Is There a Formula for The 'Perfect' Christmas Tree?

The other week I was waiting for my tram, reading Metro, and I see this article - Treegonometry: Formula for perfect Christmas tree discovered

In other word, a set of equations that will tell you, for example, how much tinsel, or how many baubles are 'required' for the 'perfect' Christmas tree.

Here is the press release, with the actual formulas.

Now, I have a few issues with this. For one thing, it's a bit silly. Surely the perfect tree is a matter of personal preference, and in that sense can't be defined by a set of mathematical rules.

Plus, it seems kind of arrogant to declare that their way is the 'perfect' way, and that (implicitly) any other way is wrong.

Also, the fact that the equations are preposterously simple (perfect thing = constant*tree height) makes me suspicious of them. But, worse than that, it really bothers me that I have no idea how they were derived.

But I'm starting to rant.

The equations in question were actually from a 'study' by the Sheffield University Maths Society (student from my university), and were commissioned by Debenhams (department store). So, I dunno, maybe I'm just bitter that no-one's ever commissioned me to do maths.

Anyway. If they'd said that these were equations for an 'ideal' Christmas tree, then I'd consider that more reasonable. You can say, for example, that the ideal Christmas tree should strike a balance between too many and too few decorations, etc.

So, to that end, I'm going to have a crack at deriving some, more general, 'Equations for an Ideal Christmas Tree' of my own.


Lights

Before we start, there's one simplification we need to make - we will assume that Christmas trees can be approximated to a smooth, regular, right-circular cone - height h, and base radius r.

There's going to be a certain degree of error as a result of this approximation, but I'm not going for perfect.

Okay. So, for lights, there's actually a justifiable basis for an ideal - the ideal tree should have its lights evenly distributed.

Or, to put it another way, you ideally want to wrap your lights such that you have a (roughly) constant light surface density (lights per unit surface area); because you don't want your tree to be covered in clumps and bald spots.

This seems like a vague point; how do you decide on the correct 'density'?

Well, we actually have two constraints to work from: firstly, we have to wrap a single line around our conical shape, and second, the distance (s) between each pair of adjacent lights on a string will be fixed.

From this, we can layout a (hypothetical) grid on the surface of the tree, so that all the lights are equally spaced out.

So, how do we do that?

1) Draw yourself a cone (tree)
2) Draw a grid
3) Draw lights at the points where the grid lines meet
4) Draw in the proper row lines - these represent the actual string of lights

In this cases, I've drawn the grid a little tight, so you'd probably want two strings of lights to get this layout (hence the two colours).

And if you like, you can play with the distances between rows, and the angles, to get some slightly different layouts

The best grid to try for would be a triangular grid, where the distances between each light and the six nearest are the same

So, let's say we've picked a grid. How many light will we need?

There's a nice short-cut to working this out, without having to worry about lengths of spirals on conic surfaces. To do this, we take advantage of the regular grid layout to work out the lights density (lights per unit area).

Each section of grid is roughly a diamond, with all sides about the same length

So with a bit of trigonometry, you can get an approximate equation for the area of this diamond

Then, the density is one over that area (since there's one light per grid diamond).

We then multiply this density by the total conic surface area (excluding the base) to get the total number of lights needed

And, finally, multiplying that by the separation between lights (s), we get the total length needed:

Easy. Though, somewhat more complicated than the L = pi*h equation, derived by SUMS.

So, then, for the triangular grid (theta=60), you get

Cool.

The only problem then is getting the lights to line up on a grid on the actual tree. So... good luck with that.

Realistically, all of this is mostly irrelevant anyway - you can't go out and ask for, say, exactly 5.83m of lights; you buy your lights in pre-cut lengths. Can't get hold of the 'perfect' length of light? Then your tree is imperfect, and you should feel bad.

Anyway. The point is, if you can get your lights more or less even - so that there aren't any clumps or bald spots -, then as far as I'm concerned, you're on to a winner.


Tinsel

What's interesting about their equation for tinsel is this factor of 13/8. Now, again, I don't know how these equations were derived, so I don't know if this was intentional; But, 8 and 13 are consecutive Fibonacci numbers. Why is this important? Well, the Fibonacci sequence is closely related to spirals.

In particular, there's this thing you see in nature; for example, if you look at the spirals on a pineapple, the spirals going in one direction might be 13 and the number in the opposite direction might be 8. Or the numbers might be 21 left and 13 right... The point is, the numbers of spirals on a pineapple are always consecutive Fibonacci numbers (or sometimes Lucas numbers).

