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Re: [Phys-l] How did Newton estimate the Gravitational constant?

Newton didn't. Cavendish did; in one of the most famous experiments of all
time. Newton knew of the proportions, but not the value of 'G'. Cavendish
performed a very delicate and intricate experiment to actually measure the
attraction between large masses in his lab. He's the one who found 'G'.

-Daryl Taylor

On Thu, Oct 28, 2010 at 6:00 AM, Brian Blais <> wrote:


A student asked me this question in class yesterday, and I wasn't sure
(haven't looked at the Principia in a long time, but always found the
arguments a bit hard to follow). I imagine he could do it from a rough
estimate of the mass of the Earth, mass of the Moon, and distance to the
Moon. With the Moon's period you could get a value for G. Is this how he
did it? I know that the direct measurement wasn't done until later, by

Further, did he have any way of estimating the distance to the Sun? I
couldn't think of one that was available at his time, but he was more clever
than I. :)



Brian Blais <>

Forum for Physics Educators

Daryl L Taylor
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