Golden spiral
In geometry, a golden spiral is a logarithmic spiral whose growth factor is , the golden ratio. That is, a golden spiral gets wider (or further from its origin) by a factor of for every quarter turn it makes. Approximations of the golden spiral. There are several comparable spirals that approximate, but do not exactly equal, a golden spiral. For example, a golden spiral can be approximated by first starting with a rectangle for which the ratio between its length and width is the golden ratio. This rectangle can then be partitioned into a square and a similar rectangle  and this rectangle can then be split in the same way. After continuing this process for an arbitrary number of steps, the result will be an almost complete partitioning of the rectangle into squares. The corners of these squares can be connected by quarter-circles. The result, though not a true logarithmic spiral, closely approximates a golden spiral. Another approximation is a Fibonacci spiral, which is constructed slightly differently. A Fibonacci spiral starts with a rectangle partitioned into 2 squares. In each step, a square the length of the rectangle's longest side is added to the rectangle. Since the  ratio between consecutive Fibonacci numbers approaches the golden ratio as the Fibonacci numbers approach infinity, so too does this spiral get more similar to the previous approximation the more squares are added, as illustrated by the image. Spirals in nature. Approximate logarithmic spirals can occur in nature, for example the arms of spiral galaxies - golden spirals are one special case of these logarithmic spirals, although there is no evidence that there is any general tendency towards this case appearing. Phyllotaxis is connected with the golden ratio because it involves successive leaves or petals being separated by the golden angle; it  also results in the emergence of spirals, although again none of them are (necessarily) golden spirals. It is sometimes stated that spiral galaxies and nautilus shells get wider in the pattern of a golden spiral, and hence are related to both and the Fibonacci series. In truth, spiral galaxies and nautilus shells (and many mollusk shells) exhibit logarithmic spiral growth, but at a variety of angles usually distinctly different from that of the golden spiral. This pattern allows the organism to grow without changing shape. Mathematics. A golden spiral with initial radius 1 is the locus of points of polar  coordinates formula_1 satisfying formula_2 The polar equation for a golden spiral is the same as for other logarithmic spirals, but with a special value of the growth factor : formula_3 or formula_4 with being the base of natural logarithms, being the initial radius of the spiral, and such that when is a right angle (a quarter turn in either direction): formula_5 Therefore, is given by formula_6 The numerical value of depends on whether the right angle is measured as 90 degrees or as formula_7 radians; and since the angle can be in either direction, it is easiest to write the formula  for the absolute value of formula_8 (that is, can also be the negative of this value): formula_9 for in degrees, or formula_10 for in radians. An alternate formula for a logarithmic and golden spiral is: formula_11 where the constant is given by: formula_12 which for the golden spiral gives values of: formula_13 if is measured in degrees, and formula_14 if is measured in radians. With respect to logarithmic spirals the golden spiral has the distinguishing property that for four collinear spiral points A, B, C, D belonging to arguments the point C is the projective harmonic conjugate of B with  respect to A, D, i.e. the cross ratio (A,D;B,C) has the singular value −1. The golden spiral is the only logarithmic spiral with (A,D;B,C) = (A,D;C,B). Polar slope. In the polar equation for a logarithmic spiral: formula_3 the parameter is related to the polar slope angle formula_16 : formula_17 In a golden spiral, being formula_8 constant and equal to formula_19 (for in radians, as defined above), the slope angle formula_16 is: formula_21 hence: formula_22 if measured in degrees, or formula_23 if measured in radians. Its complementary angle formula_24 in radians, or formula_25 in degrees, is  the angle the golden spiral arms make with a line from the center of the spiral.