A repeat episode from April 2006.

Right around the time when Shakespeare was alive there was another guy in Germany by the name of Johannes Kepler.

He was quite the guy.

Wikipedia tells me that he wrote science fiction.  He must have put his imagination to good use in the realm of science fact as well because if you recognize his name at all, it is because he came up with mathematical formulae that finally explained to all those guys who had been trying to figure out how the stars and planets moved around up there in the sky, what was going on.

Even more remarkable was the fact that he was blind as a bat and couldn’t see them himself.

He comes into my little story here because it was he, in explaining not how planetary motion worked but how light bent through lenses, it was Kepler who coined the term focus.

If you look at wikipedia at Kepler’s laws of planetary motion, you’ll see that the authors use the word focus to describe the points around which celestial bodies orbit. So today the word focus has a geometric meaning.

Figuratively we all use the word.

I have to focus on the job at hand.  If you have glasses you know that optically focus has to do with bringing the light to a focal point in the back of your eye. It was something along these lines that Kepler was thinking when he borrowed this word from Latin.

If you take a magnifying glass outside into the sunshine you can focus the sun’s rays on a tiny little point and actually start a fire.

In Latin, focus means hearth or fireplace.

Of course Johannes Kepler was writing in German, actually no, he’d have been writing in Latin.  So it was 14 years after his death that the word appeared in English.  In this case the mathematical sense was retained, but instead of appearing in a document about planets or optics, the word appeared in someone’s diary referring to acoustics and a particular place where sound seemed naturally amplified.