Converting km to miles using Fibonacci?
It is quite obvious that my parents know me well. For my birthday, they got me DVDs from a course on number theory. This meant I was awake far too long last night watching the first set. In the process, I learned a wicked kewl way of converting kilometers to miles using the Fibonacci series.
The Fibonacci sequence starts with 1,1 as the “seeds.” Then it follows the rule, “The next number in the Fibonacci sequence is the sum of the previous two numbers.” So the sequence begins:
1, 1, 2, 3, 5, 8, 13, 21, 34, 55
The 2 comes from adding the previous two numbers (1 + 1). The 3 comes from adding the previous two numbers, which now are (1 + 2). The 5 follows the same rule, only now the numbers are (2 + 3). Etc.
Now it happens that the ratio between numbers in this sequence approaches the golden ratio, and at infinity it is equal to the golden ratio. The golden ratio is defined as the Greek letter phi (which doesn’t show up here, so I’ll use x instead) and can be expressed as x = 1 + 1/x. This gives us a recursive equation, one that includes itself within the definition. However, this equation can still be solved by multiplying both sides by x2 = x + 1. This gives us a quadratic equation, which means we want all the terms on one side. That gives us x2 – x – 1 = 0. Using the binomial theorem, we can solve this to show that x = (1 +/- sqrt(5))/2.
[In the previous equation, “sqrt(5)” stands for the square root of 5.]
Now even without a calculator, we know that the square root of 5 is just a little more than 2, since the square root of 4 is exactly two. So if we subtract the square root of 5 from 1, we will end up with a negative number as our end result. But we only want the positive version, which is x = (1 + sqrt(5))/2 so we can safely ignore the negative version.
In any case, we know that 1 + a number that is a little bigger than 2 gives us a number that is a little bigger than 3. And if we divide that number by 2, we will end up with a number a little bigger than 1.5. And the golden ratio begins 1.608…
Now it just happens that 1.6 is pretty close to the ratio between a kilometer and a mile. In other words, we have about 1.6 km per mile. So if the ratio between the numbers in the Fibonacci series in about 1.6, then we can use the Fibonacci series to convert km to miles. Let’s start easy by reprinting the Fibonacci series we had above:
1, 1, 2, 3, 5, 8, 13, 21, 34, 55
If we want to know how many miles are in 13 km, we find 13 on the series and then look for the previous Fibonacci number, which is 8. So there are about 8 miles in 13 km. And if we measure it, we see that 13 km is roughly 8.08 miles.
But what if we want to know how to convert a number that is not part of the Fibonacci series? Well, it turns out that you can add up Fibonacci numbers to make other numbers. So suppose we want to know how many miles are in 20 km. What you do is start with the closest Fibonacci number that is smaller than the number you’re looking for. We see in this case that that would be the number 13.
So keep 13 in mind. Now if you subtract 13 from 20, you’re left with 7. Let’s find the closest Fibonacci number smaller than 7, and that’s 5. So keep 5 in mind too. Finally, if we subtract 5 from 7, we’re left with 2, which is itself a Fibonacci number. So we end with 2.
So we’ve pulled out 13, 5, and 2. And if you’ll notice, 13 + 5 + 2 = 20, our original number. So 20 is composed of those three Fibonacci numbers. What we do to convert it to miles, then, is to find the next lowest Fibonacci number for each of those composite numbers, and add them together. So the next smallest from 13 is 8, the next smallest from 5 is 3, and the next smallest from 2 is 1. 8 + 3 + 1 = 12.
So there are about 12 miles in 20 km. And by measurement, we see that 20 km is actually about 12.4 miles, so we’re pretty close.
Now you may be wondering how the above works. Well, we know that the ratio between Fibonacci numbers is the golden ratio, and the ratio between miles and km is also close to the golden ratio. This means that so long as we can express the ratio of any number as a sum of Fibonacci numbers, we can use the previous “trick” to convert them.
To demonstrate how this works, let’s use a simpler ratio: 1/2. Suppose we wanted to know what half of 20 is. Obviously, we know that 20/2 = 10. But we can express 20 as 8 + 12 too. If we then take half of each of those numbers, we get 4 +6. 4 + 6 = 10, which is identical to 20/2. Or suppose we expressed 20 as 18 + 2. Half of 18 = 9, and half of 2 = 1. 9 + 1 = 10.
Because the ratio is the same, then no matter how you compose 20, dividing each of those numbers by the same ratio before you add them together is equivalent to finding the value of the original number divided by that ratio. So the same thing happens with Fibonacci numbers, except the ratio is the golden ratio instead of a half. Therefore, if we can compose a number as the sums of Fibonacci numbers (and if I am not mistaken, I believe all natural numbers can be expressed as the sums of Fibonacci numbers), then we can use the same technique as shown above to find a conversion between km and miles.
Now I should point out that while there are many things in nature that seem to use the golden ratio, the fact that the relationship between km and miles is close to the golden ratio is actually a matter of serendipity rather than due to some natural law of the golden ratio. Apparently, the mile was defined by the English Parliament in 1592 as being 1760 yards. The survey mile was derived as being 8 furlongs (each furlong being 10 chains, each chain being 4 rods, and each rod being 25 links—yeah, you figure it out). There is no intrinsic pattern or order to any of these gradations, hence the continual demand in science to use the metric system.
A meter, on the other hand, was originally defined as the length of a pendulum with a half-period of 1 second. It was later changed to being 1/1000000 of the distance between the North Pole and the equator. In 1983, the definition was changed so that a meter is now officially the distance light travels through a vacuum in 1/299792458 seconds. This number was formed by using an updated version of the previous definition of a meter and measuring the speed of light as 299792458 meters per second; but it has now become the benchmark itself.
So these two methods, one complete arbitrary and seemingly random, and the other apparently strict measurements of distance covered through time, somehow happened to form a ratio fairly close to the golden ratio.