And you get the same effect on other things, like the spirals of seeds in the head of a sunflower, or the seeds on a strawberry, or the spirals on a pinecone, or a cauliflower, or all sorts of things. Hell, maybe the branches on a Christmas tree form Fibonacci spirals.

Vi Hart explains it better than me.

So maybe that has something to do with that pre-factor. Or maybe not. It's an interesting tidbit, though.

Anyway.


The thing with tinsel is different people like to do tinsel differently - some like to elegantly drape it across the outer branches, others like to wrap up their tree light they're restraining a hostage. It's a matter of preference. But it's going to affect the amount of tinsel you'll need.

Where tinsel differs from lights is, you're not trying to set up a grid, or get an even surface density. Rather, in this case, you'd probably want to wrap it such that the rows are more horizontal, with a roughly constant vertical separation.

Here's where things get messy; the equation for the length of a spiral on the surface of a cone is given by

I know, right? Maybe you would be better using the SUMS equation for this one.


Extras

The star/angel is going to be some fraction of the height of the tree.

I don't know what the ideal value of the fraction (alpha) would be, but the 10th they came up with seems reasonable.

Baubles, I haven't a clue how they came up with those numbers. The factor of sqrt(17) makes me think some geometry was probably involved, but I dunno.

You would probably want to figure it out as some ideal ornament surface density, Db (like with the lights). In this case, the number of baubles needed would be something like

In fact, if your baubles are all, more or less, the same, you can lay them out on a grid, like the lights. Though, this time, you'd want to have them a little more spaced out, since baubles are much bigger than fairy-lights. But at least this time you don't have the separation constraint.


Thoughts

Here are various things Ben Goldacre, of Bad Science, has said on the subject of commissioned, 'perfect' formulas. Here is an article by mathematician, Simon Singh. Here is an article on BBC News. And here is a collection of such formulas on Apathy Sketchpad.


To be honest, I wouldn't bother with any of this; their equations, or mine. I mean, would you really want a tree that was 'perfectly' decorated? Cold and artificial are the words that come to mind.

And, frankly, I'm not sure my equations would actually work in practice.

I'd say, use your best judgement on how much of everything you'll need, and just do your own thing. Have fun with it!


Our Christmas tree is imperfect. In fact, it's gloriously imperfect. No, seriously, it's a mess.

My ex's family used to construct these massive, elaborate, works-of-art trees; with yearly colour schemes, and matching baubles, and everything. By comparison, she described our tree as kitsch.

But it's adorned with all the baubles, and tinsel, and decorations we accumulated over the last 20-odd years; at least, the ones that haven't been lost or broken. And, in a sentimental sort of way, it is perfect.

Well, okay, not perfect. But, damn it, it's ours.





Oatzy.


[I want you to know, I had no part in decorating that tree.]

[...And, yes, that's a weeping angel on top.]

Sharing a Burger Between Three

There are several ways to divide a burger evenly between three people.

Probably the best is to cut it radially (like a pizza). It can be tricky working out exactly where to make the cuts, but if you can pull it off, then all the pieces will be roughly identical (topping distribution notwithstanding).

But for the sake of arguing, lets say you want to divide the burger by making two parallel cuts: Where, then, do you make the cuts so that all three people get the same amount of burger?

NB/ This gets quite maths-heavy, so if you're not interested in that sort of thing, feel free to skip right to the end for the solution.



Geometry

For simplicity, we're going to consider the burger as a circle, and make the cuts so that each chunk has the same area. The two cuts are going to be the same distance from, and parallel to, the central axis of the burger, so we only need to consider the position of one of the cuts.

Here's the set-up

We work out the area of the cut-off as the area of the circular segment, minus the area of the triangle.


- Aside: Radians

Radians are basically an alternative way of measuring angles. For maths and physics they're generally more useful than degrees.

They're relatively easy - there are 2pi radians in a full circle, so 2pi radians = 360 degrees

1 radian = 180/pi = 57.3 degrees
1 degree = pi/180 = 0.017 radians, etc.

*    *    *

Back to the circle; with angle x in radians, the area of the circular segment is

The area of a triangle is half base times height..


