Astronomy in Ancient Greece

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In ancient times astrology and astronomy were the same thing. Among Greece's most famous astronomers were Anaximander (611-546 B.C.), who stated all the heavenly bodies were discs; Eudoxus of Cindus (408-355 B.C.), who asserted the planets moved in concentric spheres around the Earth; and Aristarchus (live around 280 B.C.), who hypothesized that the sun not the Earth was the center of the universe. Complicated mathematical equations were developed to explain the movements of heavenly bodies in an effort to make astrology seem like a science.

Joseph Castro wrote in Live Science: “Most of what's known about early Greek astronomy comes from various literary texts, such as Aratus of Soli's Phaenomena, a poetic text that describes the Greek constellations known by the third century B.C. However, these valuable documents only date as far back as the Classical period of Ancient Greece, which lasted from the fifth to the fourth century B.C. [Source: Joseph Castro, Live Science, October 27, 2014 ***]

“To learn about how the ancient Greeks viewed the night sky before then, researchers must rely on visual depictions of the sky, such as those found on ceramic pottery — but these artifacts are relatively rare, and what's left of them generally only show one or two constellations. For example, one of the oldest constellation images from Greece comes from a pottery fragment from the Late Geometric period (760 to 700 B.C) found at a site on the island of Ischia in Italy, but it only depicts what may be the constellation Boötes ("the Herdsman"). ***

Great Astronomical Discoveries Made by the Ancient Greeks

Planets Orbit the Sun: Gareth Dorrian and Ian Whittaker wrote: Aristarchus of Samos (310–230 B.C.) argued that the Sun was the “central fire” of the cosmos and he placed all of the then known planets in their correct order of distance around it. This is the earliest known heliocentric theory of the solar system. “Unfortunately, the original text in which he makes this argument has been lost to history, so we cannot know for certain how he worked it out. Aristarchus knew the Sun was much bigger than the Earth or the Moon, and he may have surmised that it should therefore have the central position in the solar system. “Nevertheless it is a jawdropping finding, especially when you consider that it wasn’t rediscovered until the 16th century, by Nicolaus Copernicus, who even acknowledged Aristarchus during the development of his own work. [Source:Gareth Dorrian, Post Doctoral Research Fellow in Space Science, University of Birmingham and Ian Whittaker, Lecturer in Physics, Nottingham Trent University, The Conversation, April 24, 2020]

Size of the Moon: One of Aristarchus’ books that did survive is about the sizes and distances of the Sun and Moon. In this remarkable treatise, Aristarchus laid out the earliest known attempted calculations of the relative sizes and distances to the Sun and Moon. “It had long been observed that the Sun and Moon appeared to be of the same apparent size in the sky, and that the Sun was further away. They realised this from solar eclipses, caused by the Moon passing in front of the Sun at a certain distance from Earth.

“Also, at the instant when the Moon is at first or third quarter, Aristarchus reasoned that the Sun, Earth, and Moon would form a right-angled triangle. As Pythagoras had determined how the lengths of triangle’s sides were related a couple of centuries earlier, Aristarchus used the triangle to estimate that the distance to the Sun was between 18 and 20 times the distance to the Moon. He also estimated that the size of the Moon was approximately one-third that of Earth, based on careful timing of lunar eclipses.

While his estimated distance to the Sun was too low (the actual ratio is 390), on account of the lack of telescopic precision available at the time, the value for the ratio of the size of the Earth to the Moon is surprisingly accurate (the Moon has a diameter 0.27 times that of Earth). Today, we know the size and distance to the moon accurately by a variety of means, including precise telescopes, radar observations and laser reflectors left on the surface by Apollo astronauts.

Earth’s Circumference: Eratosthenes (276–195 B.C.) was chief librarian at the Great Library of Alexandria, and a keen experimentalist. Among his many achievements was the earliest known calculation of the circumference of the Earth. Pythagoras is generally regarded as the earliest proponent of a spherical Earth, although apparently not its size. Eratosthenes’ famous and yet simple method relied on measuring the different lengths of shadows cast by poles stuck vertically into the ground, at midday on the summer solstice, at different latitudes.

“The Sun is sufficiently far away that, wherever its rays arrive at Earth, they are effectively parallel, as had previously been shown by Aristarchus. So the difference in the shadows demonstrated how much the Earth’s surface curved. Eratosthenes used this to estimate the Earth’s circumference as approximately 40,000 kilometers. This is within a couple of percent of the actual value, as established by modern geodesy (the science of the Earth’s shape).

