Definition:Ovals of Cassini/Focus
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Definition
Consider the cvals of Cassini defined as the locus $M$ described by the equation:
- $P_1 M \times P_2 M = b^2$
for constant $b$
Each of the two points $P_1$ and $P_2$ can be referred to as a focus of the ovals.
Linguistic Note
The word focus is of Latin origin, hence its irregular plural form foci.
It was introduced into geometry by Johannes Kepler when he established his First Law of Planetary Motion.
The word in Latin means fireplace or hearth, which is appropriate, considering the position of the sun.
The pronunciation of foci has a hard c, and is rendered approximately as foke-eye.
Beware the solecism of pronouncing it fo-sigh, which is incorrect.
Sources
- Weisstein, Eric W. "Cassini Ovals." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/CassiniOvals.html