Calculator Accuracy & Usefulness

[Jim Horn]

While it is often said that slide rules got men to the moon, such calculation often required far more precision than any slide rule could provide. But what they did provide was a fast way to get an answer to ~3 or 4 digits of accuracy when the alternatives were far less available, slower, etc. So a 'slip-stick' could quickly give you an answer which, if it was one that was promising (i.e. it showed you *could* reach orbit / the moon / some other very difficult problem), the alternatives would then be used to determine the more exact (and needed) answer.

Many of us from those prehistoric days had our CRC Handbook of Mathematical Tables or similar listings of logarithms, trig functions, etc., often including logs of trig functions and other aids to computation. The good ones included interpolation constants for each entry so any value could have its function evaluated to high precision with nothing else but pencil and paper. Of course, some had access to computers running custom code to solve the hardest problem (high order Runge-Kutta numerical solutions to find lunar trajectories, for example). But before doing the gruesome work, slide rules were the standard way to get you in the right direction as well as checking the final answer of more exact calculation to ensure it wasn't way off due to some programming or input data error.

Around 1978 or so I took a US Air Force course on satellite orbital perturbation analysis. Needless to say, the final exam involved some really messy equations for which I was as prepared as any with my HP-67 programmable calculator. But for laughs, when sitting for the exam I pulled out my pocket Post / Hemmi 1461 Versalog II slide rule and said I was ready for the exam - which got quite a few chuckles. Of course, the '67 was used and blazed through the problems.

Right up until its batteries died (typical charge was about 2 hours of use) as I had studied so hard the night before I had forgotten to plug it in to charge. So the Post came out again and I was able to complete the test with it.

Mercifully, another student finished early and loaned me his calculator which I used to redo all the answers. None of the slide rule answers were wrong though the new answers carried more digits. But the second time (with the calculator) was far easier...