“Later, another scientist called Posidonius (135–51 B.C.) used a slightly different method and arrived at almost exactly the same answer. Posidonius lived on the island of Rhodes for much of his life. There he observed the bright star Canopus would lie very close to the horizon. However, when in Alexandria, in Egypt, he noted Canopus would ascend to some 7.5 degrees above the horizon. Given that 7.5 degrees is 1/48th of a circle, he multiplied the distance from Rhodes to Alexandria by 48, and arrived at a value also of approximately 40,000 kilometers.

Ancient Greeks and Babylonian Astrology and Astronomy

Cosmic systems — Ptolemy to Copernicus

The Egyptians refined the Babylonian system of astrology and the Greeks shaped it into its modern form. The Ancient Greeks were skeptical about astrology. They wondered, for example, why twins born under the same astrological conditions had different fortunes, and why animals weren't ruled by the same cosmic powers as humans. [Source: "The Discoverers" by Daniel Boorstin,∞]

Astrology as we know it originated in Babylon. It developed out of the belief that since the Gods in the heavens ruled man's fate, the stars could reveal fortunes and the notion that the motions of the stars and planets control the fate of people on earth. The motions of the stars and planets are mainly the result of the earth’s movement around the sun, which causes: 1) the sun to move eastward against the background of the constellations; 2) the planets and moon to shift around the sky; and 3) causes different constellations to rise from the horizon at sunset different times of the year.

The Babylonians were the first people to apply myths to constellations and astrology and describe the 12 signs of the zodiac. The Egyptians refined the Babylonian system of astrology and the Greeks shaped it into its modern form. The Greeks and Romans borrowed some of their myths from the Babylonians and invented their own. The word astrology (and astronomy) are derived from the Greek word for "star."

The names and shapes of many the constellations are believed to date to Sumerian times because the animals and figures chosen held a prominent place in their lives. It is thought that if the constellations originated with the the Egyptians were would ibises, jackals, crocodiles and hippos — animals in their environment — rather than goats and bulls. If they came from India why isn’t there a tiger or a monkey. To the Assyrians the constellation Capricorn was “ munaxa” (the goat fish).

The Greeks added names of heroes to the constellations. The Romans took these and gave them the Latin names we use today. Ptolemy listed 48 constellations. His list included ones in the southern hemisphere, which he and the Mesopotamians, Egyptians, Greeks and Romans couldn’t see.

Book: “ Astrology: A History” by Peter Whitfield (Abrams, 2001).

Influence of Babylonian Astronomy on Ancient Greek Science

John Burnet wrote in “Early Greek Philosophy”: “The other source from which the Ionians were supposed to have derived their science is Babylonian astronomy. It is certain, of course, that the Babylonians had observed the heavens from an early date. They had planned out the fixed stars, and especially those of the zodiac, in constellations. That is useful for purposes of observational astronomy, but in itself it belongs rather to mythology or folklore. They had distinguished and named the planets and noted their apparent motions. They were well aware of their stations and retrograde movements, and they were familiar with the solstices and equinoxes. They had also noted the occurrence of eclipses with a view to predicting their return for purposes of divination. But we must not exaggerate the antiquity or accuracy of these observations. It was long before the Babylonians had a satisfactory calendar, and they kept the year right only by intercalating a thirteenth month when it seemed desirable. That made a trustworthy chronology impossible, and therefore there were not and could not be any data available for astronomical purposes before the so-called era of Nabonassar (747 B.C.).

The oldest astronomical document of a really scientific character which had come to light up to 1907 is dated 523 B.C., in the reign of Cambyses, when Pythagoras had already founded his school at Croton. Moreover, the golden age of Babylonian observational astronomy is now assigned to the period after Alexander the Great, when Babylon was a Hellenistic city. Even then, though great accuracy of observation was attained, and data were accumulated which were of service to the Alexandrian astronomers, there is no evidence that Babylonian astronomy had passed beyond the empirical stage. [Source: John Burnet (1863-1928), “Early Greek Philosophy” London and Edinburgh: A. and C. Black, 1892, 3rd edition, 1920, Evansville University]

