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Galileo’s Astrolabe, Museo Galileo in Florence

Galileo’s Astrolabe, Museo Galileo in Florence

Wednesday, 31 October 2012
By Giorgio Strano
Giorgio Strano

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10 min read

The scientific instruments collected by the two grand-ducal families of Tuscany, the Medici and the Hapsburg-Lorena, are preserved at the Museo Galileo: Institute and Museum of History of Science in Florence.

Astronomy, cosmography, geography, surveying, navigation, chemistry, electro-magnetism, are only part of the wide array of disciplines represented by more than 1,000 items on display and about 3,000 items in storage. In particular, as similar Renaissance collections in Western and Central Europe, the Medicis’ includes reliable and beautiful pieces made by the most skilled craftsmen of the epoch. Astronomical time-keepers form a significant part of it.

The idea of using the course of the Sun and the stars to tell time emerged when ancient societies recognised the periodicity of the heavenly phenomena. Sundials of many shapes, horary quadrants and astrolabes were the most accurate time-keepers from late antiquity up to the 18th century. Clepsydras, sandglasses and graduated candles merely replaced the astronomical devices when the weather was cloudy. In addition, when mechanical clocks appeared in Medieval monasteries and churches, they had errors up to several minutes per day. Such a relevant lack of accuracy was caused by imperfectly shaped gears, attrite, dust, differences of temperature and humidity, as well as the increase of the motive force due to the unwinding of the ropes or chains holding the driving weights. For a few centuries, the astronomical devices, and especially the sundials, remained indispensable tools used to re-set the clocks at noon.

The Arabic world had a pivotal role

In the evolution of the astronomical time-keepers, the Arabic world had a pivotal role. During the 8th century, the Muslims inherited the scientific tradition of the ancient Greek and Hellenistic cultures. The flourishing cities of Baghdad and Damascus became advanced centres of power and learning. The most eminent mathematicians worked there, often in the direct service of the ruling caliphs. The scientific masterworks of the past were translated into Arabic, and profoundly studied. In addition to the caliphs’ interest in horoscopes, which were believed to be of great help in making political decisions, religion was the other propelling factor of astronomical studies.

The Muslims must pray five times a day, at very precise hours. The mosque’s astronomer, called the muwaqquit, had to rely upon the most precise time-keepers to inform the muezzin when it was time to climb the minaret and call others to pray. Until the end of the 19th century, the muwaqquit had at his disposal two astronomical instruments: the solar quadrant and the planispheric astrolabe. The latter was perhaps invented in the Hellenistic epoch; nevertheless, the first evidence about its existence is a treatise written by the Syrian monk Severus Sebokht (7th century). As basic as it may appear in its early version, the astrolabe was greatly improved by the Muslim astronomers.

The planispheric astrolabe is a very sophisticated analogical calculator. Its many functions include time-keeping by reproducing the sky’s appearance for any given day. To do this, the instrument is composed of five main parts:

  1. the “mater”, a hollow disk of brass with the limb graduated on the front and back.
  2. the “plate”, a thin disk fitting the mater and representing the grid of the azimuthal coordinates of the sky drawn for the latitude of the place of observation. Especially if the owner of the instrument had to travel frequently, a single astrolabe might contain several interchangeable plates drawn for different latitudes.
  3. the “web”, an open-work rotatable representation of the most bright stars, indicated by pointers, and the course of the Sun along the Zodiac, indicated by an eccentric circle.
  4. the “alidade”, a rotatable arm with two drilled sights, used to point at the Sun or at a star for determining its altitude above the horizon.
  5. the “index”, a rotatable ruler indispensable for reading the results of the calculations.
Egnazio Danti (attributed), the so-called ''Galileo’s astrolabe'', Florence, ca. 1565 (Museo Galileo, inv. no. 3361) © Museo Galileo
The old way to recount the history of science

Between the 10th and the 11th centuries, the planispheric astrolabes entered the Latin world. Here they were used in the original form, or re-adapted by erasing the Arabic inscriptions and re-engraving new Latin ones. Only around the 14th and the 15th centuries did the Europeans gain enough knowledge to build their own astrolabes, firstly equalling and then surpassing the Arabians’ skills. The Medici collection very well tells this story by including both Arabic and European planispheric astrolabes. A collection in the collection is formed by the astrolabes brought to Florence by Robert Dudley, Duke of Northumberland, who left the court of Queen Elisabeth I for the court of the Grand-Duke Cosimo II de’ Medici.

A very special piece of the exhibition of the Museo Galileo is the so-called “Galileo’s astrolabe”. In addition to its intrinsic value, it is also a witness of the old way to recount the history of science. In order for a piece to gain importance, it had to receive the name of a celebrated scientist of the past. In this case, Galileo Galilei (1564-1642), court natural philosopher of Cosimo II, neither made nor owned the instrument. He merely found it in the “Room of Mathematics” of the Uffizi Gallery, the place where the Medicis’ scientific instruments were gathered as a section of the art collection. Its large size (84 cm in diameter) made Galileo guess that the instrument could be useful in making very precise calculations. Therefore, he asked the Grand-Duke permission to have it on loan. Cosimo II agreed, and Galileo used the astrolabe, possibly in his personal quarters. When the instrument returned at the usual place, it was improperly called “Galileo’s astrolabe”, an epithet due to the mere touch of the great scientist. The first testimony of this labelling is documented in the Uffizi Gallery’s inventory of 1654, twelve years after Galileo death.

A considerable size of the device

The instrument’s appearance is quite peculiar, especially when compared to the typical planispheric astrolabes. Every modification to the traditional set up depends upon the considerable size of the device. Since it is too large to be used portably, “Galileo’s astrolabe” is mounted on an octagonal walnut table attached to a wooden column with a three-leg base. In order to measure the altitude of the Sun or a star above the horizon, the instrument must, however, be placed in the vertical plane. It is of course impossible to keep “Galileo’s astrolabe” vertically in the usual way, that is, by hanging it at the thumb by means of a suspension ring. The table is therefore connected to the wooden column by a metal hinge, and can be blocked by a hook in three different positions: horizontal, inclined at about 45 degrees, and vertical. Moreover, as it is also indispensable to direct the instrument towards any point of the horizon, the wooden column can rotate around a vertical inlaying axle.

This special mount caused other modifications. Being fixed on the walnut table, “Galileo’s astrolabe” has no back. All the astronomical information and functions typical for the back of the astrolabes, as the calendar showing the position of Sun along the Zodiac, are transferred to the front of the astrolabe. Moreover, there is no distinction between the mater and the plate. Both are skilfully engraved on the same sheet of brass. Finally, the alidade with its drilled sights, usually placed at the back of an astrolabe, is moved in front of the web, and plays the additional role of the index.

Besides these modifications, the considerable size allowed for several advantages. The web is large enough to include 41 star pointers. The course of the Sun is divided in the 12 signs of the zodiac, each sign being in turn divided into 30 degrees. The grid of the azimuthal coordinates is calculated for the very precise latitude of 43° 40′ north, the same of Florence. The limb of the mater displays a large graduated scale divided into the very small portions of 5 arc-minutes by means of transversal points. This method to divide graduated scales is usually attributed to the Danish astronomer Tycho Brahe (1546-1601), who described it in his work De mundi aetherei (1588).

Egnazio Danti (attributed), the so-called ''Galileo’s astrolabe'', Florence, ca. 1565 (Museo Galileo, inv. no. 3361) © Museo Galileo
The maker could have been Egnazio Danti

An obvious question presents itself. If not Galileo, who made the so-called “Galileo’s astrolabe”? Regrettably, no document exists to identify the very skilled craftsman of this remarkable instrument. In 1977, the astronomer Guglielmo Righini suggested that the maker could have been Egnazio Danti (1536-1586), the renown cosmographer of the Grand-Duke Cosimo I de’ Medici. The astronomical dating of the astrolabe is around 1563, and the ornamental motifs of the web have evidences to reinforce this attribution. In 1995, however, the historian of scientific instruments Gerald L’Estrange Turner suggested that the astrolabe comes from the workshop of Giovanni Battista Giusti, who flourished in Florence in mid-16th Century. The punch-marks of the inscriptions of “Galileo’s astrolabe” are identical to those on other instruments made by Giusti. Unfortunately for Turner, Mara Miniati, curator emeritus of the Museo Galileo, discovered in the archival documents of Florence that, during the 16th century, different workshops made use of the same punch-marks. These are therefore irrelevant to identify the author of an instrument.

The initial attribution of “Galileo’s astrolabe” to Egnazio Danti remains the most likely. Not only did Danti write a thorough treatise on the making and functioning of the planispheric astrolabe, but was also one of the very few astronomers which implemented creations with graduated scales divided by transversal points years before they became widespread thanks to Tycho Brahe. Indeed, under the auspices of Cosimo I, Danti installed a couple of large instruments on the façade of the church of Santa Maria Novella in Florence. A large marble quadrant on the right of the façade should have had transversal points, as shown by Danti’s preparatory drawing dating 1570, conserved in the Gabinetto Disegni e Stampe of the Uffizi Gallery.

This intriguing story is only one of many that can be told by choosing any single instrument from Museo Galileo in Florence.

Museo Galileo: Institute and Museum of History of Science
Piazza dei Giudici 1, 50122 Firenze, Italy
> www.museogalileo.it
Telephone: +39-055-26.53.11

The ancient art of time measurement
The art of time measurement is one of the great advances of humankind. This centuries-old heritage is rightfully conserved in numerous, though often little-known, museums. So as to bring these riches to life, HH Magazine presents some of the most exceptional pieces, chosen for their technical significance, historical importance, or for their beauty. This regular feature trains the spotlight on a timepiece which has been chosen and described by the curator of the horological collection of one of the approximately two hundred public or private museums which conserve clocks and watches in their collections.

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