Astronomy through the Ages- In Celebration of World Astronomy day
It seems that our love for the stars is almost instinctive. Archaeological records show that astronomy is one of the first natural sciences developed by early civilisations all over the globe. Therefore, it only seemed fitting for me to recount some of the most amazing stories about the development of our understanding of the cosmos around us for World Astronomy Day on 18 May. It is perhaps forgotten how central the work by people from across the world thousands of years ago is to the work being done today, and even more so in a very Eurocentric retelling of scientific innovation. This is why I will be endeavouring throughout this article to cover as many different cultures as possible.
Although ancient astronomers could perform only limited investigations of the sky, using rudimentary aids to the human eye, humankind had already begun the measurement of the positions of celestial bodies, trying to work out our position in the universe.
The first documented records of systematic astronomical observations date back to the Assyrian-Babylonian period around 1000 BCE. In the 8th and 7th centuries BCE, Babylonian astronomers developed a groundbreaking empirical approach to astronomy—later described as the first "successful attempt at giving a refined mathematical description of astronomical phenomena" and that "all subsequent varieties of scientific astronomy, in the Hellenistic world, in India, in Islam, and in the West ... depend upon Babylonian astronomy in decisive and fundamental ways.”
Babylonia offered uniquely fertile conditions for the development of astronomy. First, celestial phenomena held immense social significance: signs in the sky were believed to convey divine warnings to the king, foretelling events such as wars, crop failures, or epidemics. This ritualised celestial divination was already well-established by the early 2nd millennium BCE and was practised long before the rise of personal astrology. Unlike the Greeks, whose early omens stemmed from dreams or the behaviour of birds and oracles, the Babylonians focused their cosmic messages on state-level interpretations.
Second, a dedicated astronomical bureaucracy emerged. Temple scribes—often priests—systematically observed the sky every night, recording what they saw in extensive diaries. These observations, though not highly precise, were consistently collected over centuries. Babylon's stable technology for recording data—clay tablets—allowed these records to be preserved and recopied in temples, providing the longevity and accumulation of data necessary for meaningful analysis. This infrastructure for long-term, cumulative data collection was unparalleled in the early Greek world.
The Babylonians began studying and recording their belief system and philosophies dealing with the ideal nature of the universe, applying internal logic to predictive planetary models. Using the sexagesimal system, they divided the 360-degree sky into 30-degree segments, giving rise to the 12 zodiacal signs. It is also often forgotten that they were the first civilisation known to possess a functional theory of the planets. The oldest surviving planetary astronomical text—the Venus tablet of Ammisaduqa—records the motions of Venus, and though the extant copy dates to the 7th century BCE, the original observations likely date back to the second millennium BCE.
By the 7th century BCE, the Babylonians were compiling detailed astronomical diaries, documenting events such as Venus’s reappearance from solar conjunction or Jupiter’s retrograde motion. They discovered that while planetary behaviour did not repeat annually, it followed long-period cycles: Venus's retrogradation pattern repeated every 8 years, Mars's every 47 years, and Saturn’s every 59 years. These findings led to the development of goal-year texts, in which predictions for a target year were based on records from specific prior cycles. For instance, Venus’s behaviour in 2025 could be forecast based on its motion in 2017.
By around 300 BCE, Babylonian temple scribes had constructed sophisticated arithmetical models to predict planetary positions. Each planet was associated with characteristic intervals in the zodiac that corresponded to events like the onset of retrogradation. For example, Jupiter’s retrograde onsets were predicted to occur at 30° intervals in its “slow zone” and 36° intervals in its “fast zone.” These mathematical schemes allowed astronomers to forecast planetary phenomena centuries into the future.
Despite only fragments of Babylonian astronomy surviving—mainly in the form of clay tablets containing diaries, ephemerides, and computational procedures—their contribution to astronomy and the philosophy of science was nothing short of pivotal. From this cradle of civilisation in Mesopotamia, in the southern part of present-day Iraq, astronomy revolutionised early scientific inquiry. Cuneiform tablets studied by researchers from the University of Tsukuba even recorded phenomena such as unusually red skies between 680 and 650 BCE—possibly geomagnetic storms—underscoring the extraordinary observational detail and longevity of Babylonian astronomical records.
