How did we find out our Earth rotate around Sun

Sun and its planets

We all know that our Earth moves around Sun from our science textbooks. It is a quite simple idea known to any person who has been to school. But, how we came to the conclusion that Earth is move around Sun? As a science communicator when I put this question in front of student, teachers or any other layman then it becomes difficult for them to imagine this. Obviously textbooks most of the times never discuss why we consider earth revolving around sun. Even it was very hard to imagine to our ancestors also that Earth moves around sun. An idea of moving earth needed much more evidences. Because even today we experience that sun rises at our east horizon and sets at west horizon. Even moon, stars and planet follow the same thing, they rises in east and sets in west.

Many Astronomers from various ancient civilizations have given idea of moving earth around sun. One argument every time has denied this idea that if earth moves around earth then why we don’t feel it? This question was remained unanswered till Galileo’s experiment with inertia. Earth have two kinds of motions (there are several but here we are taking two) one is on its axis from west to east and second one is rotating around sun. Unable to visualize this two motions, scholars asks many questions that if earth move around its axis then if anyone jumps in air should have landed on some other place towards west. Bird, animals all should have get affected from such motion. If earth moves around sun then we should have seen stars at different angle. But, we just don’t simply see this. Hence it is not possible that earth is moving around sun. They said, we see sun going east to west. So, sun is moving.

Putting earth to the center was seems logical. All sky was revolving around our planet and we could predict about sun, moon and stars position precisely. Many leading philosophers like Aristotle was in favor of earth centered universe. Aristotle believed that earth is at the center of the universe and it is surrounded by the seven crystals carrying celestial bodies like Moon, Mercury, Venus, Sun, Mars, Jupiter and Saturn. At the end there is crystal of stars. These crystal are transparent and perfectly spherical. These all crystals rotate around earth. It was perfect picture of universe.

Aristotelian view of Geo centric universe in which crystal spheres of planet revolve around earth

Aristotle gave philosophical model. His model was nothing to do with god created universe. But, this perfect model of earth centered universe then adopted by many religious organizations in the world. After that it was projected as the universe was created by God for his special creation ‘MAN’ and god gave special position for his children in the universe. Everything in the universe must rotate around Earth. In coming ages speaking against position of earth was offence in western world. It was like questioning on authority of god. Still time to time some philosophers, mathematician put doubt on earth centered universe not only in western world but in every civilization. Aristarchus of Samos, Aryabhata from India put forwarded Sun centric models.

Earth centric model predicted position of sun and stars in sky precisely but it could not predict motion of planets in sky, especially retrograde motion. Planets shows two kind of motions in our Earth’s sky. One is diurnal motion in which they moves from east to west in sky. Other one is direct motion which is from west to east in sky. Planets direct motion observed over the period of long time on the stars background. But sometimes planets stops and moves towards west direction for few days and then stops and again follow their direct motion. Ancient astronomers define this phenomenon by saying retrograde motion is depend on planet’s will. Today it seems to be ridiculous explanation but for that time it was acceptable.

Retrograde Motion of planet Mars

Some astronomers try to solve this problem. Ptolemy (110-160 CE) came with a model. He mention this model in his book ‘Almagest’. In this model, Earth placed at the center of the universe and Sun and Moon moving around Earth. Planets are also moving around Earth in circular orbit called as Deferent and to explain their retrograde motion he put small sub-orbit in planets orbit. He named those sub-orbits Epicycles. He said when planets moves in Epicycles at that time they show retrograde motion. Ptolemy’s Earth centered model a.k.a. geocentric model could make accurate positions of Sun, Moon and with some accuracy position of planets also. But, it was very complex model and calculating positions of planets and moon was very tedious. Even though for centuries this model was accepted by astronomers. Because there was no