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Astronomical Compendium
Signed by Tobias Volckmer
Dated 1584; Brunswick
Gilt brass; 70 x 70 x 17 mm

The compendium consists of a central box for the compass with front and back covers. The covers are hinged on opposite sides of the box. The decorative scheme uses fruit in corners and free space, though the sides have strapwork and bound wreaths. Most of the letters and numbers are punched rather than engraved.

The front cover carries a circle of degrees 90 to 0 to 90 to 0 to 90, divided to 10 and 2, and numbered 10, 30, 50, 70, 90. The lower semicircle with the scale of degrees is occupied by a shadow square with its elements marked 'VM. RECT.' or 'VM. VER.', divided to 6 and 2, and numbered by 6. The upper semicircle has a rectangular grid with points marked 6 and 12. An alidade would originally have been present.

The instrument is signed within the shadow square: 'THOBIAS VOLCKMER BRAVNSWEIGSENSIS FACIEBAT ANN CX 1584'.

On the reverse of the cover is a horizontal dial with circular hour scales for latitudes 45?, 48? and 51?. All are subdivided to half hours; the inner and outer rings are numbered 4 to 12 to 8, while the middle ring is marked IIII to XII to VIII. A folding brass gnomon survives but is now detached and cannot be adjusted for latitude. It may be a later replacement. The whole dial is twisted slightly so that north is inclined westwards. The purpose of a small stud at 1 o'clock on the outer hour scale is unclear.

Two supporting struts enable the front cover to be held open.

Facing the dial is the compass. The bowl has a silvered base plate with engraved needle indicator; the needle itself and the glass are missing. The bowl rotates within a disc and a pointer enable its direction to be set. The disc carries a circular scale of compass directions, with eight arrows marked in German for the principal directions and a further division of eight lines. Beyond the compass scale is an hour ring 1 to 24. The circumference of the disc carries an unnumbered scale of degrees divided by 10 and 2, with additional graduations to correspond to the odd hours.

The combination of compass bowl and disc is held in the box by two retaining clasps and can be removed. Six studs permit the disc to be easily rotated.

On the inside of the back cover is a nocturnal with lunar volvelle. The nocturnal is constructed for a star whose right ascension is the same as the sun's on about 20 August. The folding portion of the nocturnal's index arm is now lost.

The nocturnal plate has an outer calendar scale with abbreviated names of the months in which each month is divided by 10 and 2, and numbered by 10 (adjusted for the length of the month). Inside the calendar is a scale of hours I to XII, I to XII for the moon. The innermost scale on the plate is revealed only by a cut-away portion of the volvelle and is for the age of the moon 0 to 29[1/2].

The volvelle has two scales of hours for time telling by the nocturnal: 1 to 24 and 1 to 12, 1 to 12. The main pointer is at 12 / 24 while the cut-away pointer for the age of the moon is as 12 / 12. A circular aperture reveals the phases of the moon, and the centre of the volvelle carries a diagram of astrological aspects for opposition, trine, quadrature and sextile.

The back cover carries a quadrant of degrees 0 to 90, divided by 10 and 2, and numbered by 10. A hole at the vertex of the quadrant would have been used to attach a plumb bob (now missing). Inscribed within the quadrant is a shadow square, its sides marked 0 to 12 'VMBRA RECTA' and 'VMBRA VERSA', divided to 3 and 1, and numbered by 3. The shadow square encloses a square grid similar to that on the front cover. An empty shield in a corner was presumably supplied to accommodate a coat of arms.

Three of the four sides carry a scale of equal parts 0 to 12, divided to 3, 1 and 1/2, and numbered by 3. Each of the three units is of a different length.

Stephen Johnston

Museum of the History of Science, Oxford
Inventory number 34940

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