The Song of Urania

In this episode we examine the work of two Roman astrologers to see how Roman astrology worked in practice: Marcus Manilius, who wrote Astronomica, and Firmicus Maternus, who wrote the Matheseos.

As Rome expanded to the East, the cultural influence of the Greeks deepened, and this included a strong interest in astrology. Despite resistance from conservative Romans, by the Imperial Era, astrology played a critical role in Roman politics, both as a tool to support conspirators attempting to assassinate emperors, and wielded by emperors as a way to eliminate challenges to their power.

This month the full moon falls on Three Kings' Day, traditionally a day that celebrates the adoration of the magi, so we investigate the famous Star of Bethlehem story. Through the centuries there have been dozens of astronomical and astrological theories put forward as to what the Star of Bethlehem was. We look at a few of the more prominent theories and their shortcomings, finishing with what is plausibly the leading candidate, the astrological theory of Michael Molnar, though as we'll see, this explanation, too, is not without its problems.

Rome, the great empire of the Mediterranean, is not known for its astronomy. But while it lagged behind other civilizations, it is a mistake to think that they were entirely uninterested in the subject. Astronomical references permeated the Roman calendar and one of Rome's longest lasting contributions to Western civilization, the Julian calendar, was devised by the Roman astronomer Sosigenes.

During the half millennium of the Roman Era in Ancient Greece, only two astronomers made any major advances. At the beginning of the Roman Era, Posidonius measured the size of the Earth and discovered the relationship between the Moon and the tides. But the triumph of Greek astronomy came 300 years later with the intricate planetary model of Ptolemy, which stood unchallenged for some 1400 years.

In the year 1900 a team of sea sponge divers stumbled upon a shipwreck from the late Hellenistic Era. Among the statues, coins, and jewellery, the salvage crew pulled out a small box covered in moss. Initially ignored, the contents of this box proved to be the most sophisticated mechanical device that survives from the ancient world. The Antikythera mechanism computed all the known motions of the heavens and its complexity was described by one scholar as "like finding a jet engine in King Tut's tomb."

After briefly examining the astronomy of Timocharis and Aristyllus, who developed the first known stellar catalog, we turn our attention to Hipparchus, who I claim was the greatest astronomer of ancient times. Hipparchus made major developments in virtually every area of astronomy known to him. His measurement of the lengths of the year and month were of unprecedented accuracy, he measured the distance to the Moon, and he developed a star catalog that was an order of magnitude larger than the earlier catalog of Timocharis and Aristyllus, which he was able to use to discover the precession of the equinoxes.

In the Hellenistic Era the astronomer Apollonius of Perga (maybe) developed the model of epicycles and deferents that was to dominate Western astronomy for more than 1500 years. Around the same time, Eratosthenes, who was the head librarian at the Library of Alexandria, developed a novel technique to measure the circumference of the Earth and arrived at a suspiciously accurate result.

At the dawn of the Hellenistic Age, two Greek astronomers developed radical new cosmologies. Heraclides of Pontica proposed that the Earth rotated on its axis and that Mercury and Venus revolved around the Sun instead of the Earth. Aristarchus of Samos went further and proposed that all the planets, including the Earth, revolved around the Sun. In addition, Aristarchus made the first quantitative measurement of the distances to the Sun and the Moon, along with their sizes.

Two of Plato's students were notable astronomers. We looked at one of them, Eudoxus, in the last two episodes. In this episode we turn to Plato's other student, Aristotle. Aristotle embellished Eudoxus's model of planetary motion, but also developed a comprehensive physics and cosmology that ultimately became the standard model of the universe during the High Middle Ages.