In the French Maritime Alps, in the Vallée des Merveilles, thousands of petroglyphs dating from the Bronze Age have been discovered (c. 2900–1800 BCE). The culture left images of the objects that were important to it, including one clear image of the Sun—a circle with rays coming from it—and, perhaps more controversially, archaeologists have also identified two representations of the Pleiades, shown through clusters of small cupules carved into the rock. In Saxony-Anhalt, Germany, the sky disk of Nebra, a circular bronze plate with areas of applied gold foil, is much clearer as astronomical imagery and dates from about 1600 BCE. Its golden images include the crescent Moon, probably the Sun (or perhaps the full Moon), and a cluster of seven small gold dots that almost certainly do represent the Pleiades.
Astronomical connections are also apparent in a number of prehistoric monuments and graves. In several Stone Age cultures, burial chambers often faced east. Stonehenge (c. 3000–1520 BCE) was aligned so that its principal axis coincided with the direction of sunrise on the summer solstice. Some other astronomical alignments in Stonehenge, such as with the Moon’s most southerly rising and most northerly setting point, are accepted by many archaeoastronomers. This shows that astronomy had a very strong personal connection with many ancient societies.
Curiosity alone did not inspire the earliest astronomers: astronomy and astrometry- the science of charting the sky and one of the oldest branches of astronomy -were practical sciences too. Monitoring the motions of stars and planets in the sky was the best tool to track time and led to the development of calendars and advanced mathematics, which were fundamental for agriculture, religious rituals and navigation.
In North Africa, the Egyptians developed one of the first known solar calendars (the Babylonians relied on a Lunar-based calendar). A potential blend between the two that has been theorised by some historians after the adoption of a crude leap year by the Babylonians, after the Egyptians developed one. It must be noted that the Babylonian leap year shares no similarities with the leap year practised today and involved the addition of a thirteenth month as a means to recalibrate the calendar to better match the growing season.
At this time, astronomy was a matter of the highest order, as the skies were the realms of the Gods. It therefore seemed fitting for Babylonian priests to be the ones responsible for developing new forms of mathematics, and they did so to better calculate the movements of celestial bodies. One priest, known as Nabu-rimanni, stands as the first documented Babylonian astronomer. He was a priest for the moon god and is credited with writing lunar and eclipse computation tables as well as other elaborate mathematical calculations. The computation tables used were organised in seventeen or eighteen tables that document the orbiting speeds of planets and the Moon.
Contributions made by the Chaldean astronomers later on in the Neo-Babylonian period include the discovery of eclipse cycles and saros cycles (a period of about 18 years between repetitions of solar and lunar eclipses), and many accurate astronomical observations. For example, they observed that the Sun's motion along the ecliptic was not uniform, though they were unaware of why this was; it is today known that this is due to the Earth moving in an elliptic orbit around the Sun, with the Earth moving swifter when it is nearer to the Sun at perihelion and moving slower when it is farther away at aphelion. However, whereas Greek astronomers expressed "prejudice in favor of circles or spheres rotating with uniform motion", such a preference did not exist for Babylonian astronomers.
Knowing our place in the stars has also helped countless generations of people across the world understand where they were on Earth. One example was how the positions of the stars helped guide Polynesian voyages. Voyagers measure the angles between stars and the horizon using their hands. The width of your pinkie finger at arm's length is roughly one degree, or double the angular diameter of the Sun or Moon. These explorers realised that stars, as opposed to planets, hold fixed celestial positions year-round, changing only their rising time with the seasons. Each star has a specific declination and can give a bearing for navigation as it rises or sets. Polynesian voyagers would set a heading by a star near the horizon, switching to a new one once the first rose too high. Then a specific sequence of stars would be memorised for each route they took, allowing for safe passage at night. The Polynesians also took measurements of stellar elevation to determine their latitude. The latitudes of specific islands were also known, and the technique of "sailing down the latitude" was used. For example, the top and bottom stars of the Southern Cross are separated by six degrees. When the distance between those stars is equal to the bottom star's altitude above the horizon, your northerly latitude is 21 degrees, that of Honolulu or when the bright stars Sirius and Pollux set at exactly the same time, your latitude is 18 degrees South: the latitude of Tahiti. Some star compass systems specify as many as 150 stars with known bearings, though most systems have only a few dozen.
