I am sending this to the sundial list ([email protected]) and scientific instrument list ([EMAIL PROTECTED]), where nocturnals have been discussed recently, in the hope that someone may be able to throw light on a mystery. I have two books which illustrate the same object in the Musees Royaux d'Art et d'Histoire, Brussels. They agree that it is possibly French. That's about the limit of the agreement.
The first book, "Scientific Instruments in Art and History" by Henri Michel (Barrie and Rockliff, 1967), says the object is an eighteenth century lunar dial, which can also be used as a sundial. The second, "The World's Great Clocks and Watches" by Cedric Jagger (Hamlyn, 1977) says it is a nocturnal (telling time from the position of the Pole star) dated c1582. Somewhat incompatible descriptions! My inclination would be to trust Mr Michel, especially as Jagger also shows a portable horizontal sundial with hours increasing clockwise which was "made for use in Australia"! But that date of 1582 sounds confident and Jagger was the Keeper of the Collection for the Worshipful Company of Clockmakers. Whereas Michel's book was translated by the then Curator of the Museum of the History of Science, Oxford and the Librarian of the Royal Astronomical Society. Heavyweights all round, then. I will try to describe the object. It is made of gilt brass, and is portable. The 37x28cm base has a compass, spirit levels and adjustable feet, so it can be set to the meridian and levelled. >From the centre of the base a post sticks up vertically. It supports an assembly which can be tilted according to the latitude (says Michel) so that its axis is polar. The assembly, which looks rather like a wide-brimmed hat, comprises: - a brass disc, about 25cm diameter, free to rotate on its (polar) axis, graduated anticlockwise in 2x12 hours and minutes - a crescent 'gnomon' of sheet brass fixed below the disc at 12 o'clock, its plane being radial. - a fixed pointer by which the orientation of the above disc can be read to the nearest minute. - a short tube with an angled top, supporting a smaller brass disk. The angle between the two disks is 23.5 degrees. Although neither book says so, it seems that the tube/small disc can be rotated either with or independently of the large disc. - the smaller brass disk with an alidade. The disc is graduated clockwise with dates (and signs of the Zodiac) with the end of December at the point nearest to the large disc. The alidade can be set to the current date, and thus to the sun's declination. - the alidade, which carries a foresight and a backsight. These have small holes in them, and notches in their top edges. Michel's description of the device as a sundial is convincing but incomplete: set the base horizontal and in the meridian, set the inclination of the disc assembly to the latitude, set the alidade to the date, rotate the whole assembly until sunlight passes through the sight holes, read the time from the fixed pointer. I believe this omits a vital step: It must be possible to turn the big disc to align the 'gnomon' with the alidade, otherwise it will tell sidereal time. But what a lot of work. Why not merely rotate the big disc until the gnomon's shadow is a thin line? Also, aligning the gnomon to the alidade would be much easier if the gnomon were above, not below the disc. Michel says that to use as a moondial, you do indeed rotate the big disc until the gnomon's shadow is a thin line, then use a scale on the base to adjust for the moon's phase. This seems totally plausible. The base has a circular scale graduated 1 to 28 or so and a cardioid design which might represent the moon's phase. Jagger does not describe how to do so, but says the dial was "intended to show the hours during the night by observing the position of the Pole star in relation to certain other stars." I can see no way that this could be true. There is no way to align any part of the device with the pole star or the pointers of the big dipper. Both authors say it is probably French, but the only words visible in their illustrations are the month and Zodiac names in English! So it seems to have been made for an English-speaking user. And what date could it be: 1582 or 18th century? From its decorative style I am inclined to the former. The photographs are too indistinct to be sure of the date of the solstice, but it seems to be about December 20th, which would indicate it was for the Gregorian calendar. 1582 was the very year this was introduced into Roman Catholic countries, but not England. Quite a puzzle. I trust this will reach someone who can solve it. Regards Chris Lusby Taylor Email: [EMAIL PROTECTED] (Formerly [EMAIL PROTECTED])
