introduction

Giovanni de Dondi ASTRARIUM ( # 37 )

Introduction

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In 1371 the Republic of Venice and the city of Padua were engaged in a long debate over their borders. In order to avoid an open conflict, they agreed to an arbitration and Padua sorted out Johannes de Dondi ab horologio . The above to witness how his fellow citizens highly regarded this distinguished scientist.

Giovanni de Dondi owed his fame to his boundless knowledge and to his treaties “DE BALNEIS PATAVINIS” , ”DE AERARUM NATURE” , “MODUS VIVENDI TEMPORE PESTILENTIALIS” - dealing with plague prevention – and to his Astrarium.

It is worth of interest to remark that even Giovanni’s father, Jacopo built a clock for the city tower and this clock - surely built before 1354 - must have been particularly meaningful if the city Lordship granted Jacopo the appellative “ab horologio” (of the clock), appellative then inherited by his son Giovanni, who, at the time he built his astrarium in 1364, was already known as” Magistro Johanne de roloio” (Professor John of the clock - roloio, from horologio) .

Anyhow just in watching his astrarium we should admit Giovanni deserved by himself both the appellatives .

Even if the first mechanical clocks appeared at the time Giovanni de Dondi was active, timekeeping still relied on sundials.

Just the idea of an instrument with such the complexity of the astrarium must have been a Cyclopean achievement - which, by the way it would be still now .

Giovanni was absolutely innovative and as usual it happens when dealing with the new, he was not entirely understood by his contemporaries when he first brought in the drawings the perspective ; not certainly the fine perspective of renaissance painters, but certainly a sort of perspective which nicely let the people to figure out the space depth.

He was ahead of his time choosing the moment the meridian crosses the sun as the starting point to measure the time.

At his time, people made use of the italic hours, the new day starting at the sunset which unfortunately is an instant which varies with the vary of the seasons -...hence not a practical system.

To reduce friction he first understood the gear teeth must have a rounded shape, recalling in the design the cycloidal curve, Giovanni hence made use of a teeth shape which was discovered a couple of centuries later.

Primum mobile

Today ,to cut a 365 teeth wheel might be a reasonably easy task if we have a dividing head fitted with a 73 holes plate, certainly to cut one with 183 (first number) with no numerically controlled device it might be a very hard task.

An harder one in an amateur workshop, so much to be almost an impossible achievement.

Giovanni de Dondi in his “Tractatus Astrarii” gives very detailed explanations how to cut both wheels.

Incidentally, when I cut my 183 toothed wheel I relied on those explanations.

For a long time my bad conscience suggested to cut only 182 teeth, being sure that nobody would have ever checked the number, but finally Giovanni's ghost who fluttered above me took the advantage and he convinced me to cleverly read his Tractatus.

I wonder many times what pressed me to build this astrarium and the only answer I find is my desire to understand. When I saw a replica of the astrarium in London Kensington Science Museum I was not able to clearly understand the seven dials and of no use were the explanations provided in the small glass showcase.

I then started a search which carried me to the purchase, among other books,

Giovanni de Dondi's drawing depicting a couple of gears pear shaped

the “Tractatus astarii “, where I discovered a new and very fascinating world.

With my inadequate and modest words I will try to share with you this world, synthesizing here in a short article the description of the astrarium and the experiences in building my replica.

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