Seleucus of Seleucia was a Hellenistic astronomer, mathematician, and physicist from Seleucia on the Tigris who is best known for his staunch defense of the heliocentric theory, proposed by Aristarchus of Samos, as well as his studies on the causes of tides. While none of his original writings survive, later ancient authors reference his contributions, confirming his role as one of antiquity’s most forward-thinking scientists.
Seleucus lived during the height of Hellenistic scientific advancements, when Greek, Babylonian, and Indian knowledge converged. We trace his intellectual lineage back to the rich astronomical traditions of the Babylonians, who meticulously recorded celestial events for centuries. The Greek historian and geographer Strabo referred to him as a “Chaldean,” a term that in ancient times denoted Babylonian astronomers.
Seleucus of Seleucia is one of the ancient astronomers known to have explicitly defended the Greek astronomer Aristarchus’ heliocentric theory. He argued that the Earth orbits the Sun while rotating on its axis. Aristarchus proposed this idea around the 3rd century BC. However, the Greek astronomer Ptolemy’s geocentric model dominated science for a millennium. Seleucus supported Aristarchus’ model and attempted to prove it through logical reasoning and mathematical calculations.
One of the primary sources mentioning Seleucus’s defense of heliocentrism is the Greek historian Plutarch. In his work “On the Face in the Moon’s Orb” (De Facie in Orbe Lunae), Plutarch states:
“Seleucus demonstrated the truth of the hypothesis of Aristarchus.”
Although Plutarch does not specify how Seleucus proved the heliocentric model, he might have used observational evidence, such as the apparent motion of planets and the behavior of tides, to argue for a rotating Earth.
Apart from his astronomical work, Seleucus made significant contributions to understanding tides. He was the first scientist known to correctly hypothesize that the Moon’s influence caused the tides and that their variation depended on the Moon’s position relative to the Sun.
Strabo, the Greek geographer, acknowledges Seleucus’ contributions in his Geography:
“Seleucus the mathematician attributes the phenomenon of the tides to the Moon and states that their height varies with its position relative to the Sun.”
This explanation was remarkably advanced for its time. It linked celestial mechanics with Earth’s natural phenomena. This concept would not be fully understood until the 17th century and British physicist Isaac Newton’s gravity experiments.
Seleucus likely relied on empirical observations and mathematical reasoning to support his claims. Some scholars, like the Austrian-American Otto Neugebauer, speculate that he might have observed how water bodies behaved under different lunar phases. He noticed patterns that supported his lunar-tidal hypothesis. Furthermore, his advocacy of the heliocentric model suggests that he may have recognized retrograde motion as an illusion. He believed the Earth’s movement was the cause rather than an inherent feature of planetary motion.
His work demonstrated a strong grasp of physics, astronomy, and geometry. This expertise made him one of the few ancient scientists to argue for a model of the cosmos.
Despite his significant contributions, Seleucus’ work was not widely accepted during the Middle Ages. The dominant geocentric model, championed by Aristotle and later formalized by Claudius Ptolemy in the Almagest, remained the prevailing cosmological framework until the Copernican Revolution in the 16th century.
However, the rediscovery of heliocentrism and the acknowledgment of its ancient proponents, including Aristarchus and Seleucus, highlight the continuity of scientific thought across millennia. Modern scholars like the Italian Lucio Russo regard Seleucus as one of the earliest defenders of a heliocentric universe and the first to propose a physical mechanism for tides.
Seleucus of Seleucia is a remarkable figure in ancient science, bridging the intellectual traditions of Babylon and Greece. His bold support for heliocentrism and innovative tidal studies illustrate the depth of scientific inquiry in the Hellenistic period. Although his works have been lost, references to his ideas in ancient texts confirm his status as a pioneering thinker. Additionally, his insights anticipated many principles of modern astronomy and physics.
