Astronomy in Ancient Rhodes
by Antonios D. Pinotsis
Section of Astrophysics, Astronomy and Mechanics, Department of Physics, University of Athens, Greece
The development of Sciences and Arts in ancient Rhodes started as early as the fourth century B.C. Astronomy, mathematics, geometry, meteorology, geography and philosophy flourished in the ancient cities of the island. Rhodes became an important Astronomical center of Antiquity, especially during the 2nd and 1st century B.C., similarly to Alexandria. Astronomical observations of several celestial phenomena were carried out and their detailed description as well as explanation was given. The first meteorological observations ever were also carried out on the island and were used to produce the first known calendars, the so-called Parapegmata.
The School of Art in Rhodes was considered among the most important ones in that era, particularly in the 2nd and 1st century B.C. A great number of statues and pieces of art decorated the city of Rhodes. Among the most well-known were Kolossos of Rhodes, which was made to honor the god Sun and which was about 31m tall, as well as Laokοon’s complex, toro Farnese (Φαρνέσειος Ταύρος) and others.
Rhodes was a significant centre of naval and military technology, able to resist the attack of Demetrius and reached its peak between 304 B.C and 150 B.C. The finest ships in antiquity, carrying many scientific instruments and weapons were built in the island’s shipyards. Rhodes at that time was the place where many war-machines such as the “polybolos” were developed.
We studied the coins in ancient Rhodes during the period after 408 B.C. when the city was founded. At that time, the Sun’s head without the rays was introduced as an emblem in the coins. We observed a gradual change of the rays depicted on Rhοdian coins. For the first time, we suggested that the change of the Sun’s head as depicted on coins originated from the evolution of astronomical knowledge in ancient Rhodes. Astronomical knowledge influenced the artists and resulted in the change of their perception of the Sun’s head. Indeed, the artists are influenced by the intellectual and cultural level of their age. The rays in the Sun’s head symbolize its light and heat and generally the radiating energy, properties that man does not have. They also symbolize the “life-giving source which penetrates the Earth and the heavens”, as ancient Greeks used to say.
In brief, Rhodes’ peak and eminence during Antiquity was due mainly to the development of Science, Art, technology and navy. Here, we mention, in brief, the lives and works of well-known Rhodian Astronomers, such as Hipparchus, Posidonius, Eudemus, Geminus and Attalus. The blossoming of Astronomy in ancient Rhodes was due to the simultaneous appearance of several favorable factors and conditions such as its mild climate, its democracy and financial growth which was a result of maritime trade. Many craters of the moon have their names chosen after Rhodian astronomers.
a) Hipparchus of Rhodes
Hipparchus was born at Nicea in Bithynia at 190 or 194 B.C. According to available sources, he died in Rhodes at about 120 BC. Hipparchus left from Bithynia when he was around 30 years old and settled in Rhodes. There he spent most of his life and founded his own Observatory. He wrote a great number of works on astronomy, mathematics, geography and meteorology. Unfortunately, with the exception of two works, all the works by Hipparchus were lost. The works that have reached our times are των Αράτου και Ευδόξου Φαινομένων Εξηγήσεις (Explanations for the phenomena of Aratos and Eudoxos) and Εις Αστερισμούς (on Constellations). Hipparchus used mathematics to solve astronomical and geographical problems and especially he is the first who systematically used trigonometry in his works. He is considered by many researchers as the most important astronomer and geographer of Antiquity. Ptolemy frequently cites Hipparchus' works. He constructed and improved several astronomical instruments, such as astrolabos, gnomon, dioptre etc which he used for his observations. The most important and well known of his discoveries are:
• The phenomenon of precession of the equinoxes, namely the vernal equinox and autumnal equinox slide (move) westward along the ecliptic at a speed of approximately 50΄΄ per year or (1 ½)o per century. Also, variable stars were studied for the first time by Hipparchus (about 134 B.C.) who first observed a nova.
• Based on naked-aye estimations, he compiled a catalogue including accurate positions of 1020 stars. In this catalogue, he classified the stars into six groups according to their brightness or apparent magnitude.
• In his work On the Length of the year Hipparchus calculated the length of the tropic year to be 365 days 5hrs. 55min. 12sec., while from his estimate of the Great Year we obtain for the length of the mean lunar month 29d. 12hrs. 44min. and 2.5sec.
