The Hellenistic period covers the period of Mediterranean history between the death of Alexander the Great in 323 BC and the emergence of the Roman Empire as signified by the Battle of Actium in 31 BC and the subsequent conquest of Ptolemaic Egypt the following year. The Ancient Greek word Hellas is the original word for Greece, from which the word Hellenistic was derived. At this time, Greek cultural influence and power was at its peak in Europe, North Africa and Western Asia, experiencing prosperity and progress in the arts, literature, architecture, mathematics and science, it is considered a period of transition, sometimes of decadence or degeneration, compared to the enlightenment of the Greek Classical era. The Hellenistic period saw the rise of New Comedy, Alexandrian poetry, the Septuagint and the philosophies of Stoicism and Epicureanism. Greek science was advanced by the works of the polymath Archimedes; the religious sphere expanded to include new gods such as the Greco-Egyptian Serapis, eastern deities such as Attis and Cybele and a syncretism between Hellenistic culture and Buddhism in Bactria and Northwest India.
After Alexander the Great's invasion of the Achaemenid Empire in 330 BC and its disintegration shortly after, the Hellenistic kingdoms were established throughout south-west Asia, north-east Africa and South Asia. The Hellenistic period was characterized by a new wave of Greek colonization which established Greek cities and kingdoms in Asia and Africa; this resulted in the export of Greek culture and language to these new realms, spanning as far as modern-day India. However, these new kingdoms were influenced by the indigenous cultures, adopting local practices where beneficial, necessary, or convenient. Hellenistic culture thus represents a fusion of the Ancient Greek world with that of the Near East, Middle East, Southwest Asia; this mixture gave rise to a common Attic-based Greek dialect, known as Koine Greek, which became the lingua franca through the Hellenistic world. Scholars and historians are divided as to; the Hellenistic period may be seen to end either with the final conquest of the Greek heartlands by Rome in 146 BC following the Achean War, with the final defeat of the Ptolemaic Kingdom at the Battle of Actium in 31 BC, or the move by Roman emperor Constantine the Great of the capital of the Roman Empire to Constantinople in 330 AD.
"Hellenistic" is distinguished from "Hellenic" in that the first encompasses the entire sphere of direct ancient Greek influence, while the latter refers to Greece itself. The word originated from the German term hellenistisch, from Ancient Greek Ἑλληνιστής, from Ἑλλάς. "Hellenistic" is a 19th-century concept. Although words related in form or meaning, e.g. Hellenist, have been attested since ancient times, it was Johann Gustav Droysen in the mid-19th century, who in his classic work Geschichte des Hellenismus, coined the term Hellenistic to refer to and define the period when Greek culture spread in the non-Greek world after Alexander's conquest. Following Droysen and related terms, e.g. Hellenism, have been used in various contexts; the major issue with the term Hellenistic lies in its convenience, as the spread of Greek culture was not the generalized phenomenon that the term implies. Some areas of the conquered world were more affected by Greek influences than others; the term Hellenistic implies that the Greek populations were of majority in the areas in which they settled, but in many cases, the Greek settlers were the minority among the native populations.
The Greek population and the native population did not always mix. While a few fragments exist, there is no complete surviving historical work which dates to the hundred years following Alexander's death; the works of the major Hellenistic historians Hieronymus of Cardia, Duris of Samos and Phylarchus which were used by surviving sources are all lost. The earliest and most credible surviving source for the Hellenistic period is Polybius of Megalopolis, a statesman of the Achaean League until 168 BC when he was forced to go to Rome as a hostage, his Histories grew to a length of forty books, covering the years 220 to 167 BC. The most important source after Polybius is Diodorus Siculus who wrote his Bibliotheca historica between 60 and 30 BC and reproduced some important earlier sources such as Hieronymus, but his account of the Hellenistic period breaks off after the battle of Ipsus. Another important source, Plutarch's Parallel Lives although more preoccupied with issues of personal character and morality, outlines the history of important Hellenistic figures.
