365daysDate: July 22, 2009

Title: Astronomy in Anglo-Saxon England


Podcaster: Dave Wilton


Description: The history of astronomy in the Western world usually skips over the medieval period, jumping from Ptolemy straight to Copernicus. But there were medieval European astronomers who studied and wrote about the skies, particularly in England during the Anglo-Saxon era, from c. 450–1066. This podcast addresses how medieval people viewed the sky and humanity’s place in the physical universe, as well as some misconceptions that modern people often have about the state of astronomical knowledge in the early medieval period in Europe.

Bio: Dave Wilton is a graduate student studying medieval English literature and the editor of He is also an amateur astronomer and a member of the San Francisco Amateur Astronomers.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Matthew Linkert.


Wæs þu hal! I’m Dave Wilton the editor of, a student of medieval literature, and an amateur astronomer. Over the next few minutes I’m going to review the state of astronomical work in early medieval Europe with a particular emphasis on the state of the science in Anglo-Saxon England.

The Anglo-Saxon era ran from about the year 450, when the first Germanic tribes crossed the Channel and started to subdue the native Celtic Britons, until the year 1066, when William of Normandy conquered England and installed a French aristocracy to rule over the land. The name “England,” in fact, comes from one of those Germanic tribes, the Angles, who are the Anglo in Anglo-Saxon.

Most histories of Western astronomy address the ancient Greek astronomers, ending with the work of Claudius Ptolemy in the second century Common Era and then jump to Copernicus, some fourteen hundred years later. These histories sometimes talk of medieval Arab and Chinese astronomers in the Middle Ages, but they are silent about their European counterparts. This gap has created three enduring myths about astronomy in the early medieval period. The first is that astronomy of the period was dominated by the ideas of Aristotle and Ptolemy. The second myth is that the science of astronomy did not progress during the Middle Ages. The third is that the medieval church was in conflict with science.

The idea that medieval European astronomy was in the thrall of Aristotle and Ptolemy confuses the early medieval period with later centuries. The writings of Aristotle and Ptolemy were not translated into Latin until the end of the Anglo-Saxon period and thus remained largely unknown in Europe.

Aristotle’s crystalline spheres and his fifth element of the æther played no part of early medieval cosmology. The Ptolemaic system, with its use of epicycles to explain the retrograde movement of the planets, was also unknown. Because they were not limited by Aristotelian and Ptolemaic thought, early medieval astronomers did not insist on perfect circles for the orbits of the planets; eccentric orbits were fine. And Abbo of Fleury, a tenth century French monk who taught for a while in Britain, even developed a cosmological system in which Mercury and Venus orbited the sun, as opposed to the earth, mirroring the work of Heraclides in the fourth century B.C.E. and anticipating Tycho Brahe’s cosmological system of the sixteenth century. One of Abbo’s students was a monk named Byrhtferth, from Ramsey near Cambridge, who in c. 1011 left us with a detailed commentary on astronomy called the Enchiridion, or the Handbook, which remains one of our chief sources of knowledge about the state of the astronomical art at the end of the first millennium.

The second myth is that Europeans did not advance the science of astronomy during the medieval period. They did, chiefly in the calendar and timekeeping. If we’re talking about the medieval calendar, one name stands out above all the rest. Bede. Also known as the Venerable Bede, he was born circa 673 and was a monk who spent most of his life in a monastery at Jarrow, near the modern city of Newcastle Upon Tyne. Bede is best known for writing The Ecclesiastical History of the English People, which is the source for much of our historical knowledge of the period. But he also wrote two treatises on astronomy and the calendar, the shorter De Temporibus, or Concerning Time, in circa 703 and the more comprehensive De Temporibus Ratione, or Concerning the Reckoning of Time, some twenty years later.

These treatises were used to calculate the proper date for Easter and other moveable feasts on the Church calendar. A medieval work like this is called a computus, or rimcræft in Old English, literally number-craft. Bede’s treatises are just two of many on the subject, but his are without question the most important, as they formed the basis for the medieval calendar used throughout Europe. In addition to his computus, Bede popularized the use of the Anno Domini dating system when he used it in his Ecclesiastical History. Bede, more than anyone else, is responsible for our current practice of counting years from the birth of Christ.

Early medieval astronomers like Bede calculated the length of the solar year as 365 days and six hours long, adding an extra day to the calendar every four years as we do today, and they calculated the moon’s orbital period as 27 days and eight hours—accurate to within seventeen minutes. The Anglo-Saxon lunar calendar added intercalary months as needed and skipped one day every nineteen lunar years to bring the lunar calendar into conformity with solar reckoning. Our word “leap,” that denotes this skip, was first used by Anglo-Saxon astronomers.

Clocks and hourglasses were unknown in early medieval Europe, although water clocks were introduced in the tenth century. Bede determined the period of daylight hours at different times of year and at different latitudes, calculations which could be used with sundials for daylight timekeeping. A bit earlier, in the sixth century, Gregory of Tours had written De Cursu Stellum, or The Courses of the Stars, which calculated the rising and setting times of various constellations for timekeeping at night. The primary motive for this timekeeping was to determine the proper hour during the night for monks to rise and pray, but it had other, more practical, applications as well.