For navigators near the equator, celestial navigation is simplified, given that the whole celestial sphere is exposed. Any star that passes through the zenith (overhead) moves along the celestial equator, the basis of the equatorial coordinate system.
Looking further south, Aboriginal astronomy is one of the oldest continuous astronomical traditions in the world, dating back tens of thousands of years. Indigenous Australian cultures have used the stars not only for practical purposes—such as navigation, timekeeping, and predicting seasonal changes—but also to transmit complex social laws, cultural knowledge, and Dreaming stories across generations, developing an intimate understanding of celestial phenomena, rooted in detailed observation and oral tradition.
Many Aboriginal groups recognised constellations different from those identified in Western astronomy, often shaped by the dark spaces between stars, rather than the outlines of the shapes made by them. One of the most well-known examples is the “Emu in the Sky,” formed not by stars, but by the dark dust lanes within the Milky Way. In Ku-ring-gai Chase National Park, extensive rock engravings have been found of the Guringai people who lived there, including representations of the creator-hero Daramulan and his emu-wife. An engraving near the Elvina Track shows an emu in the same pose and orientation as the Emu in the Sky constellation, which is visible against the centre and other sectors of the Milky Way background, with the The Emu's head is the very dark Coalsack nebula, next to the Southern Cross and the body and legs are that extension of the Great Rift trailing out to Scorpius.
This celestial emu aligns with real-world breeding patterns of the emu bird and is connected to seasonal knowledge and ceremonial timing. Likewise, the rising and setting of specific stars and constellations, such as the Pleiades and Orion, signalled important ecological events, such as the availability of food sources or the beginning of the wet or dry seasons. The shape and the angle of the Emu within the dark sky of the Milky Way change throughout the year, and dark spaces appear around the ‘body’ of the emu as it moves. These visual changes identify the start of the gathering season for emu eggs – a major source of seasonal protein. Fuller et al. and Leaman et al. both note that emu egg-laying coincides with the rising of the sky-emu and matches slight variations between years. Other constellations have also been found to assist in agriculture; the Yamatji people of the Wajarri language group, of the Murchison region in Western Australia, call the Pleiades Nyarluwarri. When the constellation is close to the horizon as the sun is setting, the people know that it is the right time to harvest emu eggs, and they also use the brightness of the stars to predict seasonal rainfall.
Beyond practical utility, Aboriginal astronomy is deeply spiritual. The sky is seen as a mirrored extension of the land and as a canvas of ancestral activity. Stories such as those of the Seven Sisters (associated with the Pleiades) not only explain the positions and movements of celestial bodies but also encode laws, morals, and kinship systems. These narratives are passed down through song, dance, and art, embedding astronomical knowledge in cultural identity and collective memory.
In recent years, Aboriginal astronomy has gained recognition within both academic and popular science communities. Interdisciplinary research between Indigenous elders and astronomers has revealed sophisticated understandings of phenomena such as eclipses, planetary motion, and the precession of the equinoxes. These collaborations are not only enriching Western astronomy but also fostering cultural respect and revitalising Indigenous knowledge systems. Aboriginal astronomy is a powerful example of how science and culture can intertwine, offering a holistic approach to understanding the cosmos.
The communicative efficiency of this long view’s transmission culture appears substantial. Sophisticated understandings of lunar/tidal processes and the Earth as a finite object rather than a flat infinity predate ‘western’ science by thousands of years. This capacity for communicating observations and experience enabled indigenous communities to survive in challenging environments for 40,000 years.
