The Greek astronomer Ptolemy’s Almagest was published around the year 145 in Alexandria, Egypt, while the Polish cleric Copernicus’s On the Revolutions (which we will simply call ‘Revolutions’ here) was published in 1543 from Nuremberg, Germany. Ptolemy’s vast synthesis was made possible by two thousand years of systematic observations carried out in Iraq, Syria, and Greece. Copernicus’s monumental analysis became possible thanks to fifteen hundred years of precise observations across India, China, Europe, and the Islamic world. In the five-thousand-year history of astronomy, these two books hold an exceptional place, as their social impact has been the greatest. Perhaps it is because Copernicus’s book was named ‘Revolutions’ that today the word ‘revolution’ has come to mean upheaval—the ‘Revolutions’ of Copernicus gave birth to the greatest scientific and social revolution in history. In this article, we will explore how, from the Almagest to the Revolutions, humans have tried to build a “working model” of the universe, and how the model of the Revolutions ultimately proved itself.

To begin the story of the *Almagest*, we have to start with another book: the astrology text *Enuma Anu Enlil* (meaning *When Anu and Enlil*) written in Babylon, Iraq, a thousand years before Jesus. This work drew upon nearly two thousand years of the astronomical heritage of Iraq. *Enuma* is a book of omens, meaning portents. Unlike the Greeks, the Babylonians’ focus in astrology was not to discover human fate, but to interpret omens. They identified certain regular events in the sky that could be classified as omens. They believed that if these omens could be understood in advance—that is, predicted—then proper rituals and offerings could avert impending misfortunes.

It was because the authors of *Enuma* closely observed the motions of the Moon, the Sun, and the five visible planets relative to the fixed stars over at least seven centuries that the first cosmological model could be developed—the *Almagest* being its ultimate culmination. They were concerned with reconciling the twelve lunar months (354 days) with the solar year (365 days). Six hundred years before Jesus, the Babylonians discovered the Metonic cycle—realizing that 235 lunar months (29.5 days each, totaling 6,932 days) are nearly equal to 19 solar years (365.2 days each, totaling 6,938 days). And since there are only 228 months in 19 years (which is 7 months fewer than 235), they determined that 7 extra months needed to be added across 19 years, meaning 12 years would have twelve months, and 7 years would have thirteen months.

The practice of creating ephemerides—tables listing the past and future positions of the Moon, Sun, and planets—began in Babylon. For this, they not only measured angular positions but also began calculating velocities. For example, above is a graph showing the change in the Sun’s angular velocity over a year, based on data from a Babylonian tablet dated to 132 BCE. Relative to the background of “fixed” stars, the Sun moves 360 degrees in a year. The graph shows how much distance it covers each month as velocity. We see that in the second month, the Sun’s angular velocity decreases to a minimum (around 28 degrees/month), and then increases to a maximum (30 degrees/month) in the eighth month. For six months, the velocity decreases; for six months, it increases.

They had two methods to explain exactly how the velocity changed. In one method, it was assumed that the Sun’s velocity remained constant for half the year, then shifted once, remaining constant again for the other half. In the other method, it was proposed that the Sun’s velocity steadily increased over half the year and steadily decreased over the other half. Humans first applied calculations of displacement, velocity, and acceleration properly to the Sun. Likewise, the displacement, velocity, and acceleration of the Moon and the five visible planets were also calculated by the authors of *Enuma*. However, building a cosmology—a complete model of the universe—based on these calculations happened later, in Greece.