The third myth holds that the Church was in conflict with science, but instead, early medieval scholars believed that observations of the natural world would aid in theological understanding. One of the chief promoters of this world view was Ælfric of Eynsham. One of the preeminent prose writers of the late Anglo-Saxon period, Ælfric wrote many volumes on what a good clergyman should know. As part of this curriculum, he provided an overview of astronomy and how it reflects theological principles, called De Temporibus Anni, or The Seasons of the Year, written circa 993. This work is very much like an introductory textbook, or a medieval Astronomy for Dummies. It does not contain any new insights or discoveries, but, based on the number of copies that survive, De Temporibus Anni was widely known throughout England, and it shows us that astronomy was not an esoteric subject, known to but a few cloistered monks. Alcuin of York, an eighth century monk from Northumbria who taught at the court of Charlemagne, tells us that the emperor and his daughters were keen observers of the night skies as well.

One result of this world view is that early medieval scholars rejected astrology and other mystical influences from the heavens. Astrology came from the pagan east, where theology was dominated by Manichaeism and a struggle between cosmic forces for control of the universe. But Christian thought held that one God ruled over an ordered and unchanging universe. Medieval scholars knew the signs of the zodiac, but only used them to measure the progress of the sun and moon across the sky during the year, and not for predicting the future. Of course, superstition was just as rampant back then as it is today, but the educated knew better.

The one exception to this was the appearance of comets, which they would take as a sign from God of some upcoming calamitous event, such as the death of a king—in fact the appearance of Halley’s Comet in March of 1066 was associated with William of Normandy’s conquest of England a few months later and the end of the Anglo-Saxon era.

Bede and other scholars of the day also understood and could predict eclipses, both solar and lunar—if Mark Twain’s Connecticut Yankee had actually lived, he wouldn’t have impressed or scared anyone with his blotting out of the sun. They understood that moonlight was reflected sunlight and that the phases of the moon were the result of the angle of the sun’s light on the sphere of the moon. Bede also understood that ocean tides were caused by the moon and his theory of tides was not significantly improved upon for some twelve hundred years, until Newton came along.

The Anglo-Saxons understood the relative distances to various celestial objects. For example, despite the sun and the moon appearing to be the same size in the sky, they understood that this was an illusion and that the sun was much larger, only further away. Similarly, they knew the stars to be very large, only very far away. However, they did think the stars also reflected the sun’s light and did not conceive of the sun as a star.

Early medieval astronomers also knew the relative distances of the planetary orbits and had reasonably accurate approximations of their orbital periods. They knew, for example, that Saturn was the most distant, assigning it an orbital period of 30 years—today we know it to be 29.66 years.

They had similar estimates for Mars and Jupiter, although they did not achieve consensus on the orbital periods for Venus and Mercury; various sources give different figures for the two inferior planets. Of course, they were unaware of Uranus and Neptune, which would not be discovered until the invention of the telescope.

They also used the night sky for navigation, as evidenced by the Anglo-Saxon name for Polaris, the scipsteorra, or ship star. Because the earth precesses as it rotates, Polaris was significantly further away from the pole one thousand years ago than it is today, but it was still close enough to be useful as a general guide for navigation.

Finally, we must debunk what is the perhaps the biggest myth of all. Medieval scholars did not believe the earth was flat. They knew quite well that it was round.

With few exceptions, medieval European astronomers did not leave us detailed records of astronomical observations—for example the great supernova that created the Crab Nebula in 1054 went unrecorded in Europe, although it had to have been an astounding sight to see. But from their refinement of the calendar, it is clear that they did observe and applied what they saw to their theoretical knowledge of the heavens. And this knowledge was not restricted to a handful of scholars, but astronomy was considered an essential element of educational curriculum.

I hope you’ve enjoyed this little foray into the astronomy of a thousand years ago and that it has dispelled some of the misconceptions that some of you may have had about medieval astronomy. Thanks for listening and here’s wishing you clear skies.


Ælfric. Ælfric’s De Temporibus Anni. Edited by Heinrich Henel. Early English Text Society, vol. o.s. 213. London: Oxford University Press, 1942. This is in Old English, with no modern translation.

Bede. Bede: The Reckoning of Time. Translated by Faith Wallis. Translated Texts for Historians, vol. 29. 1988; reprinted with corrections, Liverpool: Liverpool University Press, 2004. This is a modern English translation of Bede’s De Temporibus Ratione.

———. Opera De Temporibus. Edited by Charles W. Jones. Cambridge, MA: Mediaeval Academy of America, 1943. This is Bede’s De Temporibus Ratione and De Temporibus in the original Latin.

The Blackwell Encyclopaedia of Anglo-Saxon England. Edited by Michael Lapidge et al. Malden, MA: Blackwell, 1999. This is a superb resource for all things Anglo-Saxon. It contains excellent short articles on Anglo-Saxon astronomy and computus.

Byrhtferth. Byrhtferth’s Enchiridion. Edited by Peter S. Baker and Michael Lapidge. Early English Text Society, vol. s.s. 15. Oxford: Oxford University Press, 1995. Baker and Lapidge provide both the original Old English/Latin text and a modern translation.

McCluskey, Stephen C. Astronomies and Cultures in Early Medieval Europe. Cambridge: Cambridge University Press, 1998.

Stevens, Wesley M. Cycles of Time and Scientific Learning in Medieval Europe. Variorum Collected Studies Series, vol. CS482. Aldershot, UK: Variorum, 1995.

Veni Creator Spiritus, performed by the Portugeuse vocal ensemble Coral Vértice and used under a noncommercial, Creative Commons license.

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365 Days of Astronomy
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