“We shall see that Thales probably knew the cycle by means of which the Babylonians tried to predict eclipses; but it would be a mistake to suppose that the pioneers of Greek science had any detailed knowledge of Babylonian observations. The Babylonian names of the planets do not occur earlier than the writings of Plato's old age. We shall find, indeed, that the earliest cosmologists paid no attention to the planets, and it is hard to say what they thought about the fixed stars. That, in itself, shows that they started for themselves, and were quite independent of Babylonian observations, and the recorded observations were only made fully available in Alexandrian times. But, even if the Ionians had known them, their originality would remain. The Babylonians recorded celestial phenomena for astrological purposes, not from any scientific interest. There is no evidence that they attempted to account for what they saw in any but the crudest way. The Greeks, on the other hand, made at least three discoveries of capital importance in the course of two or three generations. In the first place, they discovered that the earth is a sphere and does not rest on anything. In the second place, they discovered the true theory of lunar and solar eclipses; and, in close connection with that, they came to see, in the third place, that the earth is not the center of our system, but revolves round the center like the planets. Not much later, certain Greeks took, at least tentatively, the final step of identifying the center round which the earth and planets revolve with the sun. These discoveries will be discussed in their proper place; they are only mentioned here to show the gulf between Greek astronomy and everything that had preceded it. On the other hand, the Greeks rejected astrology, and it was not till the third century B.C. that it was introduced among them.

“We may sum up all this by saying that the Greeks did not borrow either their philosophy or their science from the East. They did, however, get from Egypt certain rules of mensuration which, when generalized, gave birth to geometry; while from Babylon they learnt that the phenomena of the heavens recur in cycles. This piece of knowledge doubtless had a great deal to do with the rise of science; for to the Greek it suggested further questions such as no Babylonian ever dreamt of.”

Ancient Greek Ideas About Earth and the Universe

During Homer's time people thought the Earth was a circular disk surrounded by the river Oceanus. Herodotus (480-425 B.C.) said that this concept was completely wrong: the Earth was surrounded by desert not water. In the late fifth century B.C. Plato and the Pythagoreans argued that the sphere was the most perfect shape and it made sense that man would live on a object with such a shape. Eclipses, boats disappearing over the horizon, and the shape of the moon were all presented as evidence that the Earth was a sphere. [Source: "The Discoverers" by Daniel Boorstin,∞]

The Greeks believed that the Earth was a sphere and the heavens were a rotating spherical dome with the moving stars affixed to it. Beyond the heavenly dome was nothing, not even emptiness, and inside the dome the planets and the sun moved along their own separate paths.

Plato described the world as "round as from a lathe, having its extremes in every direction equidistant from the center, the most perfect and the most like itself of all figures...the like is infinitely better than the unlike." Aristotle aid the heavens were made of a transparent and weightless material known as "Ether" that carried the stars and planets on 55 concentrically organized spheres like "nests." The movements of the planets was described as the motions of the spheres on which the planets rested.∞

Hipparchus

Hipparchus in Raphael’s "The School of Athens"

Hipparchus (190-120 B.C.) was the greatest astronomer of the Greek era. Based in Alexandria, he is credited with using the the system of latitude and longitude to map the nightime sky based on astronomical observation superimposed on an imaginary grid on the Earth. He also discovered the procession of equinoxes, invented trigonometry, and mapped 1080 stars and placed them into six categories of brightness. Ptolemy used his data to show the Earth was the center of the solar system.

Jay Bennett wrote in National Geographic History: Hipparchus helped establish a new way of understanding the motions of the stars that persists to this day. By imagining Earth at the center of a celestial sphere, he used a coordinate system similar to latitude and longitude, which had recently been devised, to measure the precise positions of the stars. “He was arguably the greatest ancient astronomer. At least the greatest known to us by name,” says Victor Gysembergh, a science historian at the French National Center for Scientific Research. [Source Jay Bennett, National Geographic History. December 9, 2022]

Many ancient Greek scientists believed that Earth was literally at the center of the universe, and the stars and other celestial bodies rotated around it, although a model with Earth orbiting the sun was proposed in the 3rd century B.C. Although this geocentric model is incorrect, the concept, which Hipparchus used to create the first known star catalog, is still used by scientists to map objects in the sky.

