his name. This method connects the orbital parameters of a satellite (travel time and shape of the orbit) with the mass of the object it orbits.
To accomplish this, in 1976, NASA launched the Lageos-1 satellite into orbit, a large sphere made of a copper-zinc alloy covered with reflective concave surfaces, measuring 60 cm in diameter. The time delay between the transmission and reflection of laser beams was recorded to deduce the distance of the satellite from the Earth with near-absolute accuracy, establishing the value of the Earth’s mass as precisely 5.972 x 10^24 kg.
This figure is equivalent to 13 trillion Khafre Pyramids of Egypt, each weighing about 4.8 billion kg. The mass of the Earth fluctuates slightly due to the accumulation of cosmic dust and gases escaping from our atmosphere, but these small changes will not significantly affect the Earth for billions of years.
According to Kepler’s calculations, the Earth’s mass is exactly 5.972 x 10^24kg.
However, in science, there is always a need for absolutely accurate numbers, so new measurement methods are still being researched. Obtaining an exact figure for the Earth’s mass is crucial, as it helps us understand the Earth’s internal structure, the interactions between planets, and predict the orbits of satellites.
