|Alternative Fibonacci Spiral|
Alternative Fibonacci Spiral
- This is a guide to constructing arcs. The picture on the right demonstrates an alternative way of constructing the Fibonacci Spiral. Instead of using the squares to determine arcs, this uses circles to determine the arcs (their position and size)that make up the spiral. The traditional squares are overlaid to demonstrate the pattern: the ratio between circle sizes matches the ratio between square sizes.
The basic definition of a spiral is a curve that winds in a gradually widening pattern. On this page, we are going more in depth on what a spiral is--what exactly causes a spiral to be defined as a spiral--and how to construct spirals. We are also exploring the relationships between different types of spirals. Each spiral starts with a circle pattern: one circle that is tangent to a second slightly larger circle; then the second circle is tangent to a third circle, with the third larger than the second; and the pattern continues with tangent circles inside of each other. The spiral is made up of parts of each circle; each circle contains one arc that contributes to the spiral. One arc is the part of the circle from one tangent point to another tangent point. For example, one part of the spiral would be the arc from the point where the first circle is tangent with the second circle, to the point where the second circle is tangent with the third circle. The next part would be the arc from the point where the second circle is tangent with the third circle, to the point where the third circle is tangent to the fourth circle. And so on. Each arc is bigger than the previous arc so any given point is farther away from the center than the previous point. Together, the arcs form the spiral.
The bolded spiral is the well-known Archimedean Spiral, or the beginning of it. The pattern of the spiral is determined by corresponding points on right triangles that have a relationship to each other in the lengths of their hypotenuses, which is: , , , ...etc. All of the right triangles meet at a point: the center of the spiral. The points on the spiral continuously get farther away from this point.
This is the first image we started with. Shown above is a Fibonacci Spiral, in green, constructed from arcs on circles. The squares are included to show how parts of circles are used in spirals. The Fibonacci Spiral is created using 1/4 sections of circles (arcs). The radii of the circles fit the Fibonacci Sequence with the smallest circle having a radius of 1, the next largest being 2, then 3, then 5, and so on. Each circle has a radius along the length of one of the squares. The centers of the circles are rotated 90 degrees from each other at a distance that also fits the Fibonacci Sequence. That is, the center of the smallest circle is rotated 90 degrees and translated 1 unit away. Then, the next center is rotated 90 degrees and translated 2 units away. This pattern continues. This way, the circles are tangent to each other, and arcs become visible:
Shown above, on the left, the centers, shown in red, are rotated counterclockwise 90 degrees at a distance determined by the Fibonacci Sequence (1, 1, 2, 3, 5, 8, etc. units). In the middle, circles were constructed around each of the points with radii starting at the second number in the Fibonacci Sequence (1, 3, 3, 5, 8, 13, etc. units). On the right, the Fibonacci Spiral is constructed, shown in thick green. It is important to determine which direction the figure should spiral. The spiral shown above spirals counterclockwise away from the center. Each arc on the circles is 1/4 of the circle's circumference except the smallest arc, which is 1/2. The smallest arc is 1/2 of the circumference to clearly mark the beginning of the spiral. Any spiral where the radii are based on a predetermined set of numbers that start at one specific, unchangeable number and that get progressively bigger have beginnings. For example, the Fibonacci Spiral and the Archimedean Spiral start at a predetermined number--1 and respectively--and get progressively bigger. However, spirals where the radii of the circles follow a pattern like the spiral shown below do not have a constant beginning. These spirals are constructed of arcs on circles where the ratio between the radii of the first (smallest) circle and the second circle is 1:2. As the spiral continues, the pattern continues; the radius of one circle is doubled to get the radius of the next circle. These spirals have no constant beginnings because the measurement of the smallest radius can simply be halved, resulting in a smaller arc.
These spirals have no constant beginnings, but it is important to realize that they still have a center: a point that the spiral continuously travels away from. Similarly to the spirals with constant beginnings, the center of this type of spiral is located at the first point on the spiral: the beginning--which is not constant for all spirals, but it present on all spirals. For example, think of spiral with a ratio of the radii of the circles of 1:2. This first iteration of this spiral could have the smallest radius measuring 1 unit; however, it could also have a smallest radius of 1/2 units, or 1/4, or 1/8, etc. The location of the center is different in each of those cases, but the rule for finding the center of a spiral is as follows: it is the first point on the first arc of the spiral.