Hipparchus was the first astronomer to note the progression of the seasons changed in relation to the background stars. In the 17th century Sir Isaac Newton deduced that the date of certain historical events could be ascertained by reexamining Hipparchus' calculations, which Newton said were slightly off. Using Hipparchus’s record Newton was the first to record the major historical events of the Greeks, Egyptians, Persians and the Hebrews's in David and Solomon's reign in the Bible in chronological order. By reexamining calculations made by Hipparchus and comparing them with observations made by other astronomers Newton reasoned that the voyage of Jason and the Argonauts took place 43 years after King Solomon's death. [Source: "The Discoverers" by Daniel Boorstin]

Hipparchus’s Map of the Night Sky, the World's First, Was Amazingly Accurate

In the second century B.C.,Hipparchus produced an incredibly accurate map of the nighttime sky. Science historians don't know exactly how Hipparchus measured the stars. Perhaps he used a sighting tool like that used by mariners to measure the positions of the stars.[Source Jay Bennett, National Geographic History. December 9, 2022]

dioptra copy from one made in the 1st century BC

Jay Bennett wrote in National Geographic History: Hipparchus’s star catalog is the oldest known attempt to document the positions of as many objects in the night sky as possible, and it was the first time that two coordinates were used to pinpoint each object’s location. But that original catalog is lost to time, and we know of it only thanks to the writings of later scientists such as Ptolemy, who created his own star catalog around 150 A.D. and attributed an earlier one to Hipparchus. Until recently the oldest evidence for stellar coordinates from Hipparchus was an 8th-century A.D. Latin translation of a poem about the constellations that includes the coordinates as a kind of annotation. [Source Jay Bennett, National Geographic History. December 9, 2022]

Gysembergh and his colleagues revealed even older evidence of star coordinates from Hipparchus in a 5th- or 6th-century A.D. Greek version of the same poem, Phenomena, originally written by the Greek poet Aratus in the 3rd century B.C. The poem, along with the accompanying star coordinates, had been erased from a reused medieval parchment and was recovered only through multispectral imaging, which uses different wavelengths of light to highlight the removed text.

The coordinates for the four stars to the farthest north, south, east, and west of the constellation Corona Borealis are included, though one of them could not be recovered from the manuscript. They were found to be accurate to within one degree of modern values—a remarkable achievement for someone working about 1,700 years before the invention of the telescope.

Using a phenomenon called precession, which is the wobble of Earth on its axis as it rotates, the researchers were able to determine that the coordinates match the positions of the stars in Corona Borealis as seen from the island of Rhodes around 130 B.C., which is where and when Hipparchus is thought to have taken most of his observations. Hipparchus may have been amused by this, as he was also the first scientist to describe the motion of precession.

Hypothesizing Maybe How Hipparchus Charted the Stars with Ancient Tools

Jay Bennett wrote in National Geographic History: Though historians don’t know exactly how Hipparchus measured the stars, he may have used an armillary sphere, which is a mechanical device with rotating rings that represent the different parts of the celestial sphere, such as the celestial equator, an imaginary plane spreading out from Earth’s equator, and the ecliptic, or the annual path that the sun appears to follow through the sky. Hipparchus may have also used a dioptra, which is a sight that could be attached to an adjustable platform. [Source Jay Bennett, National Geographic History. December 9, 2022]

The celestial sphere, an imaginary sphere in space with Earth as its center, is the foundation of the equatorial coordinate system. The coordinate system locates stars, planets, galaxies, and more with the celestial equivalents of longitude and latitude—known as right ascension (RA) and declination (Dec), respectively.

“The dioptra was a kind of surveying instrument,” says James Evans, a physicist and science historian at the University of Puget Sound in Washington State. It “could be used for measuring angles for surveying operations, but you could imagine something like that being used also to measure angles of the sky.” The armillary sphere, which takes its name from the Latin word for bracelet or arm band, is a sphere of concentric rings that could have sights on it. “You could set this up and use it to measure angles.”

How Hipparchus determined the distance to the Moon by using a partial solar eclipse at A (Alexandria) and a total solar eclipse at H (Hellespont)

Hipparchus was likely influenced by the earlier work of Babylonian astronomers, who measured the distances of certain constellations from the ecliptic. By tracking the movements of these zodiac constellations—the constellations that sit in a part of the sky that the sun moves through over the course of the year—the Babylonians could measure the seasons and predict astronomical events such as eclipses.

Combining the Babylonian practice of measuring and predicting the movements of the stars with Greek concepts of mathematics and geometry is considered the fundamental achievement of Hipparchus. “Modern astronomy really comes from the merging of these two different approaches,” Evans says. “The Greek approach based on geometry and philosophy of nature. The Babylonian approach based on regular observation and in crunching numbers.”

The newly discovered coordinates represent only a small fraction of about 800 stars that Hipparchus is believed to have measured. In total, only a few dozen coordinates survive that have been attributed to Hipparchus, but his work appears to be more accurate than Ptolemy’s later catalog. “The sample size is admittedly small, so maybe there are errors elsewhere,” Gysembergh says. “But as it stands, he’s more accurate than Ptolemy.”

