Logarithmic spiral

In polar coordinates ${\displaystyle (r,\varphi )}$ the logarithmic spiral can be written as[1]

${\displaystyle r=ae^{k\varphi },\quad \varphi \in \mathbb {R} ,}$

or

${\displaystyle \varphi ={\frac {1}{k}}\ln {\frac {r}{a}},}$

with ${\displaystyle e}$ being the base of natural logarithms, and ${\displaystyle a>0,k\neq 0}$ being real constants.

The logarithmic spiral with the polar equation

${\displaystyle \;r=ae^{k\varphi }}$

can be represented in Cartesian coordinates ${\displaystyle (x=r\cos \varphi ,\,y=r\sin \varphi )}$ by

• ${\displaystyle x=ae^{k\varphi }\cos \varphi ,\qquad y=ae^{k\varphi }\sin \varphi .}$

In the complex plane ${\displaystyle (z=x+iy,\,e^{i\varphi }=\cos \varphi +i\sin \varphi )}$:

• ${\displaystyle z=ae^{(k+i)\varphi }.}$

Spira mirabilis, Latin for "miraculous spiral", is another name for the logarithmic spiral. Although this curve had already been named by other mathematicians, the specific name ("miraculous" or "marvelous" spiral) was given to this curve by Jacob Bernoulli, because he was fascinated by one of its unique mathematical properties: the size of the spiral increases but its shape is unaltered with each successive curve, a property known as self-similarity. Possibly as a result of this unique property, the spira mirabilis has evolved in nature, appearing in certain growing forms such as nautilus shells and sunflower heads. Jacob Bernoulli wanted such a spiral engraved on his headstone along with the phrase "Eadem mutata resurgo" ("Although changed, I shall arise the same."), but, by error, an Archimedean spiral was placed there instead.[2][3]

Definition of slope angle and sector

The logarithmic spiral ${\displaystyle \;r=ae^{k\varphi }\;,\;k\neq 0,\;}$ has the following properties (see Spiral):

• Polar slope: ${\displaystyle \ \tan \alpha =k\quad ({\color {red}{\text{constant$ !}}})}

with polar slope angle ${\displaystyle \alpha }$ (see diagram).

(In case of ${\displaystyle k=0}$ angle ${\displaystyle \alpha }$ would be 0 and the curve a circle with radius ${\displaystyle a}$.)

• Curvature: ${\displaystyle \;\kappa ={\frac {1}{r{\sqrt {1+k^{2}}}}}={\frac {\cos \alpha }{r}}}$

• Arc length:${\displaystyle \ L(\varphi _{1},\varphi _{2})={\frac {\sqrt {k^{2}+1}}{k}}{\big (}r(\varphi _{2})-r(\varphi _{1}){\big )}={\frac {r(\varphi _{2})-r(\varphi _{1})}{\sin \alpha }}}$
  

Especially: ${\displaystyle \ L(-\infty ,\varphi _{2})={\frac {r(\varphi _{2})}{\sin \alpha }}\quad ({\color {red}{\text{finite$ !}}})\;} , if ${\displaystyle k>0}$ .

This property was first realized by Evangelista Torricelli even before calculus had been invented.[4]

• Sector area: ${\displaystyle \ A={\frac {r(\varphi _{2})^{2}-r(\varphi _{1})^{2})}{4k}}}$
• Inversion: Circle inversion (${\displaystyle r\to 1/r}$) maps the logarithmic spiral ${\displaystyle \;r=ae^{k\varphi }\;}$ onto the logarithmic spiral ${\displaystyle \;r={\tfrac {1}{a}}e^{-k\varphi }\ .}$

Examples for ${\displaystyle a=1,2,3,4,5}$

• Rotating, scaling: Rotating the spiral by angle ${\displaystyle \varphi _{0}}$ yields the spiral ${\displaystyle r=ae^{-k\varphi _{0}}e^{k\varphi }}$, which is the original spiral uniformly scaled (at the origin) by ${\displaystyle e^{-k\varphi _{0}}}$.

Scaling by ${\displaystyle \;e^{kn2\pi }\;,n=\pm 1,\pm 2,...,\;}$ gives the same curve.

• Self-similarity: A result of the previous property:

A scaled logarithmic spiral is congruent (by rotation) to the original curve.

Example: The diagram schows spirals with slope angle ${\displaystyle \alpha =20^{\circ }}$ and ${\displaystyle a=1,2,3,4,5}$. Hence they are all scaled copies of the red one. But they can also be generated by rotating the red one by angles ${\displaystyle -109^{\circ },-173^{\circ },-218^{\circ },-253^{\circ }}$ resp.. All spirals have no points in common (see property on complex exponential function).

Attention: Comparing spirals with the same inclination (same value as (k)) that start at different distances (a) from the origin almost certainly turn out differently, this is due to the fact that the equation imposes the same orientation for the starting point of the spiral; it is easier to note the different orientation (mistakenly mistaking it for rotation) than the different distance (a) from the origin of the starting point.

Relation to other curves: Logarithmic spirals are congruent to their own involutes, evolutes, and the pedal curves based on their centers.

• Complex exponential function: The exponential function exactly maps all lines not parallel with the real or imaginary axis in the complex plane, to all logarithmic spirals in the complex plane with centre at ${\displaystyle 0}$:

${\displaystyle z(t)=\underbrace {(kt+b)\;+it} _{\text{line}}\quad \to \quad e^{z(t)}=e^{kt+b}\cdot e^{it}=\underbrace {e^{b}e^{kt}(\cos t+i\sin t)} _{\text{log. spiral}}\ }$

The polar slope angle ${\displaystyle \alpha }$ of the logarithmic spiral is the angle between the line and the imaginary axis.

The golden spiral is a logarithmic spiral that grows outward by a factor of the golden ratio for every 90 degrees of rotation (polar slope angle about 17.03239 degrees). It can be approximated by a "Fibonacci spiral", made of a sequence of quarter circles with radii proportional to Fibonacci numbers.

Cutaway of a nautilus shell showing the chambers arranged in an approximately logarithmic spiral. The plotted spiral (dashed blue curve) is based on growth rate parameter ${\displaystyle b=0.1759}$, resulting in a pitch of ${\displaystyle \arctan b\approx 10^{\circ }}$.

Romanesco broccoli, which grows in a logarithmic spiral

In several natural phenomena one may find curves that are close to being logarithmic spirals. Here follow some examples and reasons:

• The approach of a hawk to its prey in classical pursuit, assuming the prey travels in a straight line. Their sharpest view is at an angle to their direction of flight; this angle is the same as the spiral's pitch.[5]
• The approach of an insect to a light source. They are used to having the light source at a constant angle to their flight path. Usually the sun (or moon for nocturnal species) is the only light source and flying that way will result in a practically straight line.[6]
• The arms of spiral galaxies.[7] Our own galaxy, the Milky Way, has several spiral arms, each of which is roughly a logarithmic spiral with pitch of about 12 degrees.[8]
• The nerves of the cornea (this is, corneal nerves of the subepithelial layer terminate near superficial epithelial layer of the cornea in a logarithmic spiral pattern).[9]
• The bands of tropical cyclones, such as hurricanes.[10]
• Many biological structures including the shells of mollusks.[11] In these cases, the reason may be construction from expanding similar shapes, as is the case for polygonal figures.
• Logarithmic spiral beaches can form as the result of wave refraction and diffraction by the coast. Half Moon Bay (California) is an example of such a type of beach.[12]

• 
  A section of the Mandelbrot set following a logarithmic spiral
• 
  An extratropical cyclone over Iceland shows an approximately logarithmic spiral pattern
• 
  The arms of spiral galaxies often have the shape of a logarithmic spiral, here the Whirlpool Galaxy

• Archimedean spiral
• Epispiral
• List of spirals

• Weisstein, Eric W. "Logarithmic Spiral". MathWorld.
• Jim Wilson, Equiangular Spiral (or Logarithmic Spiral) and Its Related Curves, University of Georgia (1999)
• Alexander Bogomolny, Spira Mirabilis - Wonderful Spiral, at cut-the-knot

Wikimedia Commons has media related to Logarithmic spirals.

• Spira mirabilis history and math
• NASA Astronomy Picture of the Day: Hurricane Isabel vs. the Whirlpool Galaxy (25 September 2003)
• NASA Astronomy Picture of the Day: Typhoon Rammasun vs. the Pinwheel Galaxy (17 May 2008)
• SpiralZoom.com, an educational website about the science of pattern formation, spirals in nature, and spirals in the mythic imagination.
• Online exploration using JSXGraph (JavaScript)