Timeline of cosmological theories

Long ago, people had many ideas about space.

Some thought the Earth was a flat circle. Others thought it sat on a big ocean. Some people believed the Earth was a round ball.

Greek thinkers had even more ideas. One man thought the Earth was shaped like a tall stone pillar. He thought the Sun was a huge ball of fire.

Other thinkers said the Earth stays still. They thought everything moved around our world. They thought the stars were fixed in place.

Later, some people had a new idea. They said the Sun was at the center. This helped them understand how things move.

It is fun to learn about the sky. What do you see at night?

For a long time, people had many ideas about space.

Ancient groups had different views. Some saw a flat Earth in a cosmic ocean. Hindu hymns spoke of a "Golden Egg" that started the universe. They believed many universes exist in cycles that last forever.

Greek thinkers also studied the sky. Anaximander thought the Earth was shaped like a tall stone pillar. He believed the Sun was a huge ball of fire. He thought it sat on a wheel with holes in it.

Other Greeks had new ideas. Parmenides said the Earth was a sphere. He thought it sat at the center of everything. Plato thought the Earth was also at the center. He said the Moon and Sun moved in circles around us.

Later, some thinkers changed these views. Aristarchus of Samos said the Sun was at the center. This is called a heliocentric model. In this way, the Earth and other planets move around the Sun.

Seleucus of Seleucia used logic to prove this. He used the movement of tides to help his proof. People's ideas about the stars kept growing and changing.

Humans have wondered about the stars for thousands of years. We used to think our Solar System was the whole universe. It took a long time to see the difference between our local space and the vast cosmos.

This history is a record of how our ideas grew. We moved from simple stories to complex math. Today, we use physical cosmology to study how everything works. This science helps us understand the very beginning of time.

Early ideas often used nature to explain the sky. Mesopotamian people imagined a flat Earth in a cosmic ocean. Hindu hymns spoke of a "Golden Egg" that started everything. They believed the universe goes through endless cycles.

Some Greek thinkers had even more mechanical ideas. Anaximander thought the Earth was a tall cylinder. He imagined the Sun sat on a huge, fire-filled wheel. This wheel had holes like a flute. The Sun's light shone through these holes. This helped explain why eclipses happen.

Greek philosophers changed how we see the world's shape. Parmenides was one of the first to say Earth is a sphere. He believed Earth sat at the very center.

Later, Plato described a universe of perfect circles. He thought the Moon, Sun, and planets moved in orbits around us. Eudoxus of Cnidus used math to explain these movements. He imagined twenty-seven invisible spheres rotating at different speeds. Aristotle also believed the Earth was the stationary center. He thought everything fell toward Earth because it was the middle.

Newer ideas eventually moved the center of our system. Aristarchus of Samos proposed a Sun-centered model. This is called a heliocentric model, where the Sun is the middle. He suggested the Earth rotates on its own axis too. Seleucus of Seleucia used logic to support this idea. He looked at the tides to help prove his point. He thought the Moon's pull caused the water to move. Archimedes even tried to estimate how large the cosmos might be. He calculated it could be two light years wide. These thinkers paved the way for modern space science.

These old theories connect to what we know today. We still use circles and orbits to map the planets. Even though we know more now, we still ask the same questions. We want to know where we came from. We look at distant galaxies to find answers. The study of the universe is always growing. Every new discovery changes our view of the dark sky. We are still learning about the amazing cosmos.

Cosmology is the study of the entire universe. It seeks to explain how everything began and how it functions. For most of human history, people did not distinguish between our Solar System and the whole universe. They believed the stars and planets were simply the contents of everything that existed. It was not until the mid-17th century that scientists began to make a clear distinction between these two concepts. Today, we use the discipline of physical cosmology to study the vastness of space.

Ancient cultures used different stories to explain the cosmos. In Mesopotamia, people imagined a flat, circular Earth surrounded by a cosmic ocean. Hindu traditions, specifically in the Rigveda, described the universe's origin through the Hiranyagarbha, or "Golden Egg." This model suggested that the universe exists in endless cycles of being manifest and unmanifest. These cycles are incredibly long, lasting 311.04 trillion years each. They believed innumerable universes exist simultaneously, driven by eternal desires.

Greek philosophers introduced more mechanical ways to view the world. Anaximander proposed that the Earth was a cylinder shaped like a pillar. He thought the Earth floated still in the center of the infinite without any support. In his view, the universe was a system of hollow, concentric wheels filled with fire. These wheels had holes, similar to those on a flute. The Sun was the fire seen through a hole in the farthest wheel. This wheel was twenty-seven times larger than the Earth.

Other Greek thinkers focused on the shape of the Earth and its position. Parmenides was among the first to declare the Earth is a sphere. He believed the Earth sat at the center of the universe. The Pythagoreans held a different view, suggesting a "Central Fire" powered the planets. They believed the Earth and Moon orbited this fire, which stayed hidden from view. To keep their math perfect, they even added a tenth hidden body called the Counter-Earth.

Plato and Aristotle developed complex models of a geocentric universe. A geocentric model places the Earth at the center of all motion. Plato described the universe as a series of ordered circles. He believed the Moon, Sun, and planets moved in specific orbits around the Earth. Eudoxus of Cnidus later used math to refine this idea. He imagined twenty-seven invisible, concentric spheres rotating at different speeds. Aristotle expanded this by suggesting these spheres were made of a material called crystal.

Eventually, the focus shifted from the Earth to the Sun. Aristarchus of Samos proposed a heliocentric model, where the Sun is the center. He also suggested the Earth rotates on its own axis. Later, Seleucus of Seleucia used reasoning to support this Sun-centered view. He likely used the phenomenon of tides to help prove his points. He believed the Moon's attraction caused the tides. Archimedes even tried to calculate the size of the cosmos. He estimated it could be as large as two light years.

These early theories provided the foundation for modern science. We moved from seeing the universe as a collection of myths to seeing it as a system of math and physics. Understanding these historical shifts helps us appreciate how far we have come. We no longer see the Earth as a stationary center. Instead, we see ourselves as part of a much larger, moving system. The history of cosmology is a journey from the small to the infinite.