Appian of Alexandria wrote a history of the Roman empire that includes information of some Hellenistic kingdoms. Other sources include Justin's epitome of Pompeius Trogus' Historiae Philipicae and a summary of Arrian's Events after Alexander, by Photios I of Constantinople. Lesser supplementary sources include Curtius Rufus, Pausanias and the Byzantine encyclopedia the Suda. In the field of philosophy, Diogenes Laër
The Ancient Greek language includes the forms of Greek used in Ancient Greece and the ancient world from around the 9th century BCE to the 6th century CE. It is roughly divided into the Archaic period, Classical period, Hellenistic period, it is succeeded by medieval Greek. Koine is regarded as a separate historical stage of its own, although in its earliest form it resembled Attic Greek and in its latest form it approaches Medieval Greek. Prior to the Koine period, Greek of the classic and earlier periods included several regional dialects. Ancient Greek was the language of Homer and of fifth-century Athenian historians and philosophers, it has contributed many words to English vocabulary and has been a standard subject of study in educational institutions of the Western world since the Renaissance. This article contains information about the Epic and Classical periods of the language. Ancient Greek was a pluricentric language, divided into many dialects; the main dialect groups are Attic and Ionic, Aeolic and Doric, many of them with several subdivisions.
Some dialects are found in standardized literary forms used in literature, while others are attested only in inscriptions. There are several historical forms. Homeric Greek is a literary form of Archaic Greek used in the epic poems, the "Iliad" and "Odyssey", in poems by other authors. Homeric Greek had significant differences in grammar and pronunciation from Classical Attic and other Classical-era dialects; the origins, early form and development of the Hellenic language family are not well understood because of a lack of contemporaneous evidence. Several theories exist about what Hellenic dialect groups may have existed between the divergence of early Greek-like speech from the common Proto-Indo-European language and the Classical period, they differ in some of the detail. The only attested dialect from this period is Mycenaean Greek, but its relationship to the historical dialects and the historical circumstances of the times imply that the overall groups existed in some form. Scholars assume that major Ancient Greek period dialect groups developed not than 1120 BCE, at the time of the Dorian invasion—and that their first appearances as precise alphabetic writing began in the 8th century BCE.
The invasion would not be "Dorian" unless the invaders had some cultural relationship to the historical Dorians. The invasion is known to have displaced population to the Attic-Ionic regions, who regarded themselves as descendants of the population displaced by or contending with the Dorians; the Greeks of this period believed there were three major divisions of all Greek people—Dorians and Ionians, each with their own defining and distinctive dialects. Allowing for their oversight of Arcadian, an obscure mountain dialect, Cypriot, far from the center of Greek scholarship, this division of people and language is quite similar to the results of modern archaeological-linguistic investigation. One standard formulation for the dialects is: West vs. non-west Greek is the strongest marked and earliest division, with non-west in subsets of Ionic-Attic and Aeolic vs. Arcadocypriot, or Aeolic and Arcado-Cypriot vs. Ionic-Attic. Non-west is called East Greek. Arcadocypriot descended more from the Mycenaean Greek of the Bronze Age.
Boeotian had come under a strong Northwest Greek influence, can in some respects be considered a transitional dialect. Thessalian had come under Northwest Greek influence, though to a lesser degree. Pamphylian Greek, spoken in a small area on the southwestern coast of Anatolia and little preserved in inscriptions, may be either a fifth major dialect group, or it is Mycenaean Greek overlaid by Doric, with a non-Greek native influence. Most of the dialect sub-groups listed above had further subdivisions equivalent to a city-state and its surrounding territory, or to an island. Doric notably had several intermediate divisions as well, into Island Doric, Southern Peloponnesus Doric, Northern Peloponnesus Doric; the Lesbian dialect was Aeolic Greek. All the groups were represented by colonies beyond Greece proper as well, these colonies developed local characteristics under the influence of settlers or neighbors speaking different Greek dialects; the dialects outside the Ionic group are known from inscriptions, notable exceptions being: fragments of the works of the poet Sappho from the island of Lesbos, in Aeolian, the poems of the Boeotian poet Pindar and other lyric poets in Doric.
