Hipparchus of Nicea (l. c. 190 - c. 120 BCE) was a Greek astronomer, geographer, and mathematician regarded as the greatest astronomer of antiquity and one of the greatest of all time. He is best known for his discovery of the precession of the equinoxes and contributed significantly to the field of astronomy on every level.
He was born in Nicea, a city in the region of Bithynia in Asia Minor (modern-day Turkey), and is said to have died in Rhodes; nothing else is known of his life. Almost all of his works have been lost and his contributions are only known through the works of later writers including Strabo (l. 64 BCE - c. 24 CE), Pliny the Elder (l. c. 23-79 CE), Ptolemy (l. 100-170 CE), Pappus of Alexandria (l. c. 290-c. 350 CE), and Theon of Alexandria (l. c. 225 - c. 405 CE). The Almagest of Ptolemy (l. 100-170 CE), informed largely by Hipparchus’ work, became the standard astronomical text for almost 2,000 years. Theon notes that part of his commentary on Ptolemy’s work was prepared by his daughter Hypatia of Alexandria (l. c. 370-415 CE).
Hipparchus drew on the work of earlier Greek thinkers such as the Pre-Socratic Philosophers, Aristarchus of Samos (l. c. 310 - c. 230 BCE), Eratosthenes (l. 276-195 BCE), and Archimedes of Syracuse (l. c. 287-212 BCE) as well as from Babylonian and Egyptian sources which he examined according to his own exacting methodology in order to confirm accuracy. He regularly examined the conclusions of his predecessors to make sure they were correct and seems to have often been a harsh critic.
He is said to have either invented or developed trigonometry, created the most complete star chart of his time, calculated the position, size, and orbit of the Sun, Moon, and planets, and is one of the strongest contenders for the honor of being the inventor of the Antikythera Mechanism (also known as the Antikythera Device), considered the world’s first analogue computer. In the present day, he is one of the most highly regarded ancient Greek astronomers and is considered the greatest in all of antiquity by many scholars.
Babylonian, Egyptian, & Pre-Socratic Astronomy
Although Hipparchus is known to have drawn on Mesopotamian and Egyptian astronomical works, he was not the first Greek astronomer to do so. The Pre-Socratic philosophers of the Ionian School are said to have had access to Babylonian and Egyptian texts prior to c. 585 BCE. These thinkers lived and worked in the Greek colonies of Ionia, on the west coast of Asia Minor (modern-day Turkey), and were given their collective name Ionians by Aristotle (l. 384-322 BCE) in his summary of their work. He listed them as including:
- Thales of Miletus (l. c. 585 BCE)
- Anaximander (l. c. 610-c. 546 BCE)
- Anaximenes (l. c. 546 BCE)
- Heraclitus of Ephesus (l. c. 500 BCE)
- Anaxagoras (l. c. 500 - c. 428 BCE)
- Archelaus (l. c. 5th century BCE)
The philosopher Pythagoras (l. c. 571-c. 497 BCE) is often included as an Ionian as he was from the neighboring island of Samos. Thales is said to have studied in Babylon and Pythagoras is referenced as traveling and learning from wisemen in Egypt. Scholar T.L. Heath comments:
It is natural to ask how far the Greeks were indebted for the beginnings of their astronomy to the more ancient civilizations of Babylon and Egypt, not to speak of China…It was the Babylonians and Egyptians with whom the Ionian Greeks were in direct relations. (xiii-xiv)
The Ionians focused their efforts on discovering the First Cause of existence and this pursuit led them to astronomical observation and speculation. Plato (l. 428/427-348/347 BCE) in his Theaetetus relates how Thales was mocked by a servant-girl for falling into a well while he was gazing upwards observing the stars, “declaring that he was eager to know the things in the sky, but that what was behind him and just by his feet escaped his notice (174a). His observations and calculations, however, enabled him to accurately predict the solar eclipse of 28 May 585 BCE.
Thales’ works, like those of the others, only exist in fragments but later writers suggest they relied heavily on the earlier astronomical work of Egypt and, especially, of Mesopotamia. By the second millennium BCE the Babylonians are known to have been charting the heavens with Venus as the starting point. The Ionians were working with previously established systems of observation but, as Heath observes:
So far as we can judge, the Greeks were original and independent (1) in their cosmological speculations and (2) in their theories of astronomy. (liv)
The early work of the Ionians was then developed by later writers such as Philolaus of Croton (l. c. 470 - c. 385 BCE), Plato, and Aristotle. Aristotle’s astronomical theories became the standard against which any new claims were measured and was the most influential in directing, and also limiting, Hipparchus’ observations.
