Drawing a Pentagon

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Introduction: Drawing a Pentagon

This is a companion to the Dodecahedron Calendar project.  Here I demonstrate how to draw a perfect pentagon.  You will need a ruler and a compass.

Step 1:

First draw the horizontal and vertical lines.

Step 2:

Then draw the circle centered on the crosshairs. Do not fold the compass after the circle is drawn. The side of the pentagon will be 1.176 times the radius.

Step 3:

Without adjusting the compass, place the point of the compass on the circle where it crosses the horizontal line. Now draw arcs on the previous circle above and below and connect those points.

Step 4:

Now center the compass on the crosshair made from the bisector and draw an arc from the top of the circle down to the horizontal line. BTW, you've just drawn a golden ratio.

Step 5:

At the end of this step, do not close the compass. You will need that distance to make four more arcs. Putting the point of the compass at the top of the circle draw an arc from where the last arc intersected the horizontal line out to the circle.

Step 6:

Now move around the circle using each arc as the center of the next arc.

Step 7:

And finally, draw lines from each intersection to form a pentagon.
To be honest, I've never gotten this to work the first time. There are just too many places to make a few thousandths of an inch mistake and they all add up. What I have to do is place the center of the compass at the arc intersection just above the horizontal line on the right side, then adjust the compass in or out by one fifth of the distance I'm off from the top of the circle. Then start over from the top of the circle and work my way around again. Even though this sounds like proof that the process is wrong I've done the math and it should work if not for human error.

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Tips

1 Questions

The distance

I am not sure what you're asking here, but yesterday I did this same construction, using pencil-paper-compass-and-straightedge, and I uploaded a picture of this to the Share/I-made-it section of this instructable.

The picture has the various points labeled, (A,B,C,D, etc.) and I wrote a comment to go with it that mentions the length of some of these various line segments.

By the way, if you want to see how the pros do this, the pages at Wolfram,

http://mathworld.wolfram.com/Pentagon.html

or Wikipedia,

https://en.wikipedia.org/wiki/Pentagon

might have the answers you are seeking.

9 Comments

You didn't mentioned in step 3 the extra line comes from where. ??

You could also use the following formula wich does not use the sin function.

t=r*sqrt(1+{(sqrt(1.25)-.5)^2})

http://ru.wikipedia.org/wiki/%D0%9F%D1%80%D0%B0%D0%B2%D0%B8%D0%BB%D1%8C%D0%BD%D1%8B%D0%B9_%D0%BF%D1%8F%D1%82%D0%B8%D1%83%D0%B3%D0%BE%D0%BB%D1%8C%D0%BD%D0%B8%D0%BA

5 replies

Yes, you are right.

Estoy luchando para dibujar un pentágono en AutoCAD, dada la longitud del lado. Por supuesto, todo lo que se requiere es el radio del círculo dentro del cual el pentágono es conocido:


Donde R es el radio y t es la longitud del lado.

Bill, I tried to understand your formula, but without success. Maybe during the day I could to think something.

In AutoCAD, one must inscribe a polygon inside a circle of a certain radius. For a pentagon, I know the length of a side only, do not know radius. So, using the formula above I can calculate the radius if I know the length of a side. T=1.175*R; also R=0.851*T.

En AutoCAD, hay que inscribir un polígono dentro de un círculo de un radio determinado. Para un pentágono, sé la longitud de sólo un lado, no sé radio. Así, el uso de la fórmula I anterior puede calcular el radio si conozco la longitud de un lado. T = 1.175 * R; también R = 0,851 * T.

Untitled.jpg

Very interesting, Bill, your deduction of the relation between R and T. Many persons would need to know it.

Very nice, thank you. I have a cabinet full of compasses, will give your method a try. Also, if I know the manual method I can figure out how to do it in AutoCAD!