After the conquests of Alexander the Great in the late 4th century BCE, a new international dialect known as Koine or Common Greek developed based on Attic Greek, but with influence from other dialects. This dialect replaced most of the older dialects, although Doric dialect has survived in the Tsakonian language, spoken in the region of modern Sparta. Doric has passed down its aorist terminations into most verbs of Demotic Greek. By about the 6th century CE, the Koine had metamorphosized into Medieval Greek. Ancient Macedonian was an Indo-European language at least related to Greek, but its exact relationship is unclear because of insufficient data: a dialect of Greek; the Macedonian dialect (or l
Aristotle was a philosopher during the Classical period in Ancient Greece, the founder of the Lyceum and the Peripatetic school of philosophy and Aristotelian tradition. Along with his teacher Plato, he is considered the "Father of Western Philosophy", his writings cover many subjects – including physics, zoology, logic, aesthetics, theatre, rhetoric, linguistics, economics and government. Aristotle provided a complex synthesis of the various philosophies existing prior to him, it was above all from his teachings that the West inherited its intellectual lexicon, as well as problems and methods of inquiry; as a result, his philosophy has exerted a unique influence on every form of knowledge in the West and it continues to be a subject of contemporary philosophical discussion. Little is known about his life. Aristotle was born in the city of Stagira in Northern Greece, his father, died when Aristotle was a child, he was brought up by a guardian. At seventeen or eighteen years of age, he joined Plato's Academy in Athens and remained there until the age of thirty-seven.
Shortly after Plato died, Aristotle left Athens and, at the request of Philip II of Macedon, tutored Alexander the Great beginning in 343 BC. He established a library in the Lyceum which helped him to produce many of his hundreds of books on papyrus scrolls. Though Aristotle wrote many elegant treatises and dialogues for publication, only around a third of his original output has survived, none of it intended for publication; the fact that Aristotle was a pupil of Plato contributed to his former views of Platonism, following Plato's death, Aristotle developed an increased interest in natural sciences and adopted the position of immanent realism. Aristotle's views on physical science profoundly shaped medieval scholarship, their influence extended from Late Antiquity and the Early Middle Ages into the Renaissance, were not replaced systematically until the Enlightenment and theories such as classical mechanics. Some of Aristotle's zoological observations found in his biology, such as on the hectocotyl arm of the octopus, were disbelieved until the 19th century.
His works contain the earliest known formal study of logic, studied by medieval scholars such as Peter Abelard and John Buridan. Aristotle's influence on logic continued well into the 19th century He influenced Islamic thought during the Middle Ages, as well as Christian theology the Neoplatonism of the Early Church and the scholastic tradition of the Catholic Church. Aristotle was revered among medieval Muslim scholars as "The First Teacher" and among medieval Christians like Thomas Aquinas as "The Philosopher", his ethics, though always influential, gained renewed interest with the modern advent of virtue ethics, such as in the thinking of Alasdair MacIntyre and Philippa Foot. In general, the details of Aristotle's life are not well-established; the biographies written in ancient times are speculative and historians only agree on a few salient points. Aristotle, whose name means "the best purpose" in Ancient Greek, was born in 384 BC in Stagira, about 55 km east of modern-day Thessaloniki.
