# Your Body Ruler - A User's Manual

(Now part of A View from the Back of the Envelope.)

The idea: you can measure with your body.
And... you will never forget to bring your bodyruler with you. :) Though sometimes I forget my calibration...

## Your Body Ruler A User's Manual

Congratulations on being a human ruler!
Throughout history, people have been using themselves to measure length. You can too!

## An example

How big is the screen/paper in front of you?
My screen is about 4.5 palms by 3.5 palms.
(Yes, there are now fingerprints on my screen. :)
Having calibrated myself, I know my palm is 0.07 meters (7 cm, 2.75 in).
So my screen is something like 0.31 by 0.24 m (12.4 by 9.6 in).
Measuring with an "ordinary ruler", I get 0.32 m and 0.25 m. (12.5 by 9.5 in.)
So my palms quickly gave a nice measurement.

## Calibrating Yourself

A first step is to choose your measures, and measure your choices.

Many body distances can be used as measures, but you dont need all of them. Many have similar size (for example, hand and palm), so you can just choose ones you prefer, perhaps ones with numbers easier to remember. Also, some are related (1 palm can be 4 fingers). Feel free to use other measures, or to modify these (like using little finger rather than middle finger). The important thing is consistency, using them the same way each time.

Before using any new measuring instrument, one needs to calibrate it by comparing it with something else, something of known measure. For your bodyruler, you can use an "ordinary ruler". Averaging can be used to reduce error. For instance, measure out 3 palms and divide by 3, rather than just measuring a single palm.

Here are some measures you might use. I've highlighted my favorites, and included some roman names.

 Measure Explanation my personal calibration Your's will differ. finger finger width (digitus) 2 cm, 3/4 in "nail" width of thumb's nail. (It's about finger width.) ("nail" historically also names some longer units.) 1.5 cm, 5/8 in inch thumb width, "mesouret at the rut of the nayll" 2.5 cm, 1 in half finger fingertip to (middle of) second joint (choose a finger (historically, it was middle)) 5.5 cm, 2 1/8 in finger length fingertip to knuckle 11 cm, 4 1/4 in palm width of 4 extended fingers (palmus) ("palm" is also know as "hand".) 7 cm, 2 3/4 in (at middle joint) 8 cm, 3 in (at knuckles) hand length hand length, heel to fingertip 19 cm, 7.5 in hand span hand width, from outspread thumb to little-finger (Convenient, but because stretch varies, a bit less consistent than these others.) 22 cm, 8.5 in foot foot length, heel to toe (pes) Good for walking short distances. 27 cm, 10.5 in (bare) 30 cm, 12 in (sneakers) cubit elbow to fingertip (cubitus) 47 cm, 18.5 in (~0.5 meter) yard nose to fingertip, with arm straight out to side, head facing front Good for measuring rope and fabric. 92 cm, 36 in (3 ft) (1 meter if I turn my head) fathom fingertip to fingertip, arms out stretched 178 cm, 68 in height the usual 182 cm, 72 in step one step ~80 cm, ~30 in (~3 ft) pace two steps (passus) Good for walking long distances. ~150 cm, ~60 in (~1.5 meter)

## Stuff

Leonardo da Vinci (1452-1519) writes of Roman architect-engineer Vitruvius Pollio (1st century B.C.):
"... Vitruvius declares that Nature has thus arranged the measurements of a man: four fingers make one palm. and four palms make one foot; six palms make one cubit; four cubits make once a man's height; four cubits make a pace, and twenty-four palms make a man's height..." [Klein, pg 68]

It would be nice to discuss measurement error and techniques, usage hints, historical notes on the units, averaging to create standards, perhaps also calibrate precision/errorbars, ...

## Angle

The visual height of your fist, held out in front of you, is about 10 degrees.

You can check this by starting with your arm straight out, and than walking you fists upward, hand over hand. At 9 fists, you should be pointed at the ceiling.

```                              +
+  f
+            f
Eye ddddddddddddddddd f       +
```
A brief elaboration: together, the distance from eye to outstretched fist, and the height of the fist, form two sides of a right triangle, and thus determine an angle. (say 58 cm and 9 cm, "opposite over adjacent" is 9/58, with arctan(9/58) = 9 degrees).

It would be nice to have some notes on usage, on sensitivity to fist distance, on measuring the distance to objects of known size (aircraft, people), on measuring the height of a building by walking towards it, on ...

## References

The Science of Measurement : A Historical Survey / Herbert Arthur Klein. 1988 (Under another title in 1974). Especially pages 53-73.

```History: