Mysterious Moon

Representation of the Moon on astronomical clocks had to overcome the essential problem of the moon phases. The Moon and the Sun are the most distinct moving objects in the sky and the moon phases cannot be omitted. By comparison with examples of moon phase indicators on other astronomical clocks, the uniqueness of the movement of the Moon on the Prague clock will be distinct. We are of course interested in mechanical, not electronic solutions.

A simple solution consists in the spherical shape of the Moon; a half of the sphere independently protrudes from the body of the clock. The phases are then indicated by mere rotation; at the moment of new moon, the dark side is usually covered with stars. The sphere does not have to be dependent on the movements on the astrolabe. Another similar and frequently used solution is a circular plate with painted Moon that moves behind a carved mask, which gradually uncovers or covers the Moon. It is usual for both cases that the moving symbol of the Moon is present on the astrolabe as well; but there it only indicates its position between the Zodiac Signs. The phases are thus separated. There are also other constructions where the Moon has its own additional dial for the Zodiac Signs, indicated by another pointer.

If the Moon symbol is plastic and it moves on the astrolabe without having another pointer, a technical problem arises how to indicate its phases during while it is moving. A rod with frontal tooth system and a tooth wheel is used for transmission of the movement by 90°. The rod is usually hidden in a tube, e. g. on the clock in Ulm. In a simpler solution, the rotating rod is hidden beneath a robust leading arm. It is probably this mechanism that J. Táborský describes on the Prague clock. It could be similar to one of the old astronomical clocks in Olomouc from the 16th century.

The Moon in Prague differs from all known technical solutions. Its sphere with the diameter of 130 mm is hollow and divided into two hemispheres. The surface of the casing is divided into the light and dark halves, which represent the moon phases. The energy for the rotation is obtained from the most reliable source on the Earth – its gravity. Inside, a small machine with a lead weight is hidden. Two sectional thread-shaped ledges engage in the toothed ring inside the sphere. They have adjusted angles so that they engage in the teeth more easily. Since they rotate outside the geometrical centre of the sphere, they only engage in the teeth on one side of the inner perimeter. This hidden system is entirely freely attached to in a half-cage so that in can turn depending on the movement of the Moon wheel. During the circadian circular movement of the entire Moon, the hidden weight pulls the ledges downwards and rotates them and they, through the inner teeth, shift the sphere by two teeth a day. Thus all the phases pass within one Moon cycle, just as it is in the sky. These several interconnected movements are difficult to describe and define. The ingenuity and simplicity is evident when you hold this little sphere in your hand. We hope the photographs, which have not been published so far in such detail, will help understand the function. Even in modern terms, the mechanism can be considered as a miniature.

Měsíční koule Měsíční koule vyjmutá z lože Měsíční koule s otáčecím mechanismem Otáčecí gravitační mechanismusMěsíční koule s otáčecím mechanismem

In horologist terminology, hidden mechanisms, not visible at first sight, are called mysterious. For our ancestors, who decided not to throw the clock away but to repair it in the 19th century, this hidden mechanism was a surprise. The astrolabe has not been in operation for about a century and there were no record as to who constructed the mechanism and installed it on the clock. The last repair before its discovery took place in 1788–1791 and was carried out by the director of the Klementinum observatory A. Strnad and the imperial clockmaker J. Landsberger. As documented by records and bills, the repair only dealt with the clock machine. It does not seem probable that these reputable experts would not have been interested in the non﷓functional machine of the astronomical clock standing close by. Just as another version that they constructed the mechanism in secret, did not accounted for it, did not mention it and left it as a surprise for future generations. Hidden mechanisms had already begun to be discovered in their times so they would have been able to design one and construct it.

Today, most historians accept the version that the mechanism was created during the repair in 1659, shortly after the end of the Thirty Years’ War. During this extensive repair, the chiming machine was moved from the tower down to the astronomical clock and the creation of the wooden sculptures is also dated to this time. But there is no document of the creation of the Moon mechanism here. However, an undated note made by an unknown clock﷓master from the workbook, over 250 years old, which was still stored by the clock at that time, describes its function quite precisely, so this dating is possible. If it is true, then it is a technical unique comparable to the entire original astronomical clock. Zasněžený měsíc 11.1.2010

When the unknown old mechanism was discovered in the middle of the 19th century, the effort to improve it was quite understandable. The number of the teeth inside the sphere was newly calculated and modified from 57 to 59 so as to rotate more precisely. The calculation was incorrect, which came out shortly after the inauguration on 1st January 1886. At that time, Mánes’ calendar plate had not been finished yet, as we write elsewhere. Its delayed manifestation and, de facto, second inauguration in 18th August of that year allowed sufficient time to amend the incorrectness. The original number of teeth returned quietly. 

The circumstances of the creation of the hidden mechanism are not known until today. The title of this chapter is therefore not exaggerated, the Prague Moon really is mysterious.

The figure shows that the mechanism can really suffer, especially in winter.

Comparison of the methods of representation of the Moon

In the course of years, several methods of representation of the Moon and its phases have got settled. On the figures, you see the Moon depicted, similarly to the Prague Astronomical Clock, on the ecliptic but without the mysterious rotation, the Moon represented by a sphere rotating on  a fixed spot and the Moon represented in plane and still indicating the phases. We have no choice than to admire all the creators for the technical and artistic solution, the creators of the Prague Astronomical Clock first of all.

Srovnání zobrazování Měsíce u jiných orlojů Srovnání zobrazování Měsíce u jiných orlojů Srovnání zobrazování Měsíce u jiných orlojů

Text, photo and repro: Stan. Marušák

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