# Circular slide rules

Circular slide rule is somewhat a misnomer. Often nothing is "sliding" and they do not resemble a rule in any way. Historically they were invented in 1622 by William Oughtred -- about 220 years before linear.

Some circular slide rules seem to be a familar linear rule in circular formfactor. They have nonrotating part (stator), rotating part (slide), cursor. Stator and slide carry familiar C/D scales, sometimes A, B, K, S, T, L ... etc. Such slide rules have certain advantages and disadvantages comparing to their linear brethren.

Advantages include "closed" log scales, which eliminates the necessity to reset the slide, compact design and ease of making at home (you just need to print faces on your laser or jet printer, cut them out and join them with lapel pin in the middle).

The disadvantage is that scales close to center become really congested. Also it seems that it takes more time to perform basic operations.

## Note on emulators

Those circular slide rules that are interesting enough to justify spending effort to write emulator seem to be all very different and general approach that worked so well for normal, linear slide rules does not seem applicable. That is why I resorted to writing separate emulator for each individual circular slide rule I thought brings something new to calculation technique.

### Astrolabes

Astrolabes are specialized calculators based on same principle as circular slide rules: they have parts rotating relatively to each other and by placing them in certain postions user can solve different astronomical problems, the most important of which is finding time from sun (or other star) altitude.

While being somewhat similar mechanically, astrolabes have very different mathemtical foundations. They are based on properties of stereographic projection (other projections were also used) rather than logarithmic scale.

Astrolabes have history that spans several centuries (possibly, millenias) before slide rules. There is little doubt that William Oughtred based his slide rule design on astrolabe.

## Other sites

Here are some pointers to other circular slide rule emulators.
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