Miniature cannon dial, suspended on a ring. Brass circular base with silvered hour plate marked in Arabic numerals. Brass gnomon for 45°. Burning lens held on two adjustable brackets. Little cannon painted brown and black.
ISBN
i720\qjApparent solar time was widely used through the 19th century; in such circumstances, it was convenient to let the Sun command a time signal itself. This idea had led to the development of the noon gun in the previous century. A noon goon is an apparatus combining a miniature cannon with a lens that focuses a beam of light toward the touchhole, igniting the gunpowder at noon. The example in Fig. 1, accessioned in our collections as M-441, is a French noon gun, dated from the turn of the nineteenth century. Note the solid marble base, the plumb bob to check the horizontality of the apparatus, and the compass to align the cannon with the local meridian. \n \n In order to prepare the device for the noon signal, the cannon would be loaded with a teaspoonful of black powder, with some more powder being sprinkled in the touchhole. The device had also to be adjusted for the altitude of the Sun at noon, since the latter varies with the latitude and the time of the year, being higher around the summer solstice, and lower around the winter solstice. For that purpose, the burning lens was usually attached to a frame with two arms pivoting around one quadrant (sometimes two, on either side). In M-441, the quadrant has a scale of altitude, from 0 to 90 degrees. The desired altitude for a given date could by found through a simple calculation combining the latitude of the place with the declination of the Sun provided by an almanac (the declination of a celestial object is the angle between the object and the celestial equator). The lens would be set accordingly, then it was just a matter of waiting for noon and amuse oneself with the cannon blast, run to grab lunch, or check out one's pocket watch, as the gentleman in Fig. 2 is doing (note that nowadays we use an adjusted and standardized time scale in our clocks and electronic devices, not the time taken directly from the Sun). \n \n But what if you needed to check the solar time during the morning without having to wait until noon, or after missing the gunshot? The solution was to combine a noon gun with a horizontal sundial - a type of sundial which, as the name indicates, combines a horizontal hour scale with an upright triangular gnomon (the gnomon is the piece that casts the shadow on the hour scale). This arrangement is called a cannon dial. You can see a fine example from our collections, M-284, on display at the Astronomy in Culture gallery (Fig. 3). M-284 is a French cannon dial from the late 18th century, set on a round marble base. The sundial is designed for the latitude 45 27'. The quadrant on the west side of the apparatus has a scale labeled with the French initials of the months, so that the burning lens could be adjusted for the altitude of the Sun at noon throughout the year.\n
