[Atmob-discuss] Question about the planets

[gjs at zohar.csail.mit.edu]

     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.