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parallaxicality
2008-Apr-27, 05:36 PM
This was an interesting question posed to me, one of those questions that seems blindingly obvious until you think about it. But when were Jupiter, Saturn, Uranus and Neptune known to be gas giants? I assume it had to do with measuring their masses and determining their most likely compositions. But when was that? After all, the existence of hydrogen and helium weren't known until almost 200 years after Newton.

Disinfo Agent
2008-Apr-27, 07:10 PM
That's the kind of interesting historical question that you're probably going to have a hard time finding a straight answer to. For a similar example, see this old thread (http://www.bautforum.com/questions-answers/34542-were-sun-moon-considered-planets.html).

tony873004
2008-Apr-27, 07:43 PM
It became possible to determine Jupiter's mass after the discovery of the Galilean moons, using their periods and distances. With Jupiter's size and mass, a density calculation is the next logical step. So my guess would be shortly after Galileo's discovery of its moons.

tdvance
2008-Apr-27, 08:04 PM
well, there's a minor complication--Newton proposed the law of gravity after Galileo discovered the moons. Newton was the first to estimate the masses of all the planets and the sun, though he didn't know the gravitational constant and had to estimate that.

tony873004
2008-Apr-27, 08:12 PM
I guess that would complicate things a bit :)
Ok, I change my guess to shortly after the determination of G by Cavendish in 1798: http://en.wikipedia.org/wiki/Cavendish_experiment . He used the results of his experiment to determine the density and mass of Earth. Jupiter, with observable moons would have been the next logical candidate.

Lord Jubjub
2008-Apr-27, 09:21 PM
The discovery of hydrogen and helium on Jupiter would have been the date. This was in the 1932 by Rupert Wildt (http://en.wikipedia.org/wiki/Rupert_Wildt). Jupiter was known to be massive and covered in perpetual clouds. But as late as the 1890s, it was difficult to say whether Jupiter consisted of huge masses of gas or if it had a solid surface below.

Ilya
2008-Apr-29, 08:54 PM
As late as 1957*, fairly "hard" SF writers had Jupiter with solid surface.

* "Call Me Joe" by Poul Anderson

Disinfo Agent
2008-Apr-29, 09:07 PM
Also Asimov, in the short story "Victory Unintentional".

Ilya
2008-Apr-30, 01:42 AM
Also Asimov, in the short story "Victory Unintentional".

That was back in 1940's.