It was much later, in the third century BCE, that Greek astronomers first attempted to use astrometry to estimate cosmic scales. Among other sciences, astronomy flourished at Alexandria, a Greek colony off the northern coast of Egypt, with a renowned library and museum. The dominant view of the cosmos among scientists at this time was that the universe was geocentric, with the Earth being at the centre of the Universe and everything else revolving around it, but there were some who were edging closer to the truth.
Greek astronomy can be traced back to the earliest Greek literature. In Homer’s Iliad and Odyssey, several stars and constellations are mentioned, such as Orion, Ursa Major, Boötes, Sirius, and the Pleiades. Hesiod’s Works and Days, written a generation after Homer, shows more advanced astronomical knowledge. Hesiod used the seasonal appearances of fixed stars to guide agricultural work and sea travel. This practical star-based calendar resembled Babylonian methods, although Greek astronomy at the time was less advanced overall.
The defining development in Greek astronomy was the application of geometry to celestial phenomena. Aristotle, in On the Heavens (c. 350 BCE), argued that Earth is spherical, citing evidence such as the curved shadow it casts during lunar eclipses and the variation in visible stars from different latitudes. He mentioned that some mathematicians estimated Earth’s circumference to be 400,000 stades—possibly referring to Eudoxus of Cnidus. Although the unit's exact value varied, this early estimate of Earth’s size, while not accurate, showed a growing interest in geometric cosmology.
Eratosthenes made the earliest detailed and surviving attempt to measure the Earth’s circumference in the 3rd century BCE. Observing that the Sun was directly overhead at Syene on the summer solstice but angled at Alexandria, he used the angular difference (7.2°) and the distance between the two cities (5,000 stades) to calculate a circumference of 250,000 stades—approximately 45,000 km, remarkably close to the modern value.
Also in the 3rd century BCE, Aristarchus of Samos attempted to calculate the distances and sizes of the Sun and Moon using geometry. Although his observational assumptions—like the 87° angle between the Moon and Sun at quarter Moon—were inaccurate, his methods were sound. He estimated the Sun to be about 19 times farther than the Moon and both bodies’ diameters relative to Earth. Later astronomers refined his ideas, especially for the Moon, but the vast distance to the Sun remained underestimated until the 17th century.
By 400 BCE, Greek philosophers began exploring planetary motion. Eudoxus developed a model using concentric spheres to account for the planets' various motions, including retrograde motion. He used four spheres per planet to represent daily, zodiacal, and retrograde motions, with the latter producing a figure-eight pattern known as the hippopede. Though the model was physically appealing, it lacked numerical accuracy and could not explain changes in planetary brightness. Nonetheless, it influenced cosmological thinking for centuries.
In the late 3rd century BCE, Apollonius of Perga introduced the idea of eccentric circles and epicycles—smaller circles riding on larger ones—to explain planetary irregularities like retrograde motion and seasonal speed variations. Hipparchus later applied this model to the Sun and deduced the correct amount of the Sun’s eccentricity using seasonal length measurements. This approach allowed far greater precision and remained dominant until the 17th century.
Hipparchus was instrumental in incorporating Babylonian numerical techniques into Greek astronomy. While Greek models had emphasised geometric elegance, the Babylonians focused on accurate numerical predictions. Inspired by this, Hipparchus created successful geometrical models for the Sun and Moon, although he could not do the same for the planets. Importantly, he insisted that a correct theory must match observational data. He also discovered the precession of the equinoxes—the gradual shift in star positions due to Earth’s axial wobble—by comparing his observations with earlier records. He also compiled the first stellar catalogue. A record of his work was handed down by Ptolemy, an astronomer writing three hundred years later at Alexandria, by then part of the Roman Empire.
Greek astronomy reached its peak with Claudius Ptolemy in the 2nd century CE. In his Almagest, Ptolemy synthesised prior work, especially Hipparchus’s, and introduced the equant point to explain non-uniform planetary motion. Each planet moved uniformly on an epicycle, while the epicycle’s centre moved around an eccentric circle. From the equant, these motions appeared uniform, allowing accurate predictions while preserving circular motion ideals. The Almagest included star catalogues, trigonometric tables, and planetary tables, all intended for practical use. It dominated astronomy for over 1,400 years.
