[acqq]

> as soon as we figured out the moon was just a rock 240,000 miles away, we knew in principle we could go there, even if we didn't know how far off that would be

I have problems agreeing with that specific claim, knowing that both "the rock" and the distance were known to some ancient Greeks around 2200 years ago.

https://en.wikipedia.org/wiki/Hipparchus

Hipparchus estimated the distance to the Moon in the Earth radii to between 62 and 80 (depending on the method he used, as he intentionally used two different). Today's measurements are between 55 and 64.

[laughinghan]

Holy shit, that is so impressive. They didn't even have Newton's law of gravity yet.

Once we had Newton's law of gravity though, we knew the distance, radius, mass, and even surface gravity of the moon. Would you say it's fair to say that by then we knew in principle we could go there and walk there?

(P.S. I assume you know this but the way you wrote your comment makes it seem like our measurements of lunar distance are nearly as inaccurate as Hipparchus's, when we actually know it down to the millimeter (thanks to retroreflectors placed by Apollo, actually). The wide variation from 55x to 64x Earth's radius is because it changes over the course of the moon's orbit, due to [edit: primarily its elliptical orbit, and only secondarily] the Sun and Jupiter's gravity.)

[acqq]

> The wide variation from 55x to 64x Earth's radius is because it changes over the course of the moon's orbit, due to the Sun and Jupiter's gravity

I think you’re not only wrong but even Kepler and Newton already knew that better than you:

https://en.m.wikipedia.org/wiki/Elliptic_orbit

“Strictly speaking, both bodies revolve around the same focus of the ellipse, the one closer to the more massive body, but when one body is significantly more massive, such as the sun in relation to the earth, the focus may be contained within the larger massing body, and thus the smaller is said to revolve around it.”

But maybe you have some better information?

[laughinghan]

No you're right, the Sun and Jupiter are a secondary effect to the elliptical orbit, I skimmed the Wikipedia page too quickly:

> due to its elliptical orbit with varying eccentricity, the instantaneous distance varies with monthly periodicity. Furthermore, the distance is perturbed by the gravitational effects of various astronomical bodies – most significantly the Sun and less so Jupiter

https://en.wikipedia.org/wiki/Lunar_distance_(astronomy)#Per...

[acqq]

Thanks! Now back to your other question:

> Once we had Newton's law of gravity though, we knew the distance, radius, mass, and even surface gravity of the moon.

I think it was more complicated than what you assume there. Newton published his Principia 1687 but before 1798 we didn't know the gravitational constant:

https://en.wikipedia.org/wiki/Cavendish_experiment

However...

> Would you say it's fair to say that by then we knew in principle we could go there and walk there?

If you mean "we 'could' go if we had something what we were sure we haven't had" then there is indeed a written "fiction" story published even before Newton published his Principia:

https://en.wikipedia.org/wiki/Comical_History_of_the_States_...

It's the discovery of the telescope that allowed people to understand that there are another "worlds" and that one would be able to "walk" there.

Newton's impact was to demonstrate that there is no any "mover" (which many before identified as a deity) that provides the motion of the planets but that their motions simply follow from their properties and the "laws." Before, most expected Aristotle to be relevant:

https://en.wikipedia.org/wiki/Unmoved_mover

"In Metaphysics 12.8, Aristotle opts for both the uniqueness and the plurality of the unmoved celestial movers. Each celestial sphere possesses the unmoved mover of its own—presumably as the object of its striving, see Metaphysics 12.6—whereas the mover of the outermost celestial sphere, which carries with its diurnal rotation the fixed stars, being the first of the series of unmoved movers also guarantees the unity and uniqueness of the universe."