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The Collapse of Compasses that Do Not Copy Segments, and the Lengths We Go To

Date: 04/16/2011 at 09:51:35
From: Steve
Subject: Since when do compasses copy lengths?

Having studied both ancient Euclidean geometry and more recent treatments
of the subject, I am wondering exactly when it was that compasses began to
be used to copy lengths.

I have read that Euclid disallowed compasses from doing this because in
his geometry there was no motion. To me, this restriction makes a lot of
sense both from a practical and a theoretical viewpoint: one cannot set
the compass at a certain length and then truly claim the ability to
duplicate that length anywhere other than at a segment radiating from its
current center. Anyway, Euclid covers this problem in his second

But in our current geometry textbook, my students and I encounter "Copying
a segment is easy: we just set the compass to the length of the segment,
and then copy it elsewhere on the page." Perhaps this concession comes
only after centuries' worth of students saying, "Why can't we just measure
the length and then copy it?" Obviously, this does make a certain type of
sense, but to me it seems a sour departure from Euclid's impregnable 

What really confuses me is that Euclid already HAD a solution to this
problem. Granted, that construction is somewhat involved, even difficult
-- certainly out of proportion for such a simple objective. But not only
does it WORK, it maintains our ability to do consistent geometry without
introducing this vague notion of "moving" lengths.

My current work on this problem involves the simple observation that when
we move compasses, they often shift slightly. Unless we have a drafting
compass or something, the "pick-up-the-compass-and-move-it" part will
usually involve some alteration in the setting of the compass, however

Date: 04/16/2011 at 21:49:41
From: Doctor Peterson
Subject: Re: Since when do compasses copy lengths?

Hi, Steve.

You appear to be aware of most of this, so let me just say by way of
background that I have discussed this matter previously:

  Collapsible Compass 

The key idea here is that, as far as I can see, Euclid put his second
proposition where he did so that he could talk about everything depending
only on his simple postulates, WITHOUT having to use a real collapsible
compass to do his work. See also the Guide section here: 

I can't say that I know anything about the history of compasses. Many
modern compasses (and likely many old ones) can hold their settings
reasonably well. Since using them that way is simply a shortcut to
something that CAN be done with the collapsible compass assumed in
Euclid's postulates (a la Proposition 2, to which you refer), to require
students not to trust them to do this would make constructions a terrible
chore, and would not make math any more understandable, much less

In a course built around postulates, I would certainly mention that the
postulate does not in itself allow transferring a length using a compass,
but that we can use that process as a shortcut to a longer procedure that
could be done with the so-called collapsible compass. But I would not
subject students to more than one exercise in which they actually had to
perform the long procedure of copying a segment! Once they know it can be
done, doing it would be a waste of time. I imagine that may have been true
even in Euclid's time.

Of course I should note that there are also many (cheaper) compasses that
don't hold their settings well enough to make even a single circle
accurately! In my math course for elementary teachers, most of them buy a
compass of poor quality, and I have to take some time to teach them how to
use it so that it will retain its setting (which involves holding the
compass in such a way that all forces are perpendicular to the direction
in which it opens). This is probably a good thing for teachers to have to
learn! For high school students, good compasses would be a good investment.

If you have any further questions, feel free to write back.

- Doctor Peterson, The Math Forum 

Date: 04/27/2011 at 16:25:11
From: Steve
Subject: Thank you (Since when do compasses copy lengths?)

Hi Doctor Peterson,

Thanks for your response!

I certainly agree with your statement that it would be a terrible chore to
explicitly depend on this construction any time we wanted to copy a
segment. From the perspective of logic, it is preferable to have as few
assumptions as possible, and so it does make sense (as you say) that
Euclid would position Prop 2 so early on. That way, what might normally
need to be an assumption is introduced as a "theorem" of sorts, although
an early enough one that it enters practically at the same time as the

Of course in actual practice, we would always use a compass to copy
lengths, since as you say, the alternative would make many constructions
very tedious. I guess my frustration, and concern, comes down to seeing my
students fail to appreciate the distinction between being able to copy a
segment because a compass "gets us close enough" (which is a casual
assumption of the material world) and being able to copy a segment because
of Prop 2 (which is a rigorous theorem derived from reason alone).
Distinguishing between "assumption" and "theorem" in this way likely means
more to me than it does to my students. Perhaps I should wait for them to
mature intellectually a little bit before trying to make them understand
such an abstraction!

Again, thanks.

Associated Topics:
High School Euclidean/Plane Geometry
High School Trigonometry

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