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A two-dimensional spiral may be described using polar coordinates by saying that r is a continuous monotonic function of θ. The circle would be regarded as a degenerate case (the function not being strictly monotonic, but rather constant).
Some of the more important sorts of two-dimensional spirals include:
- The Archimedean spiral: r = a + bθ
- The hyperbolic spiral: r = a/θ
- The logarithmic spiral: r = abθ; approximations of this are found in nature
- Fermat's spiral: r = θ1/2
- The lituus: r = 1/θ1/2
For simple 3-d spirals, the third variable, h (height), is also a continuous, monotonic function of θ.
For compound 3-d spirals, such as the spherical spiral described below, h increases with θ on one side of a point, and decreases with θ on the other side.
A spherical spiral (rhumb line) is the curve on a sphere traced by a ship traveling from one pole to the other while keeping a fixed angle (but not a right angle) with respect to the meridians of longitude, i.e. keeping the same bearing. The curve has an infinite number of revolutions, with the distance between them decreasing as the curve approaches either of the poles.
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