(Redirected from Golden mean
- This article is about the mathmatical ratio. For the Aristotelian concept of "golden mean" see Nicomachean Ethics.
The golden ratio is either of two irrational numbers, approximately 0.618 and 1.618, that possess many interesting properties. Shapes defined by the golden ratio have long been considered aesthetically pleasing in western cultures, reflecting nature's balance between symmetry and asymmetry and the ancient Pythagorean belief that reality is a numerical reality, except that numbers were not units as we define them today, but were expressions of ratios. The golden ratio is still used frequently in art and design. The golden ratio is also referred to as the golden mean, golden section, golden number or divine proportion.
The golden ratio was first studied by ancient mathematicians due to its frequent appearance in geometry. The golden ratio seems to have been understood and used by the Egyptians. The discovery of irrational numbers, numbers that cannot be represented as an exact ratio of two integers, is usually attributed to Pythagoras (or to the Pythagoreans, notably Theodorus) or to Hippasus of Metapontum. Euclid spoke of the "golden mean" this way, "A straight line is said to have been cut in extreme and mean ratio when, as the whole line is to the greater segment, so is the greater to the lesser".
The golden ratio is symbolized by the Greek letter φ (phi) with τ (tau) being less common.
Origin of name
The name "golden ratio" appears in the form sectio aurea, "golden section", by Leonardo da Vinci. The use of the symbol φ to represent the golden ratio was invented by the American mathematician Mark Barr and taken from the first Greek letter in the name of the Greek sculptor Phidias, who was long believed to have used the golden ratio in his designs.
Two quantities are said to be in the golden ratio, if "the whole is to the larger as the larger is to the smaller", i.e. if
Equivalently, they are in the golden ratio if the ratio of the larger one to the smaller one equals the ratio of the smaller one to their difference:
After multiplying the first equation with a/b or the second equation with (a − b)/b), both of these equations are seen to be equivalent to
This definition gives the value of stated above. Alteratively, some define the number of the golden ratio to be the so-called golden ratio conjugate (also erroneously called the silver ratio or silver mean), . The ratios and are equivalent. Also, some use the symbols τ or to designate the number called here.
A line is divided into two segments a and b. The entire line is to the a segment as a is to the b segment
The fact that a length is divided into two parts of lengths a and b which stand in the golden ratio is also (in older texts) expressed as "the length is cut in extreme and mean ratio". This can be easily visualized using a line that is divided into two segments, as in the diagram.
For those who struggle with algebra but can at least handle the idea of fractions of equal value, the inquiry that leads to the golden number is expressed this way: Is there a number smaller than one that compares to 1 as 1 compares to 1 plus the same number? The answer is yes, but there is only one number, and it is the golden number x in this example.
or, equivalently, the quadratic equation
This quadratic equation has two roots:
The difference between these two irrational numbers is exactly 1.
If a house has a rectangular "golden window" with a length of 1 unit of measurement, then its width is the golden number, about 0.618 of the unit of measurement. If the shorter side of the window is instead determined to have a width of 1 unit of measurement, then its length is 1 plus the golden number, about 1.618 units of measurement.
The golden ratio value is the only positive number that is exactly 1 less than its own square.
A startlingly quick proof of irrationality
gives a startlingly quick proof that this number is irrational: If a/b is a fraction in lowest terms, then b/(a − b) is in even lower terms—a contradiction.
φ is an irrational number, and the unique positive real number with
The formula can be expanded recursively to obtain a continued fraction for the golden ratio:
and its conjugate:
Note that the successive convergents of these continued fractions are ratios of Fibonacci numbers.
The equation likewise produces the continued square root form:
"Geometry has two great treasures: one is the theorem of Pythagoras; the other, the division of a line into extreme and mean ratio. The first we may compare to a measure of gold; the second we may name a precious jewel."
