Medieval Scientific and Philosophical Instruments
(Summer 1999)
Introduction:
For this workshop I have brought instructions and materials for
you to make three different instruments typical of the Medieval
period. Two of them, the cross-staff and the quadrant will be
useful if you come to this evening's star-gazing for measuring
the positions of celestial objects. Before we begin I would like
to show you some instruments I have made, and discuss briefly
some of the techniques I have used in making them, and relate
how my methods compare to techniques current in the late Medieval
period. You will find additional details about these instruments
and techniques on my web-site.
I have brought a number of instruments that
I have made over the years for your inspection: a cross-staff,
a navigational astrolabe, a planispheric astrolabe, an armillary
sphere, and a torquetum (see the instrument
page on my web-site). With the exception of the cross-staff,
described in the handout, all of these instruments are fabricated
from copper and/or copper alloys.
- Navigational Astrolabe:
This is the first instrument I made (c 1985). The body was cut
out from a bronze plaque and is about 1/4" thick, as compared
to authentic examples which are more like 3/4" thick in
order to give them stability on a rocking ship in the wind. In
this early example I made graduations using a protractor with
a hole cut at the center and a steel rule firmly attached along
the 0° line. I then scribed a vertical line on the astrolabe
blank and attached the protractor with a bolt through a hole
in the astrolabe's center. I could then scribe lines with the
rule, aligning the degree marks on the protractor sequential
with the vertical line. This was a simple, reasonably accurate,
and inexpensive mode of graduation. Though I have never seen
this method noted in the literature, it certainly would not be
impossible in ancient times. Who knows, since we rarely have
records from Medieval craftsmen, a variation on it may have been
used by someone.
- Planispheric Astrolabe:
This instrument is made of copper and brass, both metals commonly
used for instruments in Medieval times. The blanks were cut from
14 gauge sheet with a power saber (or jig) saw, and then trued
up to round with a file on a wood lathe. Careful,
this can be quite dangerous, as copper alloys are "sticky"
and can grab, making the file into a deadly projectile!. And,
of course, the spinning blank can act as a blade and slice a
hand if you slip. For graduating this instrument I used a
rotary table (commonly found in machine shops, though mine was
modified from a micrometer actuated transit) with a fixed rule
held above it. The metal blank was then centered and fixed on
the table and lines scribed with a knife held against the rule.
This enables slightly cleaner and deeper graduations than the
scriber. This method of graduation would not have been used until
the late 18th century with the advent of dividing engines. To
cut out the rete I used a jewelers saw and then cleaned up with
needle files. Note that this is an unlikely scenario for early
craftsmen. High quality steel required to make the fine blades
of jeweler's saws was not available until near the 19th century.
Thus the rete's of astrolabes were probably made by drilling
holes and then cutting and shaping with files.
- Armillary Sphere: The rings
were graduated as above with the planispheric astrolabe. The
meridian ring was cut from 3/8" brass plate on a metal lathe,
the horizon ring was cut from a bronze plaque with a power jig
saw, then trued up on a lathe, and the remaining rings were fabricated
by bending and joining straight stock.
- Torquetum: The various circles were cut from brass stock after
lay-out with wing-dividers sharpened to round points - you don't
want any "knife-edge" as they will then tend to pull
instead of cutting a clean arc (trammel points will work equally
well, but you need something that can be locked at a dimension).
The dividers were then used to layout the radius of the circle
to give arcs at 60° intervals (note that this is theoretically
absolutely accurate, the precision attained in laying out these
divisions is limited only by your tools and your skill!). Additional
divisions at 30° intervals were then accomplished by bisection.
Bisection of the 30° intervals will then give 15° intervals,
however, at this point division by divider becomes more problematic.
This method of division was the method from ancient times through
the 18th century, and in fact was used for observatory instruments
through the 19th century. I did the remaining division using
my "dividing engine," an extension of the method described
for the planispheric astrolabe above.
- Projects. Three
simple projects were constructed during this workshop. These
projects are illustrated along with instructions for construction
and sample calculations via the links below:
-
- © R. Paselk
- Last modified 16 August 1999
Medieval Science & Scientific
Instruments