The Fibonacci Spiral is on the left, and a spiral with a 1:2 ratio is on the right. The orange segments connect the centers (in bright green) to random points on the spiral. The orange segments get progressively longer, demonstrating that spirals originate from this exact point. However, this one point is only one example of a center of a spiral. All spirals can have more that one center from which all points get father away. The easiest example of this is the Archimedean Spiral: all of the triangles meet at one vertex. Every point on the spiral gets progressively farther away from this point. Shown below, both the Fibonacci Spiral (left) and the 1:2 ratio spiral (right) have more than one of this kind of center.
This is one type of center that all spirals have. The other type of center is the point around which the spiral revolves, which we will refer to as a rotational center. This point is the center of the circle used in creating the smallest arc on the spiral. The spiral as a whole appears to 'spiral' away from this one point.
The 1:2 spiral may look similar to the Fibonacci Spiral, but as they get bigger, the differences between the radii increase, causing the spirals' shapes and sizes to differ. From the tenth to the seventeenth iteration, the Fibonacci Spiral follows the pattern 55; 89; 144; 233; 377; 610; 987; and 1,597. The spiral with the 1:2 ratio, with the first iteration of 1 unit, follows the pattern 512; 1,024; 2,048; 4,096; 8,192; 16,384; 32768; and 65,536. Obviously, the spiral with the 1:2 ratio is much larger than the Fibonacci Spiral. But what does this mean? How can it be measured?
It is possible to compare spirals based on the areas of their arc sectors and the lengths of the arcs that make up the spiral.
A More Mathematical Explanation
Both the Fibonacci Spiral and the 1:2 ratio spiral have centers that are rotated 90° and then transl [...]
Both the Fibonacci Spiral and the 1:2 ratio spiral have centers that are rotated 90° and then translated. We explored what would happen if we rotated the centers by 180 degrees (the centers would be on a straight line) but still translated using the previous pattern (the Fibonacci Sequence and the 1:2 pattern). The outcome:
This spiral looks "tighter" than the Fibonacci Spiral; the spiral is more "controlled." The arcs that create the spiral now are 1/2 of the individual circle instead of 1/4 because the circles are tangent to each other in different places. When the centers are rotated 180°, they all lie upon a line. This line connects all of the centers and all of the tangent points of the circles. Because this line intersects each circle twice, there are only two possible places for two circles to be tangent to each other. In a spiral, three circles cannot be tangent in the same place, so the tangent points must alternate. When the line that connects the centers and tangent points is vertical, the alternation can be described like this: image the spiral on a coordinate grid. If a circle is tangent to "the bottom" (a point lower on the y-axis) of a smaller circle, then it must be tangent to a bigger circle at a point higher on the y-axis. Because of this alternation, the arcs make up 1/2 of their individual circles.
Next, instead of using the Fibonacci Sequence, we tried a 1:2 ratio. To summarize: the circle's centers are rotated 180 degrees from each other, or in other words, they are on a straight line. The radii of the circles are dependent on a 1:2 ratio. The circles are tangent to each other, so the distance between the centers is doubled each time another circle is created. Each bold arc is half of its individual circle instead of 1/4:
Spiral Lengths and Areas
Spirals can be compared in terms of size and shape, but they can also be compared using "spiral area" and "spiral length." The formula for the length of a spiral that follows a pattern (like a ratio, as opposed to the Fibonacci Sequence and the Archimedean Spiral) is...because...
Formula for area:
The length of a spiral is infinite, but it is possible to calculate the length for iterations of the spiral. The spiral length, in this case, is defined as the arc length of each arc on each circle for a certain number of circles. Below, we calculated the arc lengths of certain spirals to the 5th iteration.
1:2 ratio rotated 180 degrees
This is a 1:2 ratio spiral with a 1st iteration circle with a radius of 1 cm.
Following the formula, the length of this spiral is
The absolute value of ((180-angle of rotation)/360)*2π*(r+r+r...r) The absolute value of 180-180 is 0, but, following the rule for spirals with a 180° rotation, we will substitute 180° in.
(180/360)*2π(1+2+4+8+16) = π(31) ≈ 97.39 cm
Notice the effect on the length of the 1:2 ratio spiral when the centers are rotated 90° instead of 180°:
This is a 1:2 ratio spiral (where the smallest circle has a radius of 1) with the centers rotated 90°. Substituting the radii and the angle of rotation into the equation results in the following equation: (
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