Aristotle maintained that the Earth was the center of the universe (the geocentric model) around which the sun, moon, and planets revolved in a circular motion. Their revolutions, he claimed, were perfect circles and, because they seemed to float in air, they must have been composed of some element lighter than earth but differing from water, air, or fire which he believed only corresponded to earth. This other element he defined as ether which was changeless and perfect and these perfect heavenly spheres, owing to their nature, revolved in absolutely perfect circles at a constant speed.
A problem the earlier Greek astronomers had grappled with was why the planets seemed to move randomly, sometimes forward or backwards, sometimes appearing brighter or dimmer, and Aristotle resolved the issue by claiming this was caused by their circular motion. A planet was brighter as it drew nearer to the Earth and dimmer as its motion carried it further away. Aristotle’s arguments seemed so convincing that his model was accepted as an accurate representation of the universe and so other models which contradicted it were rejected.
The most famous example of this is the work of Aristarchus of Samos who proposed a heliocentric model in which all planets, including earth, revolved around the sun. Since this contradicted Aristotle’s model, it was rejected and when Hipparchus of Nicea investigated Aristarchus’ proposal, he basically dismissed it for this reason. A heliocentric universe did not fit the established model of ethereal planets revolving in perfect circles.
Although he was therefore working from an inaccurate model of the solar system, Hipparchus’ calculations and observations were still accurate. He drew on the earlier work of the Babylonians and Egyptians but mathematically tested their conclusions to confirm them rather than simply accepting them as a given truth. He did the same with the works of the mathematician Archimedes and the polymath Eratosthenes and, in fact, criticized Eratosthenes harshly for his observations on geography.
Hipparchus’ methodology and conclusions, therefore, even though applied to a faulty model, gave rise to his reputation as the greatest astronomer of his time and, later, the estimation that he was the greatest in antiquity. Heath gives a brief description of the astronomical advances credited to Hipparchus:
Although, in a sense, the beginnings of trigonometry go back to Archimedes (Measurement of a Circle), Hipparchus was the first person who can be proved to have used trigonometry systematically. Hipparchus, the greatest astronomer of antiquity, whose observations were made between 161 and 126 BCE, discovered the precession of the equinoxes, calculated the mean lunar month at 29 days, 12 hours, 44 minutes, 2 ½ seconds (which differs by less than a second from the present accepted figure!), made more correct estimates of the sizes and distances of the sun and moon, introduced great improvements in the instruments used for observations, and compiled a catalog of some 850 stars; he seems to have been the first to state the position of these stars in terms of latitude and longitude (in relation to the ecliptic. He wrote a treatise in twelve books on Chords in a Circle, equivalent to a table of trigonometrical sines. For calculating arcs in astronomy from other arcs given by means of tables he used propositions in spherical trigonometry. (Livingstone, 132)
Many of his conclusions are still recognized as sound in the present day, especially the discovery he is most famous for, the precession of the equinoxes. The term refers to axial precession, the change in a celestial body’s rotational axis, as applied to earth. By measuring the longitude of certain known stars and comparing the measurements with those from earlier astronomers, then comparing how long it takes the Sun to return to an equinox and to an identifiable star, Hipparchus concluded that the rate of precession was around one degree in a century and so a full cycle would be completed in approximately 36,000 years.
His discovery enabled him to better map the heavens and predict when various astronomical events were likely to take place. Establishing the precession of the equinoxes made the universe more accessible and understandable. Planets did not move randomly but according to their nature which could be calculated mathematically. As he charted the skies, he developed various instruments to assist him, one of which may have been the famous Antikythera Mechanism.
Celestial Globe & Antikythera Mechanism
Hipparchus is said to have invented or improved upon a number of astronomical instruments, among them his celestial globe. According to Ptolemy, the globe rested within a flat plane and was encircled by a scale from which a grid divided it into lines of 24 hours. He is thought to have used the globe to work out the precession of the equinoxes and determine planetary movements as well as calculating the length of a year (which was accurate to within 6.5 minutes). His conclusions led him to criticize the works of earlier astronomers in the only work of his to have survived, Commentary on Aratus and Eudoxus, which, unfortunately, does not include his calculations.
He is also thought to have invented the Antikythera Mechanism, so named because it was discovered in 1901 off the Greek island of Antikythera. The device relies on Babylonian and Egyptian astronomical principles and uses various astronomical cycles with the span of 19 years to calculate the position of the Sun, Moon, and planets as well as when an eclipse was most likely. The device worked by turning a crank on the side which caused a series of gears to move and a pointer to shift to the positions one was attempting to calculate. All one needed to know was the present year to enable accurate predictions of future celestial events.
The Antikythera Mechanism Research Project claims that the device was most likely made by Hipparchus because “he is credited with the invention or improvement of several astronomical instruments” and so it is “very tempting to associate Hipparchus as the maker of the Mechanism, especially as he was contemporaneous with date of manufacture of the Mechanism, 150 to 100 BC” (1). The other most popular choice as inventor is Archimedes whose reputation as a genius engineer and inventor is widely acknowledged. Archimedes is also said to have improved upon or invented astronomical instruments and so, until some further information comes to light, the question continues to be debated by scholars.
Mathematics & Geography
Whether Hipparchus invented or developed trigonometry, he is recognized as the first to make ample use of its principles in creating astronomical models. It is possible that, using trigonometry, he invented the astrolabe, a model of the universe used for calculating planetary movements and eclipses. Although he rejected Aristarchus’ heliocentric model, he recognized the value of his estimates of the size and distance from the Earth of the Sun and Moon and improved upon them through more precise mathematical calculations.
As he had done with the Babylonian and Egyptian works, so he did with those of his predecessors and, for the most part, was critical of their work. He recalculated the motion of the Moon and what he interpreted as the motion of the Sun and ascertained that lunar eclipses are possible every five months and solar eclipses every seven. His mathematical skills also enabled him to create his star chart which then allowed him to rate the magnitude of stars on a scale from faintest (furthest away) to brightest (closest). His star chart was one of the sources for Ptolemy’s own in his Almagest which was regarded as a reliable source on astronomy up through the European Renaissance.
Applying his mathematical principles to geography, he attacked the work of Eratosthenes in a 3-volume manuscript, Against the Geography of Eratosthenes, which is lost but largely preserved by the geographer Strabo (l. 64 BCE – c. 24 CE) in his Geography. Eratosthenes had written a 3-volume work on geography in which he tried to chart the world and provided distances between various points. Hipparchus, according to Strabo, criticized the work for a number of faults including inaccuracy in measurement. Strabo alternately agrees with Hipparchus’ criticism or rejects it as overly harsh and unconvincing. Several of his comments suggest that Hipparchus may have had a reputation for arrogance and showed little tolerance for imprecision in others’ works:
In all these arguments, Hipparchus speaks as a geometrician, though his test of Eratosthenes is not convincing. And though he prescribed the principles of geometry for himself, he absolves himself from them by saying that, if the test showed errors amounting to only small distances, he could overlook them; but since Eratosthenes’ errors clearly amount to thousands of stadia, they cannot be overlooked. (II.i.35)
Strabo further criticizes Hipparchus for the same imprecision he condemns, claiming Hipparchus fails to cite Eratosthenes directly in his criticism but only paraphrases his claims or even “invents” them to make them easier to refute (II.1.22). While Strabo approves of Hipparchus’ goal in trying to correct earlier errors he notes how “he displays his propensity for fault-finding” suggesting he attacked other writers in the same way (II.1.36). This is corroborated by Hipparchus’ surviving text criticizing Aratus and Eudoxus.
Hipparchus, in fact, seems to have followed this course with a good many of his predecessors, correcting what he saw as their errors in establishing a model of the Earth and the universe which, once adopted by Ptolemy, would provide people with their understanding of both for almost 2,000 years. Like Aristotle, Hipparchus created a standard by which future astronomical claims would be accepted or rejected.
Ptolemy’s Almagest, informed largely by Hipparchus’ work (as noted), went unchallenged until Nicolaus Copernicus (l. 1473-1543) published his On the Revolution of the Celestial Spheres, suggesting the heliocentric model of the universe proposed earlier by Aristarchus of Samos. Copernicus’s work ignited the Scientific Revolution and inspired others including Tycho Brahe (l. 1546-1601), Johannes Kepler (l. 1571-1630), and Galileo Galilei (l. 1564-1642) who provided the foundation for Sir Isaac Newton (l. 1642-1727) to build on and mathematically prove the heliocentric model as correct.
Although, as noted, Hipparchus was working from a wrong model, his calculations within that framework were remarkably accurate and he no doubt would have made even greater progress if he had not been limited by the Aristotelian model he was unable to reject. Even considering this handicap, however, his contributions to astronomy are still recognized as the most impressive in antiquity and he continues to be regarded highly today. In 2004, he was inducted into the International Space Hall of Fame at the New Mexico Museum of Space History in Alamogordo, New Mexico, USA, and a lunar crater has been named in his honor.