Ptolemy’s work overshadowed earlier models due to its precision and practicality. Though he credited predecessors like Hipparchus, he sometimes omitted the origins of his ideas. Nonetheless, his system revolutionised Greek astronomy and shaped scientific thought well into the Renaissance, combining observation, geometry, and numerical calculation in a comprehensive model of the cosmos. Commentaries were also written on his works by Pappus of Alexandria in the 3rd century CE and by Theon of Alexandria and his daughter, Hypatia, in the 4th century CE.
Some of the earliest roots of Indian astronomy can be dated to the period of the Indus Valley civilisation or earlier. Astronomy later developed as a discipline of Vedanga, or one of the "auxiliary disciplines" associated with the study of the Vedas, dating to 1500 BCE or older. The oldest known text is the Vedanga Jyotisha, dated to 1400–1200 BCE (with the extant form possibly from 700 to 600 BCE). Babylonian astronomy had also begun to travel eastward into Persia and India, where it was adapted in original ways and combined with native Indian methods. Through the early centuries of the Common Era, Greek works started to filter into Indian astronomy following the conquests of Alexander the Great, for example, by the Romaka Siddhanta, a Sanskrit translation of a Greek text disseminated from the 2nd century, literally "The Doctrine of the Romans", is one of the five siddhantas (doctrine or tradition) mentioned in Varahamihira's Panchasiddhantika which is an Indian astronomical treatise. Greek geometrical planetary theories, from the time between Hipparchus and Ptolemy, also made their way into India. Babylonian arithmetical procedures used for computing lunar and solar phenomena turn up in conjunction with a length for the solar year due to Hipparchus.
The Rig Veda is one of the oldest pieces of Indian literature. It describes time as a wheel with 12 parts and 360 spokes (days), with a remainder of 5, making reference to the solar calendar. As in other traditions, there is a close association of astronomy and religion during the early history of the science, astronomical observation being necessitated by spatial and temporal requirements of correct performance of religious ritual. Thus, the Shulba Sutras, texts dedicated to altar construction, discuss advanced mathematics and basic astronomy.
In 505 CE, Varāhamihira, a Brahmin astronomer and astrologer, approximated the method for the determination of the meridian direction from any three positions of the shadow using a gnomon (the part of a sundial that casts a shadow). This is around the time of the Indian Classical period, and by the time of Aryabhata, one of the first major mathematicians of this era, the motion of planets was treated to be elliptical rather than circular. Other ideas, such as definitions of different units of time, eccentric models of planetary motion, epicyclic models of planetary motion, and planetary longitude corrections for various terrestrial locations, were also very prominent.
Whilst European scientific endeavours mostly languished in the Dark Ages, astronomy flourished in Asia and in the Islamic world, with the emergence of the Islamic Golden Ages between the 9th and 13th centuries CE. Extensive observations were performed in the Chinese and Indian empires, including the compilation of stellar catalogues.
In the Islamic world, observations of the sky were accompanied by the study and translation of texts from ancient Greek scientists. Under the patronage of the ʿAbbāsid caliphate in Baghdad, translation efforts flourished—most notably the translation of Ptolemy’s Almagest, which was translated into Arabic on at least four separate occasions. Islamic scholars mastered the geometrical planetary theory of the Greeks and sought to improve upon it.
Islamic astronomers also built exquisite instruments to measure angles in the sky. They improved on the quadrant—a measuring device shaped as a quarter of a circle originally proposed by Ptolemy—and invented the sextant, a similar instrument in the shape of one-sixth of a circle. They used such tools to conduct precise measurements, including detecting slow, long-term changes in the heavens. For instance, in the 9th century, astronomers in Baghdad observed that the obliquity of the ecliptic had decreased slightly from the value given in the Almagest. They also noticed small changes in the lengths of the seasons, suggesting that the solar apogee slowly moved eastward—a discovery reflected in the zīj of al-Battānī.
