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Working of Solar System and Universe

Aug 20, 2022

Key Concepts

• Solar system

• Planets

• Inner planets

• Outer planets

• Models of the solar system



People studied the stars before the invention of telescopes. When they observed the sky in the night, they found out that bright lights in the sky changed their position according to other lights. In those days, astronomers called these objects planets. The name ‘planet’ comes from the Greek word ‘wanderer.’ A planet is a huge body that revolves around a star. A moon is an object that revolves around a planet. Planets and moons are parts of a solar system. A solar system comprises of a star such as the sun, and all the planets, moons and other objects revolving around it. 

1: Earth 

Planets and Orbits 

We all know that gravity is the force of attraction among all objects. Gravity causes the thing to fall on to the floor and this force keeps planets in orbit. The amount of gravitational force between the sun and the planet depends on its mass. The higher the mass of the object, the attraction will be greater between them. Distance is also one of the factors that affect gravitation. The higher the distance between the planets and the sun, the lower will be the pull of gravity. 

2: Force of gravity between the sun and the planets 

The planets revolve in orbit due to inertia. Inertia is the tendency of a moving thing to remain in motion. Gravity will pull objects, i.e., planets, towards the sun, because of the huge mass of the sun. This does not take place because the planets have inertia and they are moving. Inertia causes the planets to move in straight lines. The balance between the force of gravity and the inertia of the planets keeps the planets moving in their orbits around the sun. The movement of planets is in a straight line due to inertia, but gravity pulls the planets and makes their path in a curved shape into orbits around the sun. 

Solar system: 

  • The solar system consists of stars, the sun and everything attached to it by gravity, and the planets such as Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune; dwarf planets like Pluto, moons and many asteroids, comets, and meteoroids.  
  • There are many planetary system orbiting other stars in the milky way. 
 3: Solar system 

What is in the inner solar system? 

  • The inner planets are Mercury, Venus, Earth, and Mars are the planets located close to the Sun. So, they are called inner planets.  
  • All the inner planets rotate somewhat slowly, and none of them have rings.  
  • The largest of the inner planets is Earth. 
 4: Inner planets 

What are outer planets? 

  • Apart from the asteroid belt is another group of planets that consist of Jupiter, Saturn, Uranus, and Neptune. These planets, called as the outer planets and are very different from the inner planets.  
  • The four outer planets are similar in size to one another and are called gas giants. All these outer planets have a small metallic core and a thick layer of atmosphere.  
  • The outer planets are larger than the inner planets and their orbits are very far away from each other.  
  • All the outer planets have rings and many moons, and they spin very fast; hence a day on the outer planets is short.  
  • Away from the outer planets there are small icy worlds; one of them is the largest, called Pluto, that was once known as the ninth planet. For many years there has been a discussion among scientists, whether Pluto should be called a planet or not? In 2003, astronomers found out about a similar but slightly bigger world beyond the orbit of Pluto. In 2005, scientists found a moon orbiting this newly found world, and in 2006, the IAU (International Astronomical Union) reclassified Pluto as a dwarf planet.
5: Outer planets 

Models of the Solar System 

The first astronomers who explored the sky thought that the stars, planets, and sun revolved around Earth. This idea led to the first model of the solar system. But the model changes as scientists explore more about the working of solar system. 

The planets orbit the Sun in a counterclockwise direction as seen from above the Sun’s north pole, and all the orbiting planets are aligned to the ecliptic plane. 


Early Models 

More than 2000 years ago, the Greek philosopher Aristotle suggested a geocentric model, or the Earth centred model of the solar system. According to this model, the sun, the stars, and the planets revolved around Earth. Though this model could not explain the reason of movement of planets sometimes in the backward direction in the sky relative to the stars. This pattern is known as retrograde motion

6: Aristotle’s Geocentric model 

The three main models of the solar system that were used to calculate the positions of the planets and stars are: 

  1. The earliest Greek geocentric model as suggested by Ptolemy. 
  1. The full heliocentric model proposed by Copernicus. 
  1. The hybrid model of these suggested by Brahe. 

