Sky & Telescope - June 2021 - 72

ASTRONOMER'S WORKBENCH by Jerry Oltion

Replicating
the Great
Quadrant
History repeats itself,
beautifully!
WHEN YOU THINK OF medieval rec-

JUNE 2 021 * SK Y & TELESCOPE

S Jon Voisey and his friend Meg Cassidy take a measurement with Jon's replica of Tycho Brahe's
Great Quadrant.

observations from this instrument, Jon
intends to collect sufﬁcient data of his
own to follow Kepler's methodology and
derive the orbits of the naked-eye planets. Because Kepler used measurements
taken at opposition, and planetary
oppositions are relatively rare events,
this will take many years. So he's also
using the quadrant in the meantime to
plot the position of hundreds of stars
as Tycho did as well, leading to the ﬁrst
accurate star map, Johannes Bayer's
Uranometria, in 1603.
Jon's father is a contractor and
carpenter, so he helped Jon build the
Quadrant over the weekends of almost
two months. They started with the
curved arc, which they drew on a sheet
of plywood and cut with a jigsaw. Then
they made the internal lattice, ripping
2-by-4s down to 1½″ × 1½″ and making
cross-lap joints with a dado blade. They
built the 5″-square central column out
of plywood and the base out of plywood
and 4″ × 4″ posts. They then made an
axle out of ½″ iron pipe and hung the
quadrant from the axle. A plumb bob
also hangs from the axle and is used to
level the instrument as well as take the
reading where the plumb line crosses
the scale.
X The lattice was made with 1.5″ wooden
strips bearing cross-lap joints.

Drawing that scale was the killer.
Tycho boasted that his instrument was
accurate to 1/6 of one minute of arc.
Jon's replica is smaller, so his scale is
smaller as well, but he wanted it to
at least be accurate to within a few
arcminutes. Unfortunately, the period
methods used to create accurate scales
proved elusive, so Jon made the scale in
the drafting software program AutoCAD. After printing, he transferred the
scale to the wood by cutting through
the drawing with a razor knife, then
enhancing the marks with a Sharpie.
The scale zigzags to provide greater

GRE AT QUADR ANT REPLICA: WILL DAY; ORIGINAL QUADR ANT: ASTR ONOMIAE
INSTAUR ATAE MECHANICA; L AT TICE UNDER CONSTRUCTION: JON VOISE Y

reation, you probably think of Renaissance clothing, feasts, calligraphy, and
swordﬁghts. All of these, and more,
are the sorts of things that members of
the Society for Creative Anachronism
(SCA) engage in. It's a group dedicated
to understanding history through
engaging in it. But one area that's often
overlooked is astronomy.
When you think of scientists, you
often think of researchers breaking new
ground with new discoveries, but one
of the most important things a scientist
can do is to replicate the methods and
the results of a previous experiment.
Understanding how it was done is both
personally satisfying and scientiﬁcally
(and historically) important.
Enter Jon Voisey, known in the
SCA as Lord Jon Chesey of the Barony of Three Rivers in the Kingdom of
Calontir, who also holds a degree in
astronomy. He fused these two passions after becoming interested in how
Johannes Kepler worked out his elliptical heliocentric model of planetary
motion before the invention of the telescope. Jon wanted to recreate Kepler's
discovery, using Kepler's methods. So,
Jon, like Kepler, turned to Tycho Brahe,
whose instruments measured the night
sky with a precision unmatched in his
time (the second half of the 16th century) and whose data laid the foundation for Kepler's work.
To take his measurements, Jon
decided to build a replica of Tycho's
Great Quadrant, one of the instruments with which Tycho measured the
position of the stars and planets. Using

Sky & Telescope - June 2021

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