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

I don't remember the answer to the gravitational constant question, but there were pretty good estimates of the distance to the sun from simultaneous measurements of the angle of the sun from two locations a known distance apart. Al van Helden's book Measuring the Universe has an excellent account of this and related issues.

Richard Grandy
Rice University (former locus of van Helden)


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 Cavendish.

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