# Lituus Curve

The lituus is a transcendental curve that can be graphed with the polar equation
r = k/√θ [1]
or, equivalently,
r2 = a2/θ.

The inverse of the lituus is Fermat’s spiral — a type of Archimedean spiral (the littus is the inverse of Fermat’s Spiral only when the inversion is take as the origin). However, the lituus itself isn’t technically a spiral because its curvature doesn’t strictly increase or decrease as a function of its arc length.

The curve can also be described as the locus of the point P moving so that the area of a circular sector — a wedge of a circle with a central angle of less than π radians — remains constant.

The name Lituus means a ‘crook,’ like a bishop’s crosier (a staff without knots and curved at the top) [2].

## History of the Littus Curve

The curve was first described by Roger Cotes in a paper collection titled Harmonia Mensurarum, published in 1722 — six years after his death. He is well known for editing the second edition of Newton’s Principia [3].

Maclaurin also used the term lituus in his book Harmonia Mensurarum in 1722.

## References

Image created with Desmos.
[1] Dunham, D. Hyperbolic Spirals and Spiral Patterns. Retrieved July 30, 2022 from: https://www.d.umn.edu/~ddunham/dunbrid03.pdf
[2] Kokosa, S. Fifty Famous Curves, Lots of Calculus Questions, And a Few Answers. Retrieved July 30, 2021 from: http://facstaff.bloomu.edu/skokoska/curves.pdf
[3] MacTutor. Roger Coates. Retrieved July 29, 2022 from: https://mathshistory.st-andrews.ac.uk/Biographies/Cotes/