His father Nicomachus was the personal physician to King Amyntas of Macedon. Both of Aristotle's parents died when he was about thirteen, Proxenus of Atarneus became his guardian. Although little information about Aristotle's childhood has survived, he spent some time within the Macedonian palace, making his first connections with the Macedonian monarchy. At the age of seventeen or eighteen, Aristotle moved to Athens to continue his education at Plato's Academy, he remained there for nearly twenty years before leaving Athens in 348/47 BC. The traditional story about his departure records that he was disappointed with the Academy's direction after control passed to Plato's nephew Speusippus, although it is possible that he feared the anti-Macedonian sentiments in Athens at that time and left before Plato died. Aristotle accompanied Xenocrates to the court of his friend Hermias of Atarneus in Asia Minor. After the death of Hermias, Aristotle travelled with his pupil Theophrastus to the island of Lesbos, where together they researched the botany and zoology of the island and its sheltered lagoon.
While in Lesbos, Aristotle married Hermias's adoptive daughter or niece. She bore him a daughter, whom they named Pythias. In 343 BC, Aristotle was invited by Philip II of Macedon to become the tutor to his son Alexander. Aristotle was appointed as the head of the royal academy of Macedon. During Aristotle's time in the Macedonian court, he gave lessons not only to Alexander, but to two other future kings: Ptolemy and Cassander. Aristotle encouraged Alexander toward eastern conquest and Aristotle's own attitude towards Persia was unabashedly ethnocentric. In one famous example, he counsels Alexander to be "a leader to the Greeks and a despot to the barbarians, to look after the former as after friends and relatives, to deal with the latter as with beasts or plants". By 335 BC, Aristotle had returned to Athens. Aristotle conducted courses at the school for the next twelve years. While in Athens, his wife Pythias died and Aristotle became involved with Herpyllis of Stagira, who bore him a son whom he named after his father, Nicomachus.
According to the Suda, he had an erômenos, Palaephatus of Abydus. This period in Athens, between 335 and 323 BC, is when Aristotle is believed to have composed many of his works, he wrote many dialogues. Those works that have survived are in treatise form and were not
Athens is the capital and largest city of Greece. Athens dominates the Attica region and is one of the world's oldest cities, with its recorded history spanning over 3,400 years and its earliest human presence starting somewhere between the 11th and 7th millennium BC. Classical Athens was a powerful city-state that emerged in conjunction with the seagoing development of the port of Piraeus, a distinct city prior to its 5th century BC incorporation with Athens. A center for the arts and philosophy, home of Plato's Academy and Aristotle's Lyceum, it is referred to as the cradle of Western civilization and the birthplace of democracy because of its cultural and political impact on the European continent, in particular the Romans. In modern times, Athens is a large cosmopolitan metropolis and central to economic, industrial, maritime and cultural life in Greece. In 2012, Athens was ranked the world's 39th richest city by purchasing power and the 67th most expensive in a UBS study. Athens is a global one of the biggest economic centres in southeastern Europe.
It has a large financial sector, its port Piraeus is both the largest passenger port in Europe, the second largest in the world. While at the same time being the sixth busiest passenger port in Europe; the Municipality of Athens had a population of 664,046 within its administrative limits, a land area of 38.96 km2. The urban area of Athens extends beyond its administrative municipal city limits, with a population of 3,090,508 over an area of 412 km2. According to Eurostat in 2011, the functional urban area of Athens was the 9th most populous FUA in the European Union, with a population of 3.8 million people. Athens is the southernmost capital on the European mainland; the heritage of the classical era is still evident in the city, represented by ancient monuments and works of art, the most famous of all being the Parthenon, considered a key landmark of early Western civilization. The city retains Roman and Byzantine monuments, as well as a smaller number of Ottoman monuments. Athens is home to two UNESCO World Heritage Sites, the Acropolis of Athens and the medieval Daphni Monastery.