- Aside: Area of the Triangle

We start by splitting the triangle down the middle, so that we have two identical right angle triangles

The height, l = r*cos(x/2)

The base, b = 2*(r*sin(x/2))

So the area of the triangle is (l*b)/2 = r^2 sin(x/2)cos(x/2)

Finally, use the identity sin(2x) = 2sin(x)cos(x)
to get

*    *    *

So the area of the cut-off is

and it needs to equal a third the area of the circle = 1/3 pi r^2.

So first, we need to find x satisfying

or

Once we have a value for x, we find where to make the cut from

Intermission

The thing about this equation is it doesn't have an exact, analytical solution - to find the solution you have to use numerical methods. Well, I say you have to use numerical methods; these days you can just type the equation into WolframAlpha, and you'll get a solution like *snaps fingers*

Which is nice. I even have the WolframAlpha app on my phone. But when I thought up this question I was on holiday in Sherwood forest, where there was literally no mobile singal.

So that was out of the question. And since I'm not in the habit of carrying a scientific calculator around with me, I was stuck with the basic calculator on my phone. It looks like this:

No trig functions, no square roots, no pi button. It doesn't even do brackets, or have a memory function. Luckily, I am in the habit of carrying around a notepad and pen.

Anyway, there are two ways of working this out with only a basic calculator. The first is 'easier', but only if you know some stuff, and the numbers happen to be nice (in this case, they kind of are). The second is harder, in that it requires more number crunching, but it'll work with any numbers, and can be more precise.

Again, feel free to skip to the solution if you're not interested in the gritty details.



Method One

First of all, here's a graph of the two sides of the equation

hand-drawn with Skitch

We want to find the point at which the two graphs cross. We can see that that happens somewhere between 2pi/3 and pi (120 and 180 degrees). So, lets make a guess that it's exactly halfway between these two values: 5pi/6 (150 degrees).

For the right hand side of the equation: 5pi/6 - 2pi/3 = 0.52

NB/ I'm using pi=3.1416 (rounded to 4 decimal places). If you prefer, you could use the approximation 22/7. The result should be roughly the same.

For the left hand side of the equation, we need to work out sin(5pi/6)

At A-level, we were expected to memorise sin() and cos() of angles 0, 30, 45, 60, 90, and 180 (degrees). We were also expected to know the formulas for sin() and cos() of sums of angles. For sin(), it works like

Why is this important? Well 150 degrees = 180 - 30 (5pi/6 rads = pi - pi/6)

So

And since 0.5 is pretty close to 0.52 - less than 5% error - we can accept the convenience of that answer and say it's close enough.

So our approximate value of x is 5pi/6 = 2.618

[Incidentally, the identity for sin(2x) is just a special case of the above, with a=b=x; i.e. sin(2x) = sin(x+x) = 2sin(x)cos(x)]


Now we just need to work out l/r = cos(x/2) = cos(5pi/12)

For this one, 5pi/12 rads = 75 degrees = 45 + 30, so we can use

So

And we just have to evaluate that. But we don't have a square root button. Now, I just happen to know that sqrt(3) ~ 1.73 and sqrt(2) ~ 1.41.

But I'm just weird like that. Let's say you don't. How do you work it out?


- Aside: Square Roots

There are several ways of working out square roots with just basic operators. For two easy examples:

The first is 'Trial and Improvement' - pick a number, square it, does that give the right answer? If not, pick another number based on whether the last guess was too big or too small.

For example: sqrt(3)

1.5 -> 2.25 -> too small
1.7 -> 2.89 -> too small
1.8 -> 3.24 -> too big
1.75 -> 3.0625 -> too big
1.73 -> 2.9929 -> too small
1.74 -> 3.0276 -> too big
1.735 -> 3.010225 -> too big
1.7325 -> 3.00155625 -> too big
1.732 -> 2.999824 -> too small
etc.

The second method is the "Babylonian Method". It's more systematic, and can converge to the correct answer quicker than guessing. But it can be irritating if your calculator doesn't have a memory function.

It uses the recurrence relation

Basically, you make a guess xn. Divide the number you want to square root (S) by xn. If xn is lower than the actual square root, then S/xn will be greater than it. That means the actual root will be between xn and S/xn, so we make the next guess xn+1 the average of these two values. Repeat until x is sufficiently accurate.

For example: sqrt(2)

x0 = 1.5 -> 2/1.5 = 1.33
x1 = (1.5 + 1.33)/2 = 1.4166.. -> 2/1.4167 = 1.41176..
x2 = (1.4167 + 1.41176..)/2 = 1.41421.. -> 2/1.41421 = 1.41421..

