A well-made, highly unusual instrument that is self-orientating due to the circular disc or halo set at the equinoxs. In use, once the instrument has been leveled and adjusted for latitude, the dial is rotated on its base until the shadow of the upper edge of the halo falls on the appropriate date when local solar time will be shown by the gut-gnomon. The principle of operation is similar to that which underlies the functioning of the common pillar dials.
According to the footnote written in 1995, when this piece was originally offered at auction, only one other example of the dial was known. The other dial is in the Instituito Geografico-Militaire, Florence and is illustrated in Bulletin of the Scientific Instrument Society, No. 45, 1995, pl. 7 fig. 3
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