Landmarks of the modern era, dating back to the establishment of Athens as the capital of the independent Greek state in 1834, include the Hellenic Parliament and the so-called "architectural trilogy of Athens", consisting of the National Library of Greece, the National and Kapodistrian University of Athens and the Academy of Athens. Athens is home to several museums and cultural institutions, such as the National Archeological Museum, featuring the world's largest collection of ancient Greek antiquities, the Acropolis Museum, the Museum of Cycladic Art, the Benaki Museum and the Byzantine and Christian Museum. Athens was the host city of the first modern-day Olympic Games in 1896, 108 years it welcomed home the 2004 Summer Olympics, making it one of only a handful of cities to have hosted the Olympics more than once. In Ancient Greek, the name of the city was Ἀθῆναι a plural. In earlier Greek, such as Homeric Greek, the name had been current in the singular form though, as Ἀθήνη, it was rendered in the plural on, like those of Θῆβαι and Μυκῆναι.
The root of the word is not of Greek or Indo-European origin, is a remnant of the Pre-Greek substrate of Attica. In antiquity, it was debated whether Athens took its name from its patron goddess Athena or Athena took her name from the city. Modern scholars now agree that the goddess takes her name from the city, because the ending -ene is common in names of locations, but rare for personal names. During the medieval period, the name of the city was rendered once again in the singular as Ἀθήνα. However, after the establishment of the modern Greek state, due to the conservatism of the written language, Ἀθῆναι became again the official name of the city and remained so until the abandonment of Katharevousa in the 1970s, when Ἀθήνα, Athína, became the official name. According to the ancient Athenian founding myth, the goddess of wisdom, competed against Poseidon, the god of the seas, for patronage of the yet-unnamed city. According to the account given by Pseudo-Apollodorus, Poseidon struck the ground with his trident and a salt water spring welled up.
In an alternative version of the myth from Vergil's Georgics, Poseidon instead gave the Athenians the first horse. In both versions, Athena offered the Athenians the first domesticated olive tree. Cecrops declared Athena the patron goddess of Athens. Different etymologies, now rejected, were proposed during the 19th century. Christian Lobeck proposed as the root of the name the word ἄθος or ἄνθος meaning "flower", to denote Athens as the "flowering city". Ludwig von Döderlein proposed the stem of the verb θάω, stem θη- to denote Athens as having fertile soil. In classical literature, the city was sometimes referred to as the City of the Violet Crown, first documented in Pindar's ἰοστέφανοι Ἀθᾶναι, or as τὸ κλεινὸν ἄστυ. In medieval texts, variant names include Setines and Astines, all derivations involving false splitting of p
Pella is an ancient city located in Central Macedonia, best known as the historical capital of the ancient Greek kingdom of Macedon and birthplace of Alexander the Great. On the site of the ancient city is the Archaeological Museum of Pella. A common folk etymology is traditionally given for the name Pella, deriving it from the Ancient Greek word pélla, "stone", it appears in some toponyms in Greece like Pallini. With the prefix a - it forms the Doric apella, meaning in enclosure of stone; the word apella meant fold, fence for animals, assembly of people into the limits of the square. Τhe original meaning was "wooden bowl", it meant "stone". R. S. P. Beekes has suggested a Pre-Greek proto-form *πελσα In Antiquity, Pella was a strategic port connected to the Thermaic Gulf by a navigable inlet, but the harbour and gulf have since silted up, leaving the site landlocked. Pella is first mentioned by Herodotus of Halicarnassus in relation to Xerxes' campaign and by Thucydides in relation to Macedonian expansion and the war against Sitalces, the king of the Thracians.
It was built as the capital of the kingdom by Archelaus I, replacing the older palace-city of Aigai although there appears to be some possibility that it may have been created by Amyntas. Archelaus invited the greatest painter of the time, to decorate his palace, he later hosted the poet Timotheus of Miletus and the Athenian playwright Euripides who finished his days there writing and producing Archelaus. Euripides Bacchae was first staged here, about 408 BC. According to Xenophon, in the beginning of the 4th century BC Pella was the largest Macedonian city, it was the birthplace and seats of Philip II, in 382 BC and of Alexander the Great, his son, in 356 BC. It became the largest and richest city in Macedonia and flourished under Cassander's rule; the reign of Antigonus most represented the height of the city's prosperity, as this is the period which has left us most archaeological remains. The famous poet Aratus died in Pella c. 240 BC. Pella is further mentioned by Polybius and Livy as the capital of Philip V and of Perseus during the Macedonian Wars fought against the Roman Republic.
