Compass
and straightedge constructions have been in the geometry curriculum for a loooonng
time. These constructions were around before Euclid included them in his
textbook, *The Elements*, 2300 years ago.
The painting shown here is an illumination from a translation of that book 1600
years later. I had to do constructions in junior high; I don’t remember being
quite as unhappy about the endeavor as these monks or the woman teaching them. And
now constructions are in the Common Core, too. Why are we eternally requiring
students to construct things with these arcane and arbitrary tools?

Here
is one answer: Plutarch tells a story of Plato and his buddy Simmias of Thebes
coming back from a trip to Egypt when they encountered a delegation from the
isle of Delos. The famous oracle at their temple had told them that the Delians
and the other Greeks could put an end to their current troubles if they could
figure out how to double the temple’s altar, which was a cube. In other words,
they had to make a new cubical altar that was twice the volume. They told Plato
that they had tried to double all the dimensions, but they ended up with an
altar that was 8 times the volume, not double the volume.

Plato
told them that this was not a problem for estimation but for precise
construction. (It can be solved by finding two lengths in mean proportion, in
other words, construct a length of the cube root of 2.) Although a few
Athenians knew how to do it, the point was that the Delians should figure it
out themselves. The oracle’s motive was that the Greeks would only end their
troubles when they focused on cultivating the art of mathematical problem
solving, including making logical arguments for their constructions. This would
give them the ability to sort out their differences through *reason* rather than *force.*

This
classic problem of doubling the cube turns out to be impossible with only a
compass and straightedge, but you can do it with other Greek tools (and you can
do it with paper folding, too!). The work that occurred on
this problem led to entirely new fields of mathematical study in ancient Greece.

I
think maybe the reason we keep doing constructions is that it allows us to
engage in at least two of the Common Core’s Standards for Mathematical Practice
(SMP 1: Make sense of problems and persevere in solving them and SMP 3:
Construct viable arguments and critique the reasoning of others). By forcing
ourselves to figure out how to do something with a limited set of tools, we refine
our problem-solving skills. By forcing ourselves to justify why we know our
constructions will always work, we improve our ability to argue rationally, to
show how ideas logically rely on other ideas.

Maybe
the oracle of Delos was onto something. . . .

*Rob Ely is an associate professor in the Department of Mathematics at the University of Idaho. He likes researching student reasoning and how it relates to historical reasoning, particularly with the infinite, infinitesimal, and algebra. He likes playing hammered dulcimer, bridge, and fetch.*