• Hipparchus adopted the geocentric system. In order to explain the geocentric system by using a mathematical model, he first used for the Sun and the moon the model of eccentric circles and later the one of epicycles. Hipparchus also used the observations available at his time to interpret the apparent orbit of the Sun. He knew that the four seasons of the year corresponding to the arcs Eγ, γΕ΄, Ε΄γ΄ and γ΄E were not equal. This inequality led him to the formulation of two models about the motion of the Sun. Hipparchus first talked about the Apogee and the Perigee of the Sun’s orbit and found that its eccentricity was e = 1/24 = 0.0416, which was very satisfying, since its current value is approximately e = 0.0167. He also found that the length of the Apogee, which is the arc between the spring equinox and the point of Apogee is 65ο 30΄, a very good value as well. In that way, he explained the inequality of the four seasons of the year, since the Sun was moving with a different angular velocity along the eccentric circle (slower towards the Apogee and faster in the region of the Perigee). Also, he proved that the orbit of the Sun coincided with the ecliptic, while previous astronomers considered that this orbit was inclined towards the ecliptic. Consequently, to a first approximation, the theory of Hipparchus was equivalent to the first two laws of Kepler !
As far as the orbit of the moon is concerned, Hipparchus knew that its motion is more complex than the Sun’s motion and that it includes many abnormalities. Ηe also formulated two models about the motion of the moon. At the beginning, he considered that an eccentric circle could depict the apparent orbit of the moon and he then calculated the first abnormality of the moon’s orbit, that is its eccentricity and found it equal to e = 0.0875. This is a very good value, since its current one is e = 0.055.
• Hipparchus calculated that the inclination of the moon’s orbit in relation to the ecliptic is 5ο and therefore its greatest declination is 23ο 51΄ +5ο = 28ο 51΄ approximately. At the time of Hipparchus the inclination, the obliquity of the ecliptic, was not 23ο 27΄, like today, but 23ο 43΄ and according to Eratosthenes’ calculations 23ο 51΄ approximately. He also knew the advance of the perigee that is the line of apsides was being shifted from west to east and performed a complete rotation in a period of approximately 9 years. We now know that the rotation of the line of apsides on the plane of the moon’s orbit is 40o40΄35΄΄ per year and so it has a period of 8.85 years. This result along with other abnormalities of the moon’s motion is due to the tidal forces wielded upon the moon. Thus he was led to the conclusion that he should implement the theory of the epicycles. He was then able to determine the second abnormality of the moon’s orbit, which is due to the regressive motion of the line of apsides around its average position. Hence the moon is shifted on both sides of its average position up to a width of 1ο, 25 and with a period equal to the time between two transitions of the Sun through the line of apsides of its orbit. The latter abnormality is due to the change of the curvature of the orbit and so of the eccentricity.
• Finally, Hipparchus by his work On the Size and Distances of the Sun and moon improved Aristarchus’ estimates of the diameters and distances of the Sun and moon from the earth.
b) Posidonius of Rhodes
Posidonius was in many ways an important personality, with a rich and varied work. He was born in Apameia, a city of ancient Syria, in 135 B.C. and he died in Rhodes at about 51 or 50 B.C. at the age of 84. He was a great Stoic philosopher. He studied in Athens under the guidance of the Rhodian stoic philosopher Panaetius, leader of the stoic school of Athens, and succeeded him in that position after his death in 110 B.C. In 90 B.C. or 97 B.C. he moved to Rhodes, where he founded a stoic school at which he taught for the rest of his life. Posidonius was an eclectic Stoic, revising whatever doctrines of stoicism did not satisfy his inquisitive nature. He was an accomplished orator, but also a natural scientist, interested in astronomy, mathematics, geography, cartography, meteorology and mechanics. He used to check with different methods or to improve the results of the other scientists. Posidonius was also a qualified teacher and many students were attending his lectures. Among them were not only Rhodians but also reputed Romans, such as Cicero and Gnaius Pompeius.
He wrote over 20 books about philosophy, astronomy, mathematics, geography, meteorology and cartography which have been lost. We know about these books from extracts mentioned in works of later authors. One of them, Posidonius’ student Geminus of Rhodes cited the theories developed by his teacher in his books. From these sources we conclude that Posidonius studied physical phenomena such as the refraction of light in the atmosphere, the influence of the moon and the Sun on tides (he is considered the ‘father’ of tidal studies), developed the division of the earth in zones, estimated the diameters of the Sun and moon and their distances from the earth as well as the size of the earth. In all of his physical studies he produced innovative ideas and new experiments to test previous assumptions, never relying entirely on previous philosophers. Posidonius repeated Eratosthenes’ experiment using an astronomical method to calculate the length of a meridian of the earth. This method was different from the geometrical one of Eratosthenes. Also, he constructed a map of the then known world which reached modern times through the work of the geographer Dionisius. He followed Archimedes in creating his own planetarium and served as Rhodes’ envoy in Rome. He was admired by the Roman orator Cicero, who visited him in Rhodes sometime between 79-77 B.C. Gnaius Pompeius, the great Roman general, also visited him in 67 B.C., on his triumphant return from his campaign against Mithridates and bestowed upon him honors unheard for non-Roman citizens. The Rhodians honored him with high administrative posts and he was given the honorary title ‘the Rhodian’. The well known great geographer Strabon (and others) is referred to Posidonius “as the most universal mind after Aristotle”.