In 168 BC, it was sacked by the Romans, its treasury transported to Rome, Livy reported how the city looked in 167 BC to Lucius Aemilius Paulus Macedonicus, the Roman who defeated Perseus at the battle of Pydna:... observed that it was not without good reason that it had been chosen as the royal residence. It is situated on the south-west slope of a hill and surrounded by a marsh too deep to be crossed on foot either in summer or winter; the citadel the "Phacus,", close to the city, stands in the marsh itself, projecting like an island, is built on a huge substructure, strong enough to carry a wall and prevent any damage from the infiltration from the water of the lagoon. At a distance it appears to be continuous with the city wall, but it is separated by a channel which flows between the two walls and is connected with the city by a bridge, thus it cuts off all means of access from an external foe, if the king shut anyone up there, there could be no possibility of escape except by the bridge, which could be easily guarded.
Pella was declared capital of the 3rd administrative division of the Roman province of Macedonia, was the seat of the Roman governor. Activity continued to be vigorous until the early 1st century BC and, crossed by the Via Egnatia, Pella remained a significant point on the route between Dyrrachium and Thessalonika. In about 90 BC the city was destroyed by an earthquake. Cicero stayed there in 58 BC, though by the provincial seat had transferred to Thessalonika. Pella was promoted to a Roman Colony sometime between 45 and 30 BC and its currency was marked Colonia Iulia Augusta Pella. Augustus settled peasants there. But, unlike other Macedonian colonies such as Philippi and Cassandreia, it never came under the jurisdiction of ius Italicum or Roman law. Four pairs of colonial magistrates are known for this period; the decline of the city was rapid, in spite of being a Colonia: Dio Chrysostom and Lucian both attest to the ruin of the ancient capital of Philip II and Alexander, though their accounts may be exaggerated.
In fact, the Roman city was somewhat to the west of and distinct from the original capital, which explains some contradictions between coinage and testimonial accounts. Despite its decline, archaeology has shown that the southern part of the city near the lagoon continued to be occupied until the 4th century.. In about 180 AD, Lucian of Samosata could describe it in passing as "now insignificant, with few inhabitants", it bore the name Diocletianopolis. In the Byzantine period, the Roman site was occupied by a fortified village. Excavations there by the Greek Archaeological Service begun in 1957 revealed large, well-built houses with colonnaded courts and rooms with mosaic floors portraying such scenes as a lion hunt and Dionysus riding a panther. In modern times it finds itself as the starting point of the Alexander The Great Marathon, in honour of the city's ancient heritage; the site was explored by 19th-century voyagers including Holand, Beaujour, Cousinéry, Hahn and Struck, based o
Ancient Greek astronomy
Greek astronomy is astronomy written in the Greek language in classical antiquity. Greek astronomy is understood to include the ancient Greek, Greco-Roman, Late Antiquity eras, it is not limited geographically to Greece or to ethnic Greeks, as the Greek language had become the language of scholarship throughout the Hellenistic world following the conquests of Alexander. This phase of Greek astronomy is known as Hellenistic astronomy, while the pre-Hellenistic phase is known as Classical Greek astronomy. During the Hellenistic and Roman periods, much of the Greek and non-Greek astronomers working in the Greek tradition studied at the Musaeum and the Library of Alexandria in Ptolemaic Egypt; the development of astronomy by the Greek and Hellenistic astronomers is considered, by historians, to be a major phase in the history of astronomy. Greek astronomy is characterized from the start by seeking a rational, physical explanation for celestial phenomena. Most of the constellations of the northern hemisphere derive from Greek astronomy, as are the names of many stars and planets.