With theories and methods that had passed from the Babylonians and Greeks through Persia to India, and now coming back to the West, Islamic astronomers had access to huge amounts of complicated astronomical material. In the 9th century, Muḥammad ibn Mūsā al-Khwārizmī’s work had many elements of Indian, Persian, and Greek tables and techniques, but it helped establish an important genre of the zīj—a handbook of astronomical tables, including data for working out positions of the Sun, Moon, and planets, accompanied by directions for using them. This was built upon the ancient prototype of Ptolemy’s Handy Tables and marked a turning point in Islamic astronomy, which had previously concentrated on translating works but now began to develop new ideas. Later, al-Battānī’s zīj showed mastery of Ptolemaic planetary theory while improving values such as the magnitude and direction of the Sun’s eccentricity.
Hundreds of zījes were compiled between the 9th and 15th centuries, most following in the tradition of the Almagest and Handy Tables. One particularly influential example was the Toledan Tables, compiled around 1080 by Muslim and Jewish astronomers in Spain, finalised by Ibn al-Zarqallu, and translated into Latin shortly after. These tables reached Europe and are even mentioned in Chaucer’s Canterbury Tales.
In 1261, Nasir al-Din al-Tusi published the Tadhkira, identifying 16 major flaws in Ptolemy’s astronomical system. This sparked a wave of scholarly efforts to revise planetary models without relying on Ptolemy’s problematic equant—a mathematical point used to explain irregular planetary motion. Scholars like Qutb al-Din al-Shirazi, Ibn al-Shatir, and Shams al-Din al-Khafri developed new astronomical models aimed at solving these issues, and their innovations were widely adopted by later astronomers.
To address inconsistencies in lunar motion predicted by the Ptolemaic model, Tusi introduced the Tusi couple—a geometric device where two circles rotate to produce linear motion. He proposed this as an alternative to the equant, which inaccurately implied drastic changes in the Moon’s distance from Earth over a month, by a factor of two if calculated. In contrast, the Tusi couple preserved uniform circular motion while aligning with observed lunar behaviour.
Another scholar, Mu'ayyad al-Din al-Urdi, contributed the concept of the lemma, which modelled planetary epicycles without using the equant. Like Tusi’s couple, this was intended to offer a more accurate and physically acceptable explanation of planetary motion. The criticisms of Ptolemy extended beyond mathematical refinements: Ibn al-Haytham, in his Doubts About Ptolemy, objected to the use of abstract, immaterial constructs like the equant point as if they were physical realities—echoing philosophical critiques made earlier by the Greek philosopher Proclus.
The debate over Earth’s motion also featured in Islamic astronomy. Abu Rayhan al-Biruni (973–1048) acknowledged that Earth’s rotation could be mathematically consistent with astronomical observations, but he ultimately adhered to the geocentric model, placing Earth motionless at the universe’s centre. He treated the idea of Earth’s rotation as a question of natural philosophy rather than empirical astronomy. However, al-Biruni’s contemporary, Abu Sa’id al-Sijzi, accepted Earth’s rotation and even invented an astrolabe based on this idea. A 13th-century Arabic reference confirmed that some engineers believed the Earth moved in a circular motion, interpreting the apparent movement of the heavens as a result of the Earth's rotation, not celestial motion.
At institutions like the Maragha and Samarkand observatories, astronomers including Tusi, Najm al-Din al-Qazwini al-Katibi, and later al-Qushji explored Earth’s rotation in greater depth. Their arguments in support of Earth’s motion closely resembled those later used by Copernicus. Despite these forward-thinking insights, the Maragha school did not ultimately propose a fully heliocentric model, stopping short of abandoning geocentrism altogether. Yet, in the 16th century, Nicolaus Copernicus would incorporate mathematical models—such as the Tusi couple and the epicyclic refinements of Ibn al-Shatir—into his De revolutionibus, indicating a direct intellectual inheritance from Islamic astronomy, even though Copernicus’ sources remain uncertain.
