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Category Archives: Astronomy

Armillary Spheres: Following Celestial Objects in the Ancient World
By Ḏḥwty, Contributing Writer, Ancient Origins Astronomy is often considered to be one of the oldest branches of science. In
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The Eastern Roman Empire’s Legacy to Astronomy
By Monica Correa, Contributing Writer, Classical Wisdom Decades ago, the word “Byzantine” was used as a synonym for corruption and
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Armillary Spheres: Following Celestial Objects in the Ancient World

by October 6, 2018

By Ḏḥwty, Contributing Writer, Ancient Origins

Detail of Augustinus in betrachtendem Gebet. (1480) Sandro Botticelli. ( Wikimedia Commons )

Astronomy is often considered to be one of the oldest branches of science. In many ancient societies, astronomical observations were used not only for the practical job of determine the rhythm of life, (e.g. the various seasons of the year, the celebration of festivals, etc.) but also for the philosophical exploration of the nature of the universe as well as that of human existence. Therefore, various instruments were invented to aid the important science of astronomy. One of these instruments was called the armillary sphere.
The Function of Armillary Spheres
An armillary sphere is an astronomical device made up of a number of rings linked to a pole. These rings represent the circles of the celestial sphere, such as the equator, the ecliptic and the meridians. Incidentally, it is from these rings that the name of this device is derived from (the word armilla is Latin for “bracelet, armlet, arm ring”).
Armillary spheres may be divided into two main categories based on their function – demonstrational armillary spheres and observational armillary spheres. The former is used to demonstrate and explain the movement of celestial objects, whilst the latter is used to observe the celestial objects themselves. Therefore, observational armillary spheres are generally larger in size when compared to their demonstrational counterparts. The observational armillary spheres also had fewer rings, which made them more accurate and easier to use.
Painting of Ptolemy

Ptolemy with an armillary sphere model. (1476) Joos van Ghent and Pedro Berruguete. ( Wikimedia Commons )

The Ancient Greeks and the Armillary Sphere
The armillary sphere is believed to have originated from the ancient Greek world. The inventor of this device, however, is less than certain. Some, for instance, claim that the armillary sphere was invented sometime during the 6th century BC by the Greek philosopher Anaximander of Miletus. Others credit the 2nd century BC astronomer, Hipparchus, with the invention of this device.
The earliest reference to the armillary sphere, however, is said to have come from a treatise known today as the Almagest (known also as the Syntaxis), written by the 2nd century AD Greco-Egyptian geographer, Claudius Ptolemy. In this treatise, Ptolemy describes the construction and use of a zodiacal armillary sphere, an instrument used to determine the locations of celestial bodies in ecliptic co-ordinates. Furthermore, Ptolemy also gives examples of his use of this device for the observation of stars and planets.
The Armillary Sphere in Ancient China
Interestingly, the armillary sphere was also being developed independently in another civilization – China (albeit possibly at a later date.) The armillary sphere is said to have appeared in China during the Han dynasty 206 BC – 220 AD.)
The use of such a device may be traced to the astronomer Zhang Heng, who lived during the second half of the Han Dynasty, i.e. the Eastern Han Dynasty (25 AD– 220 AD). Originally, the structure of these spheres was very simple, consisting of three rings and a metal axis that was orientated towards the North and South Poles.
However, over the centuries, more rings were added to the spheres so that different measurements could be taken. In the courtyard of the Ancient Observatory in Beijing, for example, one can see a full sized replica of an elaborate armillary sphere produced during the reign of a 15th century Ming emperor, Zhengtong.
Armillary Sphere in Asia

Armillary Sphere in the Ancient Observatory, Beijing, China. ( Wikimedia Commons )

Armillary Spheres in the Islamic World and Christian Europe
During the Middle Ages, knowledge for the production and use of armillary spheres passed into the Islamic world. The first known treaty on this device is known as Dhat al-halaq (translated as ‘The Instrument with the Rings,’) written by the 8th century astronomer, al-Fazari.
Many Muslim astronomers wrote about the armillary sphere, though with reference to Ptolemy’s work. It may be mentioned that clear references to demonstrational armillary spheres are absent from documents of the Islamic world, whilst there is a considerable amount of evidence for the use of the observational armillary sphere.
The armillary sphere is said to have been introduced into Christian Europe by Gerbert d’Aurillac (later Pope Slyvester II.) It is assumed that d’Aurillac acquired such knowledge from Islamic Spain. It has been suggested that by the Late Medieval period, the demonstrative armillary sphere became quite a common device in European universities, as treatises on the geometry of the celestial sphere was taught in many such institutions, thus making the armillary sphere an indispensable teaching tool.
Spanish Armillary Sphere

Spanish armillary sphere. (1582) Antonio Santucci. El Escorial Monastery, Madrid, Spain ( Wikimedia Commons )

The Eastern Roman Empire’s Legacy to Astronomy

by August 21, 2018

By Monica Correa, Contributing Writer, Classical Wisdom
Decades ago, the word “Byzantine” was used as a synonym for corruption and decadence, however, the period between 395 and 1453 was also one of great scientific progress.
Byzantium, later renamed Constantinople in honor of its founder, Constantine, was a land where Latin, Greek, Islamic and Jewish traditions mixed to create a new way to study Math, History, Science and Astronomy. Consequently, there were great discoveries by dedicated scholars, such as Claudius Ptolemy, Gregory Chioniades and Nicephoros Gregoras. The scholars of this period were committed to preserve and transmit the traditions and scientific knowledge of the ancient world.
According to some research done in the last two decades, Byzantine astronomers focused in three main topics:

Figure of the heavenly bodies — An illustration of the Ptolemaic geocentric system by Portuguese cosmographer and cartographer Bartolomeu Velho, 1568 (Bibliothèque Nationale, Paris)

1. Equinoxes and Eclipses
During the Byzantine era, the Astronomic model was geocentric, meaning the consensus view was that the earth was at the center of the solar system; however, most scholars were aware of some existing errors with regards to measuring the stars and planets.
Illustration of Ptolemy

Ptolemy (Source: Blanche Marantin and Guillaume Chaudiere, Paris, 1584

A gradual improvement of methods, such as better use of the astrolabe, culminated centuries later with the introduction of the heliocentric system, which correctly placed the sun at the center of our solar system. Gregoras, who lived between 1295 and 1360, understood the mechanism of eclipses, and he calculated all the solar eclipses of the millennium up to the 13th century. He also predicted future eclipses of both the sun and the moon, constructed a prototype astrolabe, and proposed reforms to the calendar, all of which led to great progress for human kind.
2. The shape of the earth
In the text The Schemata of the Stars, Chionades draws some diagrams for solar and lunar eclipses where the earth is spherical. This provides further evidence that the Byzantines (as well as several other cultures around the world at that time) considered the earth to be spherical.

Astrolabe (Source: Masoud Safarniya)

Some years later, when Gregoras refers to the Earth in his famous work Roman History, he uses the phrase ‘below the sun’. There, indirectly he accepts its spherical shape, and he also refers to its subdivision into parallel circles and continents.
3. Models for the sun, the moon and the five (known) planets
As mentioned previously, Byzantine models for the sun and the planets are geocentric. Essentially this means: for each celestial body it is necessary to introduce a system of spheres whose axes and rates of rotation are exclusive for them.
For Chionades, Mercury and Venus are inner planets and, as seen from the earth, appear to follow the sun because they are sometimes ahead and at other times trailing the sun.
Papal letter

The first page of the papal bull “Inter Gravissimas” by which Pope Gregory XIII introduced his calendar

Also, regarding Mercury, Chioniades makes an interesting remark concerning latitude. In his writings, he explains that among the five planets, four of them have their apogees (highest point) in the northern hemisphere of the globe, except for Mercury whose apogee is in the southern hemisphere. Was this the result of observations, or was he echoing an ancient tradition? We may never know, but this description of the latitudes survived after the introduction of the heliocentric system, with both Rheticus and Copernicus making similar observations.
Years later, a mixed model with Venus and Mercury rotating around the Sun, and all of them together rotating around the earth, was introduced by Heraklides of Pontus.
The legacy that lasts until today: Our Calendar
The writings of Gregoras are especially important; today we know Byzantine astronomy owes much of its progress to him. Aware of the mistakes made by his predecessors, in 1324 Nicephoros Gregoras proposed a correction to the calculation of the date of Easter, and to the Julian calendar itself.
At that time, his beliefs conflicted with his work, so he retired from public life and his work was discredited by the church.
The calendar as we know it today was implemented by the Italian, Pope Gregory XIII in 1582. Though the so-called Gregorian calendar was named in the Pope’s honor, it was not his invention.
Established on October 4, 1582, the new calendar solved the problem that the Julian year had 11 minutes and 14 seconds more than the solar year, which had a cumulative effect to the date of the spring equinox.
Despite the fact that Gregoras didn’t live to see it implemented, it’s one of the main contributions that Byzantine Empire bequeathed to us.