the wight of the moon is 81 billion tons but how do we know this...?

Question

wight = weight

Answer 1

we dont

Answer 2

Well, the weight of the moon is 10e24 Kg. That is 13.5 billion *times* more than the 81 billion tons you stated. Perhaps you got the 81 from the fact that the earth is 81 times the mass of the moon. Could that be it?
It is known by using the Universal Law of gravitation as it applies to orbits. Knowing the distance from the earth, the speed of its orbit and the mass of the earth, its mass can be calculated to great degree of accuracy. We know this information is highly accurate because space flight trajectories to the moon (and all the other places we've sent probes), would have been seriously in error if we did not.

(Yeah, "mass," okay. Sheesh. If a person says weight and then mass, that should cover it.)

Answer 3

Somebody wrote "We just give an estimate based on observations." Isn't that all we ever have? When you step on a scale to get your weight, you observe the display on the scale. That is an observation, just like the Moon's orbit is an observation. From this observation, it can be calculated that the Moon's mass (not weight) is 7.349 x 10^22 kg, or 7.349 x 10^19 tons. That's a LOT more than 81 x 10^9 tons in your question.

Answer 4

The mass of the moon( "weight" is meaningless), is much more than 81 billion tons.

It is known very well, and can be calculated by knowing its effect on other bodies. In fact, knowing how long it takes a satellite to orbit the moon at a given distance is enough to get a very good answer, from a simple formula.

Answer 5

Mass (not weight) directly contributes the gravitational field of an object. For the moon, the Earth exerts a gravitational force on the moon to keep it in orbit. And the moon exerts a gravitational force on the Earth (much smaller of course).
Its higher math, but basically we know the speed of the moon's orbit, the Earth's mass, and the distance between the 2 so we can calculate the moon's mass.
The moon's calculated mass is 7.3 x 10^22 kg, or about .0123 Earth masses.

Answer 6

We don’t really know it. We just give an estimate based on observations. We analize the light spectrum of a planet (or the Moon on this case), and we “guess” what is made of and how much mass is in there.

Then we just convert those numbers to weight as if it was right here on Earth at 1 gravity.

That’s it.

Answer 7

The astronauts weighed it when they were there.

Just kidding. It's calculated from its effect on other bodies, most likely using orbital parameters and the law of universal gravitation.

Answer 8

Estimate the density of the moon and the circumference.

d=m/v

m=dv

Answer 9

We know it's diameter and the strength of it's gravity.