[Atmob-discuss] Question about the planets

[Terry Magrath]

Both the question posed and the answers provided show me why I am so proud
to be a member of this organization!




On 4/13/08 3:15 PM, "gjs at zohar.csail.mit.edu" <gjs at zohar.csail.mit.edu>
wrote:

> 
>      So here's a question that just occurred to me.  We all know the
>      story of how unexplained irregularities in the motion of Uranus
>      led to the discovery of Neptune.  So how come there weren't
>      irregularities in the motion of Saturn, caused by Uranus, but
>      unexplained before Uranus was discovered?
> 
> It is very difficult to say why something did not happen, but here are
> a number of factors that could have contributed to the reason.
> 
> Many discoveries are made by careful observation, long before any
> theoretical reasons for those results are considered.  For example,
> everyone was take by surprise in the late 1990's when the discovery
> was made that the expansion of the universe was accelerating, even
> though a nonzero cosmological constant could have predicted that.
> But before the observational discovery, everyone expected that the
> expansion would be slowed, rather than accelerated.
> 
> Similarly, until Uranus was discovered it was assumed that we had seen
> all of the planets.  However, once it was discovered we knew that
> there were undiscovered planets and so it was worth looking for them.
> 
> There are also more specific contributions:
> 
> Jupiter and Saturn are big enough so that the unaccounted for
> perturbations in Saturn's motion were probably not noticed before
> Uranus was discovered.
> 
> Newton's theory was published in 1687.  It is adequate to analyze the
> motions of the planets, but his vectoral formulation made it a painful
> computation.  
> 
> Uranus was discovered (officially) in 1781.  This was before Lagrange
> published his analytical mechanics (1788) based on the variational
> calculus of Euler.  The resulting perturbation analysis made it easier
> to compute the orbits to high enough precision to make good enough
> predictions without horrible calculations.
> 
> Indeed, by 1821, Bouvard, building on the work of Lagrange, published
> a perturbational analysis of the motions of Uranus that made it clear
> that there were discrepancies between the observations and the theory.
> 
> So Neptune was discovered by LeVerrier (and Adams) in 1846, based on
> these irregularities.
> 
> 
> I want to make it clear that I don't really know the answer to your
> question, but I hope that my speculations may be helpful.
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