A catalogue of 994 stars was created by Ulugh Beg of the Timurid dynasty in the fifteenth century. Ruling over Central Asia, the astronomer and mathematician constructed an enormous sextant with a radius of 36 metres in Samarkand, located in present-day Uzbekistan. Ulugh Beg's catalogue has a precision of slightly better than one degree, comparable to that of Hipparchus's compilation from several centuries before. Ulugh Beg and the many other astronomers who were active in the Islamic world kept the practice of astronomy and astrometry alive, smoothly ushering them into the modern era.
Further east, one of the main functions of astronomy was for the purpose of timekeeping. The Chinese used a lunisolar calendar, but as the cycles of the Sun and the Moon are different, leap months had to be inserted regularly. The Chinese calendar was considered to be a symbol of a dynasty. As dynasties would rise and fall, astronomers and astrologers of each period would often prepare a new calendar, making observations for that purpose. Astronomers took note of "guest stars", usually supernovas or comets, which appear among the fixed stars. The supernova which created the Crab Nebula, now known as SN 1054, is an example of an astronomical event observed by Ancient Chinese astronomers. These Ancient astronomical records of phenomena like comets and supernovae have proved exceptionally useful in our modern understanding of the evolution of the skies around us, however, astronomical reports begin to be fairly numerous only from about 200 BCE. A key area of study in China within astronomy was sunspots, which they discovered and also recorded much earlier than their European counterparts.
The later Song dynasty scientist Shen Kuo (1031–1095 CE) used the models of lunar eclipse and solar eclipse in order to prove that the celestial bodies were round, not flat. This was an extension of the reasoning of Jing Fang and other theorists as early as the Han dynasty (202 BC – 9 AD, 25–220 AD). In an essay written in 1088 CE, he reasoned:
“If they were like balls, they would surely obstruct each other when they met. I replied that these celestial bodies were certainly like balls. How do we know this? By the waxing and waning of the moon. The moon itself gives forth no light, but is like a ball of silver; the light is the light of the sun (reflected). When the brightness is first seen, the sun (-light passes almost) alongside, so the side is only illuminated and looks like a crescent. When the sun gradually gets further away, the light shines slanting, and the moon is full, round like a bullet. If half of a sphere is covered with (white) powder and looked at from the side, the covered part will look like a crescent; if looked at from the front, it will appear round. Thus we know that the celestial bodies are spherical.”
He also explained why eclipses occurred only on an occasional basis while in conjunction and opposition once a month:
“...The ecliptic and the moon's path are like two rings, lying one over the other, but distant by a small amount. (If this obliquity did not exist), the sun would be eclipsed whenever the two bodies were in conjunction, and the moon would be eclipsed whenever they were exactly in opposition. But (in fact) though they may occupy the same degree, the two paths are not (always) near (each other), and so naturally, the bodies do not (intrude) upon one another.”
During the Ming dynasty, from 1368 until 1644, the nation experienced a decrease in astronomical expansion, with astronomers during these times relying less on discovery and more on the use of astronomy. Astronomers worked in the two Astronomical Bureaus, both of which underwent many changes throughout the years since their formation. The path into the occupation was also hereditary; because of the rigidity and high level of intelligence needed for this occupation, children of astronomers were banned from pursuing other professions.
The Renaissance and Enlightenment periods marked a dramatic transformation in European astronomy. Driven by revived interest in classical knowledge, increased cross-cultural exchange, and advances in mathematics and instrumentation, this era witnessed a fundamental shift from a geocentric (Earth-centred) cosmos to a heliocentric (Sun-centred) model, eventually laying the groundwork for modern astrophysics.
The Renaissance began with a resurgence of interest in ancient Greek and Roman science, much of which had been preserved, translated, and refined by Islamic scholars. European thinkers gained access to Ptolemy’s Almagest, Aristotle’s writings, and Islamic zījes (astronomical tables), often via translations from Arabic into Latin in places like Spain and Sicily. These materials provided both the tools and the motivation to question long-held cosmological models.
