my astrarium

Giovanni de Dondi ASTRARIUM ( # 37 )

my astrarium

As a non professional enthusiastic clockmaker and having already built a few clocks, I wanted to check my skill and the challenge par excellence was obviously the Astrarium of Giovanni de Dondi.

Firstly I made a long research of the best available information. Bearing in mind my workshop facilities ,it was quite clear that if I wanted to build the astrarium by myself with no professional help, I had to reduce the original Astrarium sizes.

Mercury dial
(watch the loupe for size comparison!)

I had not chances: the maximum diameter accepted by my lathe is 250 mm, which approximately is one half of the year wheel diameter - the largest one found in the Astrarium.

I was so forced to scale the complete Astrarium by 1/2.

Venus dial

As suggested by Giovanni,I firstly built the inferior"casamento"(house), then the superior one.

As soon as I was to cut the teeth of the year wheel . I faced with the problem to give them the proper shape.

Giovanni gives no information on this matter, but looking at his sketches, teeth undoubtedly seem to be of the triangular shape . Just in few cases he clearly states they must have their tip rounded.

My fear was that if the choice was for the triangular shape, the force given by the small driving weight ( in my replica reduced by 1/2 of the actual size),due to the high frictions involved, might be not suitable to move all the devices included in the Astrarium .

inside view

inside vi

So my preference went to an epicyclical shape, the same gotten by modern cutters. I am sure that even Giovanni, if this new technology was available at his time , might have made use of it.

Building the clock was not a straightforward job : we must recall that everything made by man can be reduced to scale: not the same for what is created by nature.

It is clear that varying the balance wheel size, the gears train suggested by Giovanni de Dondi was not suitable for my reduced replica.

Resizing the balance wheel diameter (in order to keep its former inertial moment - therefore the same frequency ) I had to increase massively the external crown mass, in a way such to completely change its aesthetics. I solved this problem just varying the number of teeth in a pinion and leaving unchanged the remaining gears train.

As a matter of curiosity we must say that at de Dondi's time, screws were unknown and the astrarium was put together by soft soldering and all the other parts were fixed through pins.

....almost finished

I engraved all the names and numbers (the smallest of them is 0.0149" high) which is to say around 8,000 individual characters.

To my opinion the most beautiful dial is the one showing the sun (Primum mobile): technical solutions herein involved are simply exquisite. The most complex dial is the one showing the moon. Herein, among others things, there are two wheels that rather than circular are pear shaped.

As said, I was forced to a couple of marginal deviations from the "Tractatus Astrarii" original account . But, as Jack Lemmon is reminded in the final line of Billy Wilder's wonderful film Some Like It Hot: "nobody is perfect"!

I don't want to seem iconoclast, but I believe Giovanni de Dondi too made at least one minor error when on page 29 of the Tractatus he states a pinion must have 15 leaves, and in a sketch shows it as having 12 leaves.

I hope every clockmaker will share with me the pleasure to see Giovanni de Dondi's aim fulfilled: writing

the "Tractatus Astrarii", he wanted other clockmakers to be in the position to build what his mind had conceived.

I wish hereby to thank my new friend (as I hope he also considers me) Don Unwin who built an astrarium and did not hesitate to provide me with all the explanations I asked for
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Astrarium menu

my astrarium

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