Question

how do scientists measure the weight of a planet

Answer 1

Weight is measured by how much gravity acts on you, which is why you would weigh less on the Moon, which has less gravity than Earth, than you would at home. So scientists talk about mass rather than weight, as mass is the same no matter where you are.

To understand how we are able to calculate the mass of a planet, you have to first start with the principle called the Law of Universal Gravitation, published in 1687 by Sir Isaac Newton. Newton’s work tells us to look at how a planet affects the things around it. First, find a planet with a handy second object nearby like a moon. Second, measure the distance from the moon to the planet. Third, time one complete orbit. This gives you a moon’s speed, and the faster the moon is going the bigger the planet must be.

This only allows you to compare the relative masses of planets. To find out the actual masses of planets we had to wait for Lord Henry Cavendish’s experiment in 1797. He set up an experiment with two 150kg lead balls representing planets, and two smaller spheres, representing moons, and he measured the gravitational pull between them. Cavendish’s experiment led us to the missing piece of Newton’s puzzle, which was the value of G – the number that relates the gravitational force between two bodies to their masses and distance apart. By putting the value of G into Newton’s equation Cavendish calculated Earth’s mass to be six billion trillion tonnes, which is within 1% of our best guess today.

Answer 2

Some visitors to the wonderful planetarium at the Audubon Louisiana Nature Center wondered how we weigh planets. It's not quite as easy as reading the weight on a scale, but in some ways, it's the same basic idea. When you stand on a scale, the scale measures how hard our planet Earth's gravity pulls on you. But how hard the planet pulls depends not only on your weight but also on the planet's weight: The heavier the planet, the greater its gravitational pull. So scientists can weigh Earth by measuring how hard it pulls on something. The first person to weigh a planet was one of my heroes, a scientist named Henry Cavendish who figured out all the details needed to find how much Earth weighs. He and I both were born on October 10, but he's a little older than I am—about 225 years older. How much do you think Earth weighs? It turns out it's a little over 100 trillion trillion pounds, or about 200 billion trillion times as much as you weigh!


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