A unique manuscript lunar computer, circa 1800 unsigned

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A unique manuscript lunar computer, circa 1800 unsigned
Inscribed Instrumentum Lunarem cujus ope Locus medius Solis, Lunæ, ejusdemque Apogæim Solis Declinatio, Lunæque. Anomalia media necnon hujus Æquatio Centri, Motus horarius Parallaxis horizontalis. Diameter apparente atque Distantia a Tellurem inveniantur.
The volvelle consists of a circular cardboard base plate with, on top, three rotatable circular cardboard volvelles which are attached to each other in the centre, together with a brass pointer. A ring for suspending the volvelle is fixed to a brass construction attached to the disc. The three volvelles and the base plate are inscribed in manuscript with various scales drawn in colours. The smallest volvelle has scales for the distance of the Moon to the Earth: Distantia a Tellure in Milliaribus Geographicus (varying from 195963 to 218762 geographical miles); the apparent diameter of the Moon: Diameter apparens Lunæ (varying from 29'22" to 33'32"); the horizontal parallax of the Moon: Parallaxis horizontalis Lunæ (varying from 54' to 60'17"); the mean hourly motion of the Moon: Motus horarius Lunæ (varying from 29'34" to 36'48.5"); the equation of the centre: Æquatio Centri subtrahenda a Longitudine media Lunæ (varying from 0° to 6°18'32"); and finally, the mean anomaly of the Moon: Anomalia media, followed by the symbol of the Moon and the positions of the Apogæum and Perigæum. On the next volvelle the years from 1801 till 1932 are distributed over eleven cycles of the twelve months of the calendar, such that in each cycle the year number increases by eleven each subsequent month. The following volvelle carries the Zodiac with delicately drawn pictures and the symbols of the signs. below the Zodiac is a scale for the declination of the Sun throughout the year and 4 sequences of 3 years with 44-year intervals are inscribed: 1801-1845-1889, 1812-1856-1900, 1823-1867-1911, 1834-1878-1922. On the outside rim of the Zodiac is a calendar with the names of the months as usual, and outside this, there is another calendar scale in which the months are unevenly distributed over more than a full circle. This scale presumably has to be used in conjunction with the outer scale on the circular base plate. This latter scale gives the days, subdivided in hours. The days are numbered I till XXXI, with the number I coinciding with XXVII and 8 hours (the sidereal period of the Moon), which results in a double numbering from XXVIII til XXXI. Below this scale for days, there is a series of dates, all confined to the period 1801-1830, presumably for solar eclipses, 18.1/2in. (47cm.) diameter

See Colour Illustration and details

Lot Essay

This sophisticated lunar volvelle seems to have been designed for calculating rather accurately the orbital features of the Moon as they are usually encountered in a lunar ephemeris. By setting the various volvelles, these orbital elements can be obtained at any time and day in the period of 1801-1932. However, the precise way to operate this `lunar computer' is not clear, and neither is its possible significance for the prediction of solar eclipses. The values used for the scale of the apparent diameter of the Moon, its horizontal parallax and the equation of the centre, agree with those reported in Jerome Le Franais La Lande, Astronomie (Paris, 1792), which seem to justify a date of around 1800, as indicated by the series of years, which start at 1801.

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