Seleucus of Seleucia facts for kids

Seleucus of Seleucia was an ancient Greek astronomer and philosopher. He lived around 190 BC. He came from a city called Seleucia on the Tigris in Mesopotamia (which is now part of Iraq). Seleucus is best known for two big ideas: he believed the Earth goes around the Sun (a theory called heliocentrism), and he had a good explanation for why we have tides in the ocean.

Seleucus and the Sun-Centered Universe

Seleucus was a strong supporter of the idea that the Sun is at the center of our solar system. This idea is called the heliocentric theory. Another astronomer, Aristarchus of Samos, first suggested this. He believed the Earth spins on its own axis and also travels around the Sun.

The ancient writer Plutarch said that Seleucus was the first person to prove this sun-centered system using logical arguments. However, we don't know exactly what proofs he used. Some historians think Seleucus might have used math and geometry to figure out how planets move, much like Nicolaus Copernicus did many centuries later. Seleucus lived at the same time as Hipparchus, who was a master of trigonometry, a type of math that deals with triangles and angles. Seleucus might have used similar methods.

Seleucus also believed that the universe was infinite, meaning it goes on forever without end. Sadly, none of Seleucus's original writings have survived to today. We only know about his work because other ancient writers mentioned him. A small part of his work was later found in an Arabic translation, which was mentioned by a Persian philosopher named Muhammad ibn Zakariya al-Razi.

Understanding Ocean Tides

Seleucus also had a very good understanding of tides. He correctly figured out that tides are caused by the Moon. He explained that the Moon somehow interacts with the Earth to pull the water, causing the tides to rise and fall. He also noticed that tides change in height and strength in different parts of the world.

Some historians believe that Seleucus's ideas about tides might have helped him support the sun-centered theory. For example, the way tides change throughout the year is hard to explain if the Earth is the center of everything. Seleucus thought that tides were caused by both the Moon and a spinning motion of the Earth. This "spinning motion" could mean the Earth moving around the Earth-Moon center of mass, which is a point between the Earth and Moon that they both orbit.

The Greek geographer Strabo wrote that Seleucus was the first to say that tides happen because the Moon pulls on the Earth. Strabo also noted that the height of the tides depends on where the Moon is in relation to the Sun.

Seleucus in Ancient Writings

We know about Seleucus from the writings of several ancient Greek authors like Plutarch, Aetius, and Strabo. The Persian philosopher Muhammad ibn Zakariya al-Razi also mentioned him.

Strabo listed Seleucus as one of the most important "Chaldean" astronomers. The "Chaldeans" were ancient people from Babylonia (modern-day Iraq) who were famous for their knowledge of the stars and planets. Their ideas were very important and were translated into Greek, influencing later thinkers. Strabo mentioned other famous Chaldean astronomers like Kidenas and Naburianos alongside Seleucus.

See also

• Babylonian astronomy
• Greek astronomy
• Discourse on the Tides