Object Name: vertical disk sundial with nocturnal
Dimensions: 8.5 x 5.7 x 0.3 cm (3 3/8 x 2 1/4 x 1/8 in.)
Description: This is a vertical disk sundial with a nocturnal on the reverse. The suspension structure for the sundial becomes the handle for the nocturnal. The instrument is made of brass and designed for latitude 50°.
obverse: vertical disk sundial
The suspension structure consists of two scrolling arms engraved with acanthus leaves. These come together at the top where they are surmounted by a fleur-de-lys. There is a hole in the fleur-de-lys for the insertion of a hanging thread or ring.
The sundial is laid out on a circular plate with a folding pin gnomon. Across the top, there is a calendar scale marked in zodiacal symbols and divided every 10°. It also lists the civil date that the sun enters that sign by use of the initial of the month and the date. For example: M · 21 / [symbol of Aries] This calendar scale runs from the winter solstice to the fall equinox. Another similar calendar scale (running from the summer solstice to the winter solstice) is below the hour scale.
The hours are inscribed from 5-12-7 and divided every hour.
An inscription above the dial--"honor et gloria ipsi soli"--surrounds a crude attempt at rendering the Hebrew tetragram. To the left of the dial, there is an image of the sun with the words, "Sol cognouit occasum suum." At the right edge of the sundial, it reads, "Post tenebras / spero lucem." Other areas are filled with images of the moon, stars, and clouds.
on reverse: nocturnal
The handle of the nocturnal is similar in style to the suspension system of the sundial. Hours on the nocturnal run I-XII twice and are divided by 1/4 hours. Inside this circle, there is a volvelle with a turning knob. Concentric scales show the age of the moon, 1-29 1/2; followed by days of the months divided every 2 days; names of each month in abbreviated French; degrees of the zodiacal calendar divided every 2°; abbreviated zodiacal names. Next comes a rotating volvelle with an index arm (used to sight the stars). The index arm is ornamented with acanthus leaves and a trimmed pointer. An aperture in the volvelle reveals lunar phases. Ornamentation in the center consists of an 8-point flower formed of acanthus leaves with stars between the petals.
Function: The vertical disk dial is a form of altitude sundial. To tell the time, the dial is suspended from a cord, and the pin gnomon is hinged out so that it is perpendicular to the disk. The dial is rotated to face the sun and let the shadow fall flat on the disk. Time is read on the portion of the hour scale associated with the correct date.
The nocturnal is a device for finding time at night from the stars of Ursa Major (or sometimes Ursa Minor). After setting the volvelle for the correct date, the user sights the Pole Star through a hole in the center of the disk, and aligns the index arm with the "guide stars" of Ursa Major (aka the Big Dipper). The time is read off hour scale. The lunar scales on this example can also be used for determining the times of tides.
Curatorial Remarks: XRF analysis of the metals was done by Katherine Eremin and Sara Schechner in August 2007.