Since the complex planispheric astrolabe
is much earlier than the Mariner's astrolabe, the Mariner's astrolabe
is considered to be derived from it. The transitional form of
astrolabe would then have the altitude reading scales and an
alidade. Only later, with experience, would the cutouts appear
to reduce the effects of wind when taking readings on the open
sea.
The instrument here is modeled after one
claimed to have belonged to Christopher Columbus. It is known
the he had an astrolabe, but he apparently found it unusable
and navigated without it. Details about the original instrument,
including scale drawings, may be found in Alan
Stimson's book in References. His
scale drawings were used in making this instrument, after scaling
them up to life-size. The resulting instrument has the same dimensions
and appearance of the original, excepting the thickness of the
metal: the mater of mine is made from 14 G copper sheet rather
than the thick castings normally used in mariner's astrolabes.
In the discussion below the numbers refer
to the parts in the exploded view below:
Materials:
Mater (body of instrument-1): 14 G
copper sheet.
Alidade (2): 14 G brass sheet.
Horse (3): 10 G brass sheet.
Sights (6): 3/16" plate.
Axis pin (4) and rivet for hanger
(8): 3/8" dia brass rod.
Suspension ring (7): 1/4".
Washers (5): commercial 1/4"
brass.
Construction:
The outline of the mater (1) was transferred
from a life size photocopy of the scale drawing from Stimson's
book using dividers, after first marking the center with a deep
punch prick. It was then cut out using a high quality power "jig-saw"
with a metal cutting blade. The final outline was accomplished
by hand filing.
The sights (6) were soldered to the
alidade (2) with lead/tin solder (I had not yet mastered
silver soldering when I made this instrument).
The rivet (8) and axis pin (4)
were turned on a lathe to 1/4" leaving a 3/8" head
on one end. Two holes were then drilled through the shaft of
the pin and expanded and shaped with files to fit the horse.
Medieval Science & Scientific
Instruments