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Models of the Solar System and Their Limitations

Grade 8
Jun 10, 2023

Models of the Solar System


We can learn a great deal about the Universe and Earth by observing our nearest celestial objects. The solar system consists of planets, asteroids, comets, and the Sun. These neighboring celestial objects we can observe at night in the clear sky. The Sun and all celestial objects that are held by the Sun’s gravity are known as the solar system.

In space, all celestial objects like the sun, stars, planets, moons, and satellites exert a gravitational force on each other and it is always attractive. To understand the movement of these celestial objects, different models have been proposed.


Anyone who spends some time looking up at the sky will quickly realize that the Sun rises in the east, reaches its zenith at noon, and finally sets in the west. The Moon and the stars also rise in the east, ascend higher in the sky, and set in the west.


Ancient astronomers believed that the Earth doesn’t spin on its own but that the Sun, Moon, and stars are all rotating around it, hence the assumption of a geocentric model, in which the Earth doesn’t rotate independently of the Sun, Moon, and stars, looked entirely reasonable. After all, individuals on Earth do not feel the Earth’s rotation.

The geocentric model of the Solar system

Ancient Greek’s Geocentric Model:

The ancient Greeks thought that Earth was at the center of the universe. They thought that the sky had a set of spheres layered on top of one another. Each object in the sky was attached to one of these spheres.

Ancient Greek’s Geocentric model of the Solar System

The object moved around Earth as that sphere rotates. These spheres contained the Moon, the Sun, and the five planets they recognized: Mercury, Venus, Mars, Jupiter, and Saturn. An outer sphere contained all the stars. The planets appear to move much faster than the stars, so the Greeks placed them closer to Earth.

Ancient Greek’s Geocentric model of the Solar System 2

Aristotle (384-322BC)

This geocentric theory of the solar system was proposed by Aristotle a Greek philosopher. Others broadly accepted it since the existence of day and night on Earth served as evidence.

Plato (428-348 BC)

The geocentric concept was promoted by Plato. He found that the solar system contained orbits. Additionally, he discovered the stars and realized that they moved every night.

Plato’s Geocentric model of the Solar System

Plato was only aware that the planets’ orbits were circular rather than elliptical. Since he was certain that the planets were perfect spheres, he assumed that the planets’ orbits around the Earth would also be perfect spheres. For more than 2000 years, up until Kepler discovered elliptical orbits, his model was the one that was most universally accepted.

Ptolemy (100-168)

Ptolemy, a very well-known astronomer, studied the data collected by other astronomers including Hipparchus and Apollonius. The eccentricity (deviation of a curve or orbit from circularity) and the epicycle (a small circle whose center moves around the circumference of a larger one) were their inventions. These are the fundamentals of planet motion.

Ptolemy’s Geocentric model of the Solar System

A planet is fixed to a specific location in the solar system by a force that maintains its orbital path. He integrated their ideas and produced a model that wasn’t challenged until the sixteenth century.

Ptolemy’s Geocentric Model’s Failure:

There were numerous issues with Ptolemy’s solar system model, even though it was widely accepted and used for centuries. One issue was the periodic appearance of planets moving backward or retrograde when viewed against a stellar background, as illustrated in the picture. This model had to be changed, which resulted in its ultimate complexity.

Ptolemy’s Geocentric model failure

Copernicus (1473-1543)

The Earth-centered model was in use for many years despite being quite complex. However, a new theory was put out in 1543 by Polish astronomer Nicholas Copernicus.

Copernicus published in 1543, Copernicus’ De Revolutionibus Orbium Coelestium, which was his model (On the Revolutions of the Heavenly Bodies) outlined the heliocentric universe like what we know today.

Copernicus heliocentric model of the Solar System

Heliocentric Model:

According to Copernicus, the Moon revolves around Earth, and the Earth and the other planets revolve around the Sun.  According to this model, Earth’s rotation causes the apparent motion of the planets, stars, and Sun. This is the solar system’s heliocentric model, also known as the Sun-centered model.

He inspired Galileo to create his model, which is the currently accepted model today.

Kepler (1571-1630)

​Kepler’s solar system model was similar to Copernicus’s, but he calculated that each planet’s orbit around the sun was elliptical. He was able to do this because he kept track of Mars’ whereabouts and its separation from the Sun and Earth.

Kepler’s heliocentric model of the Solar System

Kepler developed a few equations to plot each point at the precise angle needed to generate an elliptical orbit. This was a significant solar system discovery. He produced the rule that all orbits are elliptical and that planets should be falling toward the sun but are not as they are orbiting it.

Kepler found that:

  • The planets move around the Sun in ellipses rather than circles.
  • The planets move at different speeds as they orbit the Sun. The closer a planet is to the Sun, the faster it moves.
  • As a result, the outer planets take longer to complete one orbit, while Neptune takes 164 years.

Galileo (1564 – 1642)

The solar system model that is currently recognized is Galileo’s. He found that the moon rotates as well as orbits the Earth. Additionally, he built his telescope and used it to find additional moons surrounding Jupiter. He supported Copernicus’s heliocentric hypothesis with evidence he had obtained, and his fellow scientists began to accept his theory.

Galileo’s heliocentric model of the Solar System
Models of the Solar System


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