Discusses the concept of measurement in general before exploring how we might define a unit of measure for angles, and design a tool for the purpose (which, in its refined version, is what we now call a "protractor"). Shows how to use a protractor to draw angles of various sizes, then ends by introducing the concept of the radian measurement.
8. and also the highest angle the Sun
reaches at each place on a given day,
Highest angle at Location A Highest angle at Location B
Sky simulations made with the free astronomy program Stellarium
(http://stellarium.org/)
9. we can calculate
the size of the Earth.
The first person to do this
calculation was Eratosthenes, a
librarian at the Great Library of
Alexandria, Egypt, around 200 BC.
10. Actually, almost all of astronomy
and geography depends upon
being able to measure angles.
So do many jobs, such as
surveying.
11. So, how might we measure
angles?
First, let’s review what we mean by
“angle”, and “measurement”.
12. One definition of “angle” is
“A pair of rays that
have the same endpoint”.
The endpoint,
V, is called
the vertex.
13. You’ve probably learned that
angles are related to “turns”, and
that the same set of rays can be
made by two different turns.
For example, here are …
14. the two “turns” that take us from ray
VA to ray VB.
Counterclockwise turn Clockwise turn
15. Now that we’ve reviewed what
“angle” means, what do we
mean by “measurement”?
16. A good definition is found in The
Archimedes Codex: How a
Medieval Prayer Book is Revealing
the True Genius of Antiquity’s
Greatest Scientist:*
* Reviel Netz and William Noel, Da Capo Press,
2009, p. 41.
17. “To measure is to find a
measuring tool and apply it
successively to the object being
measured. Suppose we want to
measure a straight line.
According to the authors,
18. “For instance, suppose we want to
measure your height, which is
really saying that we want to
measure the straight line from the
floor to the top of your head.
19. “Then what we do is take a line
the length of an inch [this is our
measuring tool] and apply it
successively, well over sixty times,
but probably fewer than eighty
times to measure your height.
20. “Since this is very tiresome, we
have pre-marked measuring tapes
that save us the trouble of actually
applying the [one-inch line]
successively,
21. “but, at the conceptual level,
successive application is precisely
what takes place.”
22. That definition of “measurement”
needs some explanation. For
example, when we read
“To measure is to find a measuring
tool and apply it successively to the
object being measured,”
23. we probably thought of “measuring
tools” as rulers, etc.
Actually, the authors meant
something quite different.
24. They meant that to measure a
length (that’s the example they
give), the “tool” we choose is a line
segment with a convenient length.
25. The authors mentioned “an inch” as
an example of a convenient length,
but we could use a segment of any
length we like.
84. Now that we have our angle-
measurement tool, and know how
to use it, we can look for ways to
make a more-convenient version of
it.
85. We already know that we can make
a more-convenient version of a
length-measurement tool by pre-
marking “something” at chosen
intervals, to make a tape or ruler.
138. Here are links to more information
about reflex angles:
Reflex angles
http://www.mathsisfun.com/reflex.html
Re-entrant angles
http://www.thefreedictionary.com/re-
entrant+angle
139. Besides wanting to measure
angles, there are times when we
want to (or must!) draw them.
166. Summary (cont’d)
• When we need to find a way to
measure something, we can
invent our own “tools”.
167. Summary (cont’d)
• It’s often helpful to imagine some
simple version of a tool, or of
solving any problem, then look
for a way of making that simple
tool or technique more
convenient.