It was influenced by Egyptian and Babylonian astronomy. References to identifiable stars and constellations appear in the writings of Homer and Hesiod, the earliest surviving examples of Greek literature. In the oldest European texts, the Iliad and the Odyssey, Homer has several astronomical phenomena including solar eclipses. Eclipses that can permit the dating of these events as the place is known and the calculation of the time is possible if other celestial phenomena are described at the same time. In the Iliad and the Odyssey, Homer refers to the following celestial objects: the constellation Boötes the star cluster Hyades the constellation Orion the star cluster Pleiades Sirius, the Dog Star the constellation Ursa Major Hesiod, who wrote in the early 7th century BC, adds the star Arcturus to this list in his poetic calendar Works and Days. Though neither Homer nor Hesiod set out to write a scientific work, they hint at a rudimentary cosmology of a flat Earth surrounded by an "Ocean River."
Some stars set. At certain times of the year, certain stars will set at sunrise or sunset. Speculation about the cosmos was common in Pre-Socratic philosophy in the 6th and 5th centuries BC. Anaximander described a cyclical earth suspended in the center of the cosmos, surrounded by rings of fire. Philolaus the Pythagorean described a cosmos with the stars, Sun, Earth, a counter-Earth —ten bodies in all—circling an unseen central fire; such reports show that Greeks of the 6th and 5th centuries BC were aware of the planets and speculated about the structure of the cosmos. A more detailed description about the cosmos, Sun and the Earth can be found in the Orphism, which dates back to the end of the 5th century BC, it is even older. Within the lyrics of the Orphic poems we can find remarkable information such as that the Earth is round, it has an axis and it moves around it in one day, it has three climate zones and that the Sun magnetizes the Stars and planets; the name "planet" comes from the Greek term πλανήτης, meaning "wanderer", as ancient astronomers noted how certain lights moved across the sky in relation to the other stars.
Five planets can be seen with the naked eye: Mercury, Mars and Saturn, the Greek names being Hermes, Ares and Cronus. Sometimes the luminaries, the Sun and Moon, are added to the list of naked eye planets to make a total of seven. Since the planets disappear from time to time when they approach the Sun, careful attention is required to identify all five. Observations of Venus are not straightforward. Early Greeks thought that the evening and morning appearances of Venus represented two different objects, calling it Hesperus when it appeared in the western evening sky and Phosphorus when it appeared in the eastern morning sky, they came to recognize that both objects were the same planet. Pythagoras is given credit for this realization. In classical Greece, astronomy was a branch of mathematics; this tradition began with the Pythagoreans. The study of number comprising the four arts was called the Quadrivium. Although he was not a creative mathematician, Plato included the quadrivium as the basis for philosophical education in the Republic.
He encouraged Eudoxus of Cnidus, to develop a system of Greek astronomy. According to a modern historian of science, David Lindberg: "In their work we find a shift from stellar to planetary concerns, the creation of a geometrical model, the "two-sphere model," for the representation of stellar and planetary phenomena, the establishment of criteria governing theories designed to account for planetary observations"; the two-sphere model is a geocentric model that divides the cosmos into two regions, a spherical Earth and motionless and a spherical heavenly realm centered on the Earth, which may contain multiple rotating spheres made of aether. Plato's main books on cosmology are the Republic. In them he described the two-sphere model and said there were eight circles or spheres carrying the seven planets and the fixed stars. According to the "Myth of Er" in the Republic, the cosmos is the Spindle of Nec
The Milky Way is the galaxy that contains our Solar System. The name describes the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye; the term Milky Way is a translation of the Latin via lactea, from the Greek γαλαξίας κύκλος. From Earth, the Milky Way appears as a band. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610; until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies; the Milky Way is a barred spiral galaxy with a diameter between 200,000 light-years. It is estimated to contain 100 -- more than 100 billion planets; the Solar System is located at a radius of 26,490 light-years from the Galactic Center, on the inner edge of the Orion Arm, one of the spiral-shaped concentrations of gas and dust.