prof. J. Seiradakis (CV) will give an invited talk on the subject on Friday, 11th of July
In (Pinotsis 2007) the discovery of the Antikythera treasure is reviewed. This treasure was found by Symian sponger-divers in November 1900 and among the findings was the well-known Antikythera mechanism. In the same paper, we gave an overview of results on the Antikethyra mechanism, which is a specimen of high technology from the Antiquity and has tremendous archaeological importance and interest. We also gave a description of this Mechanism and its function, use and purpose and suggested that its creator was Posidonius. An alternative hypothesis to the one we presented is that the Antikythera mechanism was created by Hipparhus or Geminus of Rhodes. The Mechanism has not been fully studied yet; it is likely that there exist other inscriptions and gears since the gears inside are not easily distinguishable.
c) Eudemus of Rhodes
Eudemus the Rhodian (350-290 B.C.) was a peripatetic philosopher and one of the most beloved students and fellows of Aristotle. Aristotle appreciated him very much. Aulus Gellius in his work Noctes Atticae mentions that Aristotle was thinking for a long time about whom he should appoint as his successor in his Lyceum, namely Eudemus or Theophrastus, The final choice of the philosopher and mathematician Theophrastus does not put into question Eudemus’ scientific value and Aristotle's appreciation for him.
After Aristotle’s death, Eudemus settled in Rhodes, where he founded a School in which he has taught the “Peripatical philosophy” of his Teacher. The foundation of this School at the beginning of the Hellenistic period contributed to the very early development of philosophy, literature and sciences. Thus, together with other contemporary intellectual men he has contributed in making Rhodes, a significant cultural center.
Eudemus was engaged in sciences and mainly in astronomy, geometry, mathematics, physics as well as in pure sciences and more specifically logic, theology, moral philosophy, metaphysics and philology. He had also taught in Aristotle’ s Lyceum.
He collaborated with Aristotle in writing memorandums in his Teacher’s works and also completed some of them. Furthermore, he took care of the publication of at least some of the works of Aristotle, especially after the death of his Teacher and looked after the manuscripts he had left behind.
Eudemus wrote many books about Astronomy, Mathematics etc. Two of his most important and great works were the History of Astronomy, and the History of Mathematics.
Many of Eudemus’ works are lost. However, later authors used his works therefore many extracts from Eudemus works reached modern times throught secondary sources.
The great geographer Strabon mentions Eudemus “as among the most remembered children of Rhodes”, who motivated the development and advance of sciences in ancient Rhodes.
d) Geminus of Rhodes
Geminus was a Rhodian Stoic philosopher, astronomer, and mathematician of 1st century BC. He was a student of Posidonius and few of his works reached modern times. He tried to popularize some of the writings of his teacher for educational purposes. These works were brief, however precise, and were used by both ancient Greeks and Romans. Geminus wrote also some classic works on astronomy, meteorology and mathematics: 1) Introduction to the Phenomena, written circa 77 B.C. In that book, which contained a traditional Greek Calendar (Parapegma), Geminus also mentions the theory of lunisolar cycles and the duration of the four seasons including the measurements made by Hipparchus. 2) A summary of the major opus of the Meteorologica of Posidonius (On meteorological phenomena). 3) Theory of mathematics and a Reference Book for Geometry (Εγχειρίδιο Γεωμετρίας).
e) Attalus of Rhodes
Attalus was a Rhodian astronomer, mathematician and writer of the Alexandrian period. He lived in Rhodes during the 2nd B.C. century and was a contemporary of Hipparchus. The scientific work of Attalus is generally unknown because his works did not reach our days neither have we sufficient information about him from other writers. He wrote scientific comments in favour of the eminent poem (book) Phaenomena of the astronomer, poet and mathematician Aratus (315-240 B.C.). In these comments, Attalus used scientific arguments to defend Aratus and the theories of the astronomer and mathematician Eudoxus of Knidus, against criticisms from certain contemporary astronomers and mathematicians. Hipparchus, in his work Των Αράτου και Ευδόξου Φαινομένων Εξηγήσεις, mentions repeatedly the comments and the arguments of Attalus which he took seriously into consideration. After studying the relevant references we concluded that Hipparchus distinguished Attalus among all those who had studied and written comments on the Phaenomena of Aratus. Hipparchus highly appreciated Attalus because he was a serious and careful scientist and also because of his deep knowledge of astronomy and mathematics. In conclusion, Attalus occupied a distinguished place among the scientists and philosophers of that period.
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