Unexpected approximations which have led to important mathematical discoveries

On a regular basis, one sees at MSE approximate numerology questions like

• Prove $\log_{{1}/{4}} \frac{8}{7}> \log_{{1}/{5}} \frac{5}{4}$,
• Prove $\left(\dfrac{2}{5}\right)^{{2}/{5}}<\ln{2}$,
• Comparing $2013!$ and $1007^{2013}$

or yet the classical $\pi^e$ vs $e^{\pi}$. In general I don't like this kind of problems since a determined person with calculator can always find two numbers accidentally close to each other - and then ask others to compare them without calculator. An illustration I've quickly found myself (presumably it is as difficult as stupid): show that $\sin 2013$ is between $\displaystyle \frac{e}{4}$ and $\ln 2$.

However, sometimes there are deep reasons for "almost coincidence". One famous example is the explanation of the fact that $e^{\pi\sqrt{163}}$ is an almost integer number (with more than $10$-digit accuracy) using the theory of elliptic curves with complex multiplication.

The question I want to ask is: which unexpected good approximations have led to important mathematical developments in the past?

To give an idea of what I have in mind, let me mention Monstrous Moonshine where the observation that $196\,884\approx 196\,883$ has revealed deep connections between modular functions, sporadic finite simple groups and vertex operator algebras.

Many thanks in advance for sharing your insights.

Solution 1:

The most famous, most misguided, and most useful case of approximation fanaticism comes from Kepler's attempt to match the orbits of the planets to a nested arrangement of platonic solids. Fortunately, he decided to go with his data instead of his desires and abandoned the approximations in favor of Kepler's Laws.

Kepler's Mysterium Cosmographicum has unexpected close approximations, and they led to a major result in science.

Solution 2:

Not sure if this is unexpected or so but I think the fast inverse square root is kinda cool. Don't think it lead to any mathematical developments though it's been implemented more widely since.