Scientific Dials
Olivia Lanni (University of Zurich)
Dialling, then, describes the art of creating a sundial to determine the time based on the position of the Sun. Those proficient in the art of dialling were referred to as “dialists” or “gnomonists.”
“Scientific dials” not only allow the user to tell the time more accurately but were even portable. Other, newer types of dials were developed that were used to measure the movements of the celestial bodies or aid in navigation through the use of fixed stars in the firmament.
The following examples comprise a selection of scientific dials.
The Mathematical Jewel
An astrolabe is an instrument used to make astronomical measurements, such as the altitudes of celestial bodies. In navigation, prior to the development of the sextant in the eighteenth century, an astrolabe could be employed to calculate latitude.
John Blagrave’s 1585 book, The Mathematical Jewel described a new type of astrolabe with a variety of navigational and astronomical applications.
The mathematical ievvel [jewel], shewing the making, and most excellent vse of a singuler instrument so called: in that it performeth with wonderfull dexteritie, whatsoeuer is to be done, either by quadrant, ship, circle, cylinder, ring, dyall, horoscope, astrolabe, sphere, globe, or any such like heretofore deuised.
Blagrave’s instrument also functioned as an armillary sphere, using the geocentric model. The Earth was surrounded by the circles of the great heavens as well as the Sun contained in the ecliptic band of the zodiac. Once it was calibrated to the latitude of the user, it could be applied to various problems of spherical astronomy, for example, to calculate the time of sunrise and sunset on any day of the year.
Read more about the construction and use of Blagrave’s “mathematical jewel” in The Catholique Planisphaer, a 1658 book by John Palmer.
Nocturnal Dials
“Nocturnal dials” or “night dials” are astronomical instruments used to tell the time at night with the aid of fixed stars like the Great Bear. Since each star completes one revolution around the Pole Star within a day, each star could be regarded as an hour-hand in a clock. In The Arte of Nauigation (1589), Martín Córtes refers to the “nocturnal dial” as a “an instrument generall for the houres of the nyght.” Córtes was a Spanish cosmographer whose career was inspired by growing up in a family of sailors, seafarers, and navigators. In his work, he states that after calibrating a dial, it could be used at night:
[W]hen you desire to know the houre, you shal turne the index of the lesse rundell in the which is written, Time, to that part of the great rundell where is marked the day in the whiche you desyre to knowe the houre: and directing your face towarde the North, you shal make the head toward the height of heauen.
After this, he provides further instructions on how to correctly aim the dial towards the northern stars and adjust the moveable arm on the dial to eventually measure the time of the night using the celestial constellations.
The Navicula
The “Navicula” (“little ship”) is a rare type of portable dial. It could be used to calculate time, latitude, the amount of daylight, and to measure the height of objects in the distance. The scale at the front is labeled with zodiacs. The moveable mast contains a scale of latitudes at regular intervals, while the other side is engraved with hour lines.
This sundial also has religious connotations. Its ship-like appearance evokes the idea of life as a pilgrimage, both on land and sea.
Read more about the Navicula in Catherine Eagleton’s 2010 book, Monks, Manuscripts, and Sundials: The Navicula in Medieval England.
Ring Dials
“Pocket dials” came in various shapes and sizes and were primarily luxury items. Though they possessed all the elements of a traditional sundial, the addition of gemstones or elaborate engravings made them fashionable and eye-catching.
One example was the “ring dial,” which once removed from the finger, could tell the exact hour of the day. The sliding piece on the outside of the ring must first be lined up with the corresponding month. A dial is engraved inside the ring and, by holding it up towards the Sun, the light can shine through a tiny hole in the ring, marking the hour on the face inside the ring.