The mass of an object tells us the amount of ‘stuff’ or matter it is made of. An object’s mass remains the same wherever the object is. Mass is measured in kilograms (kg). Or grams for small objects, and tonnes for larger objects.

Weight is different from mass. Weight tells us how heavy something feels. It is the force that acts on mass due to gravity. An object’s weight will change depending on the gravity of where it is. Weight is measured in Newtons (N).

Different places in our Solar System have different gravitational field strengths. On Earth it is 9.8 N/kg. On the Moon it is 1.6 N/kg. This means that an object with a mass of 45 kg will weigh 441 N on Earth but only 72 N on the Moon.

The weight of an object is its mass multiplied by the gravitational field strength of where it is.

It is often more useful so say how much of something there is, without worrying about where it is at the time. That is why astronomers use mass. They describe objects as more massive or less massive rather than ‘heavy’ or ‘light’.