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The model is a reconstruction of the original fragmentary
instrument, believed to date from the early sixth century A.D.,
which is also on display (Field & Wright, 1985).
The instrument comprises two practically independent parts: a
sundial for use at any latitude, and a geared calendrical device
showing the phase of the Moon, the day of the month and the
places of the Sun and the Moon in the Zodiac.
This type of sundial is attested by a several examples, some
inscribed in Latin and some in Greek (Field, 1990). None is
securely dated, but the archaeological record suggests that
these instruments were widely distributed within the Roman and
the early Byzantine empires. The design probably corresponds to
that described by the Roman author Vitruvius (late 1st century
B.C.) as “pros pan clima” (for every latitude), suggesting a yet
earlier Greek origin. |
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The dial occupies most of one face of the instrument. It
comprises a piece that is both shadow-caster and hour-scale, the
central pin of which passes first through the circular body and
then through the swinging arm at the back to which is jointed a
ring by which the instrument is hung upright.
The dial occupies most of one face of the instrument. It
comprises a piece that is both shadow-caster and hour-scale, the
central pin of which passes first through the circular body and
then through the swinging arm at the back to which is jointed a
ring by which the instrument is hung upright. Two scales on the
body enable the user to adjust the first part to the elevation
of the Sun at noon, according to the place and the time of year:
the shadow-caster is moved over a double scale of solar
declination, marked out with abbreviations of the Julian
month-names; and the arm is adjusted according to a quadrant
scale of latitude near the rim of the body. The dial is then
held up, and rotated until the shadow of the projecting part
falls along the curved scale, whereupon the user may read off
the morning or afternoon hours. Much of the rest of the face of
the dial is taken up with a reference table of place-names and
their latitudes.
The known comparable dials are smaller and are based on flat
circular discs. In this case, uniquely, a hollow box, which
takes the place of the disc, contains a geared calendrical
mechanism which is worked by turning a pointer on the face of
the dial.
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The pointer moves over a circle of seven incised heads
representing the seven days of the Judaeo-Christian week. A
ratchet inside prevents the user from turning it backwards.
Simple gearing in the ratio 7:59 rotates a disc making one turn
in 59 days which displays the day of the month (alternately 29
and 30 days in length) and an approximate representation of the
phase of the Moon, through openings in the back of the box.
Following the description of a similar instrument by al-Bīrūnī
(Hill, 1985), the remainder of the mechanism is restored to
drive indications of the places of the Moon and of the Sun in
the Zodiac. A more elaborate reconstruction might include a
display of the Moon’s nodes (enabling the user to predict the
possibility of eclipse) or mechanism whereby the position of the
shadow-caster and hour-scale is set automatically by the
calendar (Wright, 1990); but these possibilities have no
historical basis. |
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The Catalog pages on
sundials.
Bibliography :
J.V. Field & M.T. Wright, “Gears from the Byzantines: a Portable
Sundial with Calendrical Gearing”, Annals of Science, 42 (1985),
pp. 87 – 138.
D.R. Hill, “Al-Bīrūnī’s Mechanical Calendar”, Annals of Science,
42 (1985), pp.139 – 163.
J.V. Field, “Some Roman and Byzantine Portable Sundials and the
London Sundial-Calendar”, History of Technology, 12 (1990), pp.
103 – 135.
M.T. Wright, “Rational and Irrational Reconstruction: the London
Sundial-Calendar and the Early History of Geared Mechanisms”,
History of Technology, 12 (1990), pp.65 – 102.
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