Medieval Astronomical Timekeeping Instruments
For this workshop I have brought instructions and materials for
you to make two different time keeping instruments typical of
the 11th-13th centuries. Before we begin the projects I will show
you some instruments I have made which illustrate the operation
of these instruments in terms of astronomical observations and
phenomena. I will also 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
I have brought three instruments that I have
made over the years: an armillary sphere (a Ptolomaic, or Earth-centered,
model of the Universe), a planispheric astrolabe (a combination
observational tool and 2-D model of the heavens), and a torquetum
(a complex observational tool, which also illustrates different
reference frames useful for astronomical observation). Detailed
descriptions of these instruments are available via the instrument
page on my web-site. All of these instruments are fabricated
from copper and/or copper alloys.
- 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. Two
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 23 March 2001