The golden rectangle, whose sides a and b stand in the golden ratio, is illustrated below:
If from this rectangle we remove square B with sides of length b, then the remaining rectangle A is again a golden rectangle, since its side ratio is b/(a-b) = a/b = φ. Iterating this construction produces a sequence of progressively smaller golden rectangles; each dividing line marks a point on a logarithmic spiral. The curve of the spiral can be closely approximated by inscribing quarter-circle arcs in each square, while the true logarithmic spiral is expressed θ = (π/2log(φ)) * log r in polar coordinates.
Golden and logarithmic spirals. The green
spiral is made from quarter-circles tangent to the interior of each square, while the red
spiral is a logarithmic spiral
. Overlapping portions appear yellow
Since φ is defined to be the root of a polynomial equation, it is an algebraic number. It can be shown that φ is an irrational number.
The number φ turns up frequently in geometry, in particular in figures involving pentagonal symmetry. For instance the ratio of a regular pentagon's side and diagonal is equal to φ, and the vertices of a regular icosahedron are located on three orthogonal golden rectangles.
The explicit expression for the Fibonacci sequence involves the golden ratio and its conjugate. Also, the limit of ratios of successive terms of the Fibonacci sequence (or any Fibonacci-like sequence) equals the golden ratio. This means when given a Fibonacci number, multiplying it by φ approximates the next Fibonacci number, and that approximation gets better and better as the Fibonacci numbers get higher. Interestingly enough, if all the approximation errors are added up, they equal φ. Stated mathematically:
Furthermore, the successive powers of φ obey the Fibonacci recurrence. From a mathematical point of view, the golden ratio is notable for having the simplest continued fraction expansion, and of thereby being the "most irrational number" worst case of Lagrange's approximation theorem. It is also the fundamental unit of the algebraic number field and is a Pisot-Vijayaraghavan number.
The golden ratio has interesting properties when used as the base of a numeral system: see golden mean base.
The ancient Greeks already knew the golden ratio from their investigations into geometry, but there is no evidence they thought the number warranted special attention above that for numbers like π (Pi), for example. Studies by psychologists have been devised to test the idea that the golden ratio plays a role in human perception of beauty. They are, at best, inconclusive.  Despite this, a large corpus of beliefs about the aesthetics of the golden ratio has developed. These beliefs include the mistaken idea that the purported aesthetic properties of the ratio was known in antiquity. This has encouraged modern artists, architects, and others, during the last 500 years, to incorporate the ratio in their work. The Acropolis is believed to have been constructed using the golden ratio. Many buildings follow the Golden Ratio design.
It is also believed that the human body has proportions close to the golden ratio.
In 1509 Luca Pacioli published the Divina Proportione, which explored not only the mathematics of the golden ratio, but also its use in architectural design. This was a major influence on subsequent generations of artists and architects. Leonardo Da Vinci drew the illustrations, leading many to speculate that he himself incorporated the golden ratio into his work, although there is no evidence supporting this.
The ratio is sometimes used in modern man-made constructions, such as stairs and buildings, woodwork, and in paper sizes; however, the series of standard sizes that includes A4 is based on a ratio of and not on the golden ratio. It's also interesting to note that the average ratio of the sides of great paintings, according to a recent analysis, is 1.34. 
The ratios of justly tuned octave, fifth, and major and minor sixths are ratios of consecutive numbers of the Fibonacci sequence, making them the closest low integer ratios to the golden ratio. James Tenney reconceived his piece For Ann (rising), which consists of up to twelve computer-generated upwardly glissandoing tones (see Shepard tone), as having each tone start so it is the golden ratio (in between an equal tempered minor and major sixth) below the previous tone, so that the combination tones produced by all consecutive tones are a lower or higher pitch already, or soon to be, produced.
The construction of a pentagram is based on the golden ratio. The pentagram can be seen as a geometric shape consisting of 5 straight lines aranged as a star with 5 points. The intersection of the lines naturally divides each length into 3 parts. The smaller part (which forms the pentagon inside the star) is proportional to the longer length (which form the points of the star) by a ratio of 1:1.618... It is thought by some that this fact may be a reason why the ancient philosopher Pythagoras chose the pentagram as the symbol of the secret brotherhood of which he was both leader and founder.
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Last updated: 05-13-2005 07:56:04