*    *    *

Whatever way you do it, you repeat the process until you get the degree of accuracy you're happy with.

You should get the answer around l/r = 0.259



Method Two

We go back to the equation sin(x) = x - 2pi/3

We still have to find the solution numerically, we still don't have a calculator with a sin() function, and this time the numbers don't work out nicely.

So, the question is, how do we calculate sin(x)?


- Aside: Taylor Expansion

The Taylor Expansion of a function is a way of fitting a polynomial (sums of powers) to a more complicated function. It works like this

Basically, it gives a way of converting a function we can't calculate into an infinite sum of powers of x, which we can calculate.

It's usually expanded around the origin (x0=0), since the equations work out neater. But you can do it around any point, x0=a. This is useful if the value you are trying to calculate is far from x=0. The closer x is to x0=a, the quicker the sum converges.

Even though the expansion is an infinite sum, it's usually sufficient to just take the first few terms, since each additional term makes a smaller and smaller contribution to the sum.

So the trick is working out how many terms you need to include to get some desired level of accuracy.

*    *    *

In this case, I'm going to use the Taylor Expansion of sin(x) around x0=pi, since the approximate value (2.6) is nearer to pi than 0.

Here's what the expansion looks like

So, how many terms do we need to include?

Here's what the graph looks like for different numbers of terms

plotted with WolframAlpha

For a value around 2.6, it can be shown that including the first two terms is correct to ~3 decimal places; the first three terms is correct to ~5 decimal places; the first four terms to ~7 decimal places, etc.

So I would probably go to the third term (for 5dp), but only take the result to 3dp.

NB/ We shouldn't get too hung up on getting an extremely accurate value for x, since we're already getting rounding errors from the factors of pi in the expansion. Also, since we're calculating x to 5dp, we should use pi=3.14159

That means we want to solve

which can't be solved exactly.

So, for finding the correct value (without WolframAlpha), we can use any root-finding method. For what it's worth, I used Trial and Improvement; the other methods are easier with a computer.

But, note that the function is decreasing

So if the guess gives a value greater than zero, you need to increase the value of x (and vice versa).

If you run through all that (I won't go into detail), it gives a value around x=2.605


Alternatively, you could expand around x0=5pi/6 (if you know/can work-out sin and cos of 150 deg without a calculator).

In this case you'd only need up to the term in x^2 (correct to ~4dp). Using this expansion would mean solving a quadratic equation, which is easy. But using this expansion can introduce more rounding errors from the factors of sqrt(3). It's a matter of preference, I guess. The answer should be about the same.


Finally, we need to calculate cos(x/2)

Again, we use the Taylor Expansion to calculate cos(). In this case, we're doing the expansion around x0=0; the expansion is

In this case, you just keep adding terms until the result remains approximately constant to some desired degree of accuracy (3pd).

This gives a value around l/r = 0.265



So What is the Real Answer?

Once I got to somewhere where I could get at WolframAlpha, I checked the real numbers; here are the results:

The approximation of x from Method One (2.618) is an over estimate by ~0.5%, which is relatively acceptable. The approximation from Method Two (2.605) is correct to 3 decimal places, which is definitely acceptable.

And for the value of l/r

From Method One (0.259), the approximation is an under estimate by 2%, and correct to 2 decimal places, so is probably acceptable. The approximation from Method Two (0.265) is, again, correct to 3 decimal places. So that is also acceptable.

So, if the numbers happen to be convenient and you know some trigonometry, you're probably as well using Method One. If not, or if you just want more accuracy, then go for Method Two.



Applying the Results

The results are actually quite nice, in terms of practical application (dividing up a burger). The ratio of the radius (0.265) being close to one quarter, you find the cuts like this

That is, find the central axis, then find the (imaginary) line halfway between the centre and the edge - make the cut halfway between the centre and this imaginary line (maybe cut an extra hair's breadth towards the edge). Repeat on the other side.

Easy.


So, now you know. Obviously, all this applies to dividing any circular thing evenly between three people. You could probably even adapt the methods for sharing between even more people.

And in theory, You could do all this with just pen and paper (no calculator). Though you probably wouldn't want to. I know I wouldn't..


Oatzy.


[Wow, I really managed to stretch that one out.]