In spite of their philosophical variations, mathematically all these models were same. They all depend on combinations of circles moving at constant speeds in order to work. The only actual mathematical difference between them is the selection of origin and coordinate system. They were all wrong. In the beginning of the seventeenth century, Kepler finally found out that the planets moved in ellipses around the sun.  


By 150 CE, the Greek astronomer Claudius Ptolemy suggested changes to this model. He thought that planets move in small circles, known as epicycles, as they move in large circles around the Earth. These epicycles seemed to explain the reason for movement of planets appear to move backwards. 

This model was created on the ancient Greek “Principle of Uniform Circular Motion,” which said that the only motion that could go on indefinitely was heavenly motion in a circle at constant speed. It was also completely geocentric; the Earth was in the center of the universe and did not move. 

7: Order of planets in Ptolemy’s model  

There were two major circles that carried each of the planets. The process for each planet is shown in the diagram below: 

Each planet denoted by M, travelled in a little circle called an epicycle. The epicycle rotated around its center point, P, with a constant rate. The center of the epicycle (P) moved in a larger circle called the deferent. The center of the deferent is ‘C’, was not precisely on the Earth denoted by ‘E’, but it is close to the Earth. The point marked as Q was called the equant point, and EC = CQ. Here we have two circles and those circles had to rotate at constant speed. In the Ptolemaic model, the epicycle did rotate with a constant speed, thus that PM rotated at a constant speed. However, the center of the epicycle itself did something a little odd. The line QP rotate at constant speed. This means that the planet actually moved around the deferent at various speeds. (The fact is that the planets move rapidly when they are nearer to the sun, and planets slow down when they are farther from the sun.) The equant was Ptolemy’s big discovery, and though it made the model more precise, it also annoyed the others who believed in the Principle of Uniform Circular Motion. 

In this model, the epicycle was needed to get the position of the planet accurate, and to explain retrograde motion. If the epicycle rotation was more rapid than the deferent rotation, then the planet would actually go backwards when the planet was nearer to the Earth. 

 8: Ptolemy’s model of the Solar system 


In 1543 CE, a polish astronomer Nicolaus Copernicus proposed a sun centred or also called as heliocentric model of the solar system. In the Copernicus model, the planets revolved around the sun in the similar direction but at various speeds and distances from the sun. Fast moving planets crossed the slow-moving planets. Hence, planets that were moving slowly than the Earth seemed to move backward. 

 9: Copernicus model of the solar system 


At the end of the sixteenth century, Brahe suggested his own hybrid model of the solar system.  Brahe liked the Copernican model but for one thing: he did not think that the Earth could be moving without it being noticeable to people on the Earth.  He thought that the Earth had to be at rest. In his model, the Earth is at rest, and the sun moves around the Earth. But all the other planets move around the sun. The diagram below shows the orbits. Brahe used circles, and he also used epicycles. 

 10: Brahe’s model of solar system 


The Italian scientist Galileo Galilei observed four moons moving around the Jupiter. This observation suggested that objects can and do revolve around objects other than the Earth. 

Isaac Newton 

He brought the laws of physics to the solar system. Isaac Newton clarified why the planets move the way they want, by applying his laws of motion, and the force of gravitation between any two objects, allowing the force decrease with the square of the distance between the two objects.  


• Planets and moons are parts of a solar system. A solar system comprises of a star such as the sun, and all the planets, moons and other objects revolving around it.

• The amount of gravitaional force between the sun and the planet depends on the mass.

• Inertia is the tendency of a moving thing to remain in motion.

• Solar system consists of star, the sun and everything attached to it by gravity and the planets such as mercury, venus, Earth, mars, jupiter, saturn, uranus and neptune; dwarf planets like pluto, moons and many asteroids, comets, and meteoroids.

• Aristotle suggested a geocentric model, or the Earth centered model of the solar system.

• Claudius Ptolemy thought that planets move in small circles, known as epicycles, as they move in large circles around the Earth.

• Nicolaus Copernicus proposed a sun centered or also called a heliocentric model of the solar system.

• Brahe suggested his own hybrid model of the solar system.

• Galileo Galilei observed four moons moving around Jupiter.

• Isaac Newton clarified why the planets move the way they want, by applying his laws of motion.


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