Byzantine sundial calendar

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Byzantine calendar
Manufacturer (unknown)
Production years late 5th, early 6th century A.D.
Production location Byzantine Empire

A portable sundial combined with a geared calendrical mechanism. The style of the Greek lettering, the ring of incised heads, and the set of place-names, suggest an origin within the Byzantine empire in the late fifth or early sixth century A.D. The instrument shows that the use of geared mechanism in an astronomical context, first found in the Antikythera Mechanism, continued into the Byzantine era and was then adopted by the Arabs.

How it works

The large circular disc formed the basis of the sundial. A lost part, combining shadow-caster and hour scale, was fitted to the central hole, and the scales around the hole, marked with abbreviations for the Julian months, enabled the user to set it to the Sun’s declination for the time of year. At the edge of the disc is a scale of degrees for setting the instrument to the user’s latitude, and there is a table of the names of cities and provinces with their latitudes. A second hole, for the stem of the mobile by which the calendar is worked (described below), is encircled by seven incised heads representing the days of the Judaeo-Christian week. A lost rim, fixed to the back of the disc, formed a shallow box. The suspension arm lay behind the box, held in place by the stem of the shadow-caster. Its hooked end embraced the rim of the box, so that its pointer registered on the scale of latitude. When used as a dial, the instrument was held up by the ring. The other two pieces were parts of the internal calendrical mechanism. One has two small gears, of seven and ten teeth, and a seven-lobed ratchet. Its stem, which projected through the offset hole in the disc and carried a pointer indicating the day of the week, was turned to set the calendar. A pawl, engaging the seven-lobed ratchet, ensured that the user always moved the calendar forward, one step each day. Thus the smaller gear, with seven teeth, moved forward by one tooth each day. This gear engaged the larger one on the other surviving piece which, having 59 teeth, therefore made one turn in 59 days: an approximation to two synodic months. Its face, brightened by tinning, formed part of the display. The numbers 1 to 29 and 1 to 30, the days of two consecutive months, are engraved near the edge, and it has two large circular holes which were probably filled with some dark material. This wheel rotated behind two openings in the lost back of the box, showing the day of the month through a small hole and a crude representation of the phase of the Moon through a larger circular one. Behind the large wheel is a smaller one of 19 teeth. This and the second gear on the other mobile worked additional display elements. Following a description by al-Bīrūnī (c.1000 A.D.) of a similar geared calendar, these were probably indications of the places of the Sun and Moon in the Zodiac (Hill, 1985).


In the Science Museum's Records

Inv. No: 1983-1393