The stars in the innermost 10,000 light-years form a bulge and one or more bars that radiate from the bulge. The galactic center is an intense radio source known as Sagittarius A*, assumed to be a supermassive black hole of 4.100 million solar masses. Stars and gases at a wide range of distances from the Galactic Center orbit at 220 kilometers per second; the constant rotation speed contradicts the laws of Keplerian dynamics and suggests that much of the mass of the Milky Way is invisible to telescopes, neither emitting nor absorbing electromagnetic radiation. This conjectural mass has been termed "dark matter"; the rotational period is about 240 million years at the radius of the Sun. The Milky Way as a whole is moving at a velocity of 600 km per second with respect to extragalactic frames of reference; the oldest stars in the Milky Way are nearly as old as the Universe itself and thus formed shortly after the Dark Ages of the Big Bang. The Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which form part of the Virgo Supercluster, itself a component of the Laniakea Supercluster.
The Milky Way is visible from Earth as a hazy band of white light, some 30° wide, arching across the night sky. In night sky observing, although all the individual naked-eye stars in the entire sky are part of the Milky Way, the term “Milky Way” is limited to this band of light; the light originates from the accumulation of unresolved stars and other material located in the direction of the galactic plane. Dark regions within the band, such as the Great Rift and the Coalsack, are areas where interstellar dust blocks light from distant stars; the area of sky that the Milky Way obscures is called the Zone of Avoidance. The Milky Way has a low surface brightness, its visibility can be reduced by background light, such as light pollution or moonlight. The sky needs to be darker than about 20.2 magnitude per square arcsecond in order for the Milky Way to be visible. It should be visible if the limiting magnitude is +5.1 or better and shows a great deal of detail at +6.1. This makes the Milky Way difficult to see from brightly lit urban or suburban areas, but prominent when viewed from rural areas when the Moon is below the horizon.
Maps of artificial night sky brightness show that more than one-third of Earth's population cannot see the Milky Way from their homes due to light pollution. As viewed from Earth, the visible region of the Milky Way's galactic plane occupies an area of the sky that includes 30 constellations; the Galactic Center lies in the direction of Sagittarius. From Sagittarius, the hazy band of white light appears to pass around to the galactic anticenter in Auriga; the band continues the rest of the way around the sky, back to Sagittarius, dividing the sky into two equal hemispheres. The galactic plane is inclined by about 60° to the ecliptic. Relative to the celestial equator, it passes as far north as the constellation of Cassiopeia and as far south as the constellation of Crux, indicating the high inclination of Earth's equatorial plane and the plane of the ecliptic, relative to the galactic plane; the north galactic pole is situated at right ascension 12h 49m, declination +27.4° near β Comae Berenices, the south galactic pole is near α Sculptoris.
Because of this high inclination, depending on the time of night and year, the arch of the Milky Way may appear low or high in the sky. For observers from latitudes 65° north to 65° south, the Milky Way passes directly overhead twice a day; the Milky Way is the second-largest galaxy in the Local Group, with its stellar disk 100,000 ly in diameter and, on average 1,000 ly thick. The Milky Way is 1.5 trillion times the mass of the Sun. To compare the relative physical scale of the Milky Way, if the Solar System out to Neptune were the size of a US quarter, the Milky Way would be the size of the contiguous United States. There is a ring-like filament of stars rippling above and below the flat galactic plane, wrapping around the Milky Way at a diameter of 150,000–180,000 light-years, which may be part of the Milky Way itself. Estimates of the mass of the Milky Way vary, depending upon the method and data used; the low end of the estimate range is 5.8×1011 solar masses, somewhat less than that of the Andromeda Galaxy.
Measurements using the Very Long Baseline Array in 2009 found