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How to seek a prediction: Wait for a comet to appear

(Sample) Equipment: naked eye

Personnel: Any #human who attributes meaning to the appearance of a comet

Suddenly, and unexpectedly, the apparent perfection and regularity of the Heavens are disturbed by an unknown interloper: a comet. For thousands of years, myriad civilizations around the globe saw comets as supernatural signs--sometimes as portents of good news, and other times as harbingers of doom. For cultures striving to understand the world around them, the 


capricious, seemingly #random, unpredictability of comets is what imbued them with meaning: where else could comets be coming from, if not from the divine?

Some examples of famous comets in Western history include:

  • 44 BCE: An appearance of a comet soon after Julius Caesar’s death was taken as a sign that Caesar had become a god.

  • 12 BCE: The appearance of the “Star of Bethlehem” during the birth of Jesus is often interpreted as a comet (perhaps Halley’s Comet).

  • 1066 CE: An appearance of Halley’s Comet was supposedly a good omen for William the Conqueror and the Norman’s conquest of England at the Battle of Hastings (or, alternatively, a bad sign for the defeated King Harold and his Anglo-Saxons).


The change over time in humanity’s feelings about comets offers a great example the same phenomenon taking on very different meaning within the predictive systems we discuss in this course (see Framework diagram). Before Newton worked out his predictive theory of gravity, comets were almost universally thought to be signs from the gods, appearing to be #random phenomena as we mention above. Once Edmund Halley made the bold claim, using calculations based on Newton’s theory, that the “Great” Comet of 1682 (now called “Halley’s Comet!”) would return 75 years later, and that claim was posthumously verified, comets were effectively demoted (in some people’s view!) to being great demonstrations of the near-fully #deterministic nature of gravity, rather than supernatural messengers.

Meet the Experts: John Overholt, Owen Gingerich, and Sara Schechner


The Diviner's Guide experts for comets are John Overholt, Professor Owen Gingerich, and Dr. Sara Schechner.

John Overholt is the Curator of the Donald and Mary Hyde Collection of Dr. Samuel Johnson and of Early Modern Books and Manuscripts at Harvard University's Houghton Library. You can follow him on Twitter @john_overholt.

Professor Gingerich is a professor emeritus of astronomy and the history of science at Harvard, where he has taught for nearly 50 years. He is one of the world's experts in Johannes Kepler and Nicolaus Copernicus, who he wrote a best selling book about, The Book Nobody Read: Chasing the Revolutions of Nicolaus Copernicus.

Dr. Sara Schechner is the David P. Wheatland Curator of the Collection of Historical Scientific Instruments at Harvard University. Her research is on the study of the history of science, with a focus on material culture and astronomy. She is the co-author of Tangible Things: Making History through Objects, which is also a course on edX.

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