The recent study compared the newly discovered coordinates to the values found in other sources, and they generally agreed, though some discrepancies exist, perhaps the result of different measurements or changes as the figures were transcribed through the ages.

Cup Offers Insights Into Ancient Greek Astronomy

A 2,600-year-old skyphos, two-handled wine cup, currently on display at the Lamia Archaeological Museum in Greece has long been thought to depict a random assortment of animals but in actuality the piece,may contain one of the earliest Greek depictions of the constellations. Joseph Castro wrote in Live Science: “The study researchers suggested that other ancient artistic representations of animals may also portray constellations, and hold clues to what the early Greeks knew about astronomy, said study researcher John Barnes, a classical archaeology doctoral candidate at the University of Missouri. “If we go back and re-evaluate other animal scenes that might have been originally categorized as hunting scenes or animal friezes, then maybe we can find more [depictions of constellations] and get a greater understanding of how the ancient Greeks viewed the night sky," Barnes told Live Science. [Source: Joseph Castro, Live Science, October 27, 2014 ***]

“Barnes didn't set out to find ancient Greek constellation portrayals, but rather stumbled upon the curious skyphos while visiting the Lamia Archaeological Museum. The artifact, which dates back to 625 B.C., was originally discovered in a debris-filled trench next to a temple in the seventh-century acropolis of Halai, which is located about 25 miles (40 kilometers) north of Thebes, Greece. About a third of the wine cup (including one handle) is missing. What's left of the skyphos depicts an array of animals: a bull with only the back half preserved, a snake, a hare or small dog, a large dog, a scorpion, a dolphin and the front half of a panther or lion. Though the skyphos was labeled as showing a simple animal scene, Barnes immediately thought it showed something else. ***

“Animal friezes (horizontal bands of decoration) and hunting scenes are common types of decorations in ancient Greece, but the skyphos's particular collection of animals is atypical, Barnes said. For instance, the dolphin is out of place with the land animals. Additionally, scorpions are uncommon motifs that don't often show up as actual animals, and are instead represented as shield emblems. And while a dog chasing a rabbit is often seen in hunting scenes, the snake underneath the pair is unusual. ***

“What's more likely is that the animals are constellations, Barnes said: The bull is Taurus; the snake is probably Hydra (rather than Serpens or Draco, two other serpent constellations recognized by the Greeks); the rabbit is Lepus; the dog is Canis Major or Canis Minor; the scorpion is Scorpius; the dolphin is Delphinus; and the lion is Leo. ***

Ptolemy's sky

“Interestingly, Barnes added, the animals are not arranged on the skyphos in the order they appear in the sky. "If they are not arranged as they are in the night sky, then either the specific arrangement is not important, or they were arranged for another purpose," Barnes said, adding that he thinks there's a seasonal aspect to the arrangement, with the constellations separated into fall, winter, spring and summer groups, in accordance with when they rise and set throughout the year. ***

“Specifically, the bull and (presumably) other constellations from the missing third of the skyphos represent fall; the snake, rabbit and dog make up winter; the dog (again) and scorpion belong to spring; and the dolphin and lion (and perhaps other missing constellations) signify summer, Barnes added. However, the skyphos likely didn't function as an ancient calendar, and instead merely showed a generalized representation of time throughout the year, Barnes said. Barnes' analysis of the skyphos was detailed in the April-June issue of the journal Hesperia.” ***

Image Sources: Wikimedia Commons, The Louvre, The British Museum

Text Sources: Internet Ancient History Sourcebook: Greece sourcebooks.fordham.edu ; Internet Ancient History Sourcebook: Hellenistic World sourcebooks.fordham.edu ; BBC Ancient Greeks bbc.co.uk/history/; Canadian Museum of History, Perseus Project - Tufts University; perseus.tufts.edu ; MIT Classics Online classics.mit.edu ; Gutenberg.org, Metropolitan Museum of Art, National Geographic, Smithsonian magazine, New York Times, Washington Post, Live Science, Discover magazine, Natural History magazine, Archaeology magazine, The New Yorker, Encyclopædia Britannica, "The Discoverers" and "The Creators" by Daniel Boorstin. "Greek and Roman Life" by Ian Jenkins from the British Museum, Wikipedia, Reuters, Associated Press, The Guardian, AFP and various books and other publications.

Last updated September 2024