In 1543, Nicolaus Copernicus published De revolutionibus orbium coelestium (On the Revolutions of the Heavenly Spheres), proposing a heliocentric model in which the Earth and planets orbited the Sun. His model still used circular orbits and epicycles, but it fundamentally challenged the dominant Ptolemaic system and placed the Sun, not Earth, at the centre of the cosmos. Several mathematical models used by Copernicus, including the Tusi couple and modifications to planetary epicycles, closely resembled innovations developed at Maragha Observatory, particularly those by Nasir al-Din al-Tusi and Ibn al-Shatir.
Danish astronomer Tycho Brahe (1546–1601) built large instruments to take naked-eye measurements of planetary positions with unprecedented precision. His extensive data sets, particularly for Mars, provided the empirical basis needed to refine planetary models. Though Brahe rejected heliocentrism, proposing instead a hybrid “geoheliocentric” system, his observations were critical for later developments.
Using Brahe’s data, Johannes Kepler (1571–1630) formulated three laws of planetary motion, published between 1609 and 1619:
Planets orbit the Sun in ellipses, not circles.
Planets sweep out equal areas in equal times.
The square of a planet’s orbital period is proportional to the cube of its average distance from the Sun.
Kepler’s work mathematically proved that planetary motion was neither circular nor uniform, breaking away from Aristotelian physics and Ptolemaic geometry.
Galileo Galilei (1564–1642) built one of the first astronomical telescopes and made groundbreaking observations:
Mountains and craters on the Moon, challenging the idea of celestial perfection.
Moons orbiting Jupiter, proving not everything revolved around Earth.
Phases of Venus, which supported heliocentrism.
Sunspots and the Milky Way as a collection of countless stars.
Galileo’s observations, published in Sidereus Nuncius (1610), undermined the Aristotelian cosmos and provided direct visual evidence supporting the Copernican model.
The Enlightenment period brought a new emphasis on reason, empirical evidence, and the mathematical description of natural laws. Astronomy became a prime example of how rational inquiry could unravel the workings of the universe.
In 1687, Isaac Newton published Philosophiæ Naturalis Principia Mathematica, in which he unified celestial and terrestrial physics through:
The Law of Universal Gravitation, explaining why planets follow elliptical orbits.
The Three Laws of Motion, providing a mechanical basis for planetary dynamics.
Newton showed that Kepler’s laws were not just empirical but consequences of deeper physical principles. His work formed the foundation of classical mechanics and predictive astronomy.
Enlightenment astronomers also greatly benefited from technological innovation:
Reflecting telescopes (invented by Newton) improved image clarity.
Micrometres allowed precise angular measurements.
Clocks with pendulums enabled the accurate timing of celestial events.
These instruments led to detailed star catalogues, accurate lunar maps, and the measurement of planetary masses. Large state-funded observatories began to be built across Europe and its colonies too, allowing astronomers to have access to telescopes with a higher resolution and magnification. Observatories such as the ones at Greenwich (1675) and Paris (1667) were vital for the enhancement of navigation and the understanding of planetary motion. Expeditions were also commissioned to observe transits of Venus (e.g., 1761 and 1769), helping determine the astronomical unit—the distance from Earth to the Sun. Learned societies like the Royal Society also opened a forum for scientists to debate and discuss upcoming astronomical research, furthering the development of new theories.
Star Cataloguing and Stellar Science likewise were witness to a variety of advancements, such as Edmond Halley predicting the return of Halley’s Comet using Newtonian physics and James Bradley discovering the aberration of starlight, confirming Earth’s motion. William Herschel also proposed that the Milky Way was a collection of stars in a disk, as well as setting up the first deep-sky surveys.
The Renaissance and Enlightenment transformed astronomy from a philosophical speculation based on classical authority to a mathematical and observational science grounded in physical laws. These centuries marked:
The collapse of geocentrism and Aristotelian cosmology.
The rise of heliocentrism and Newtonian mechanics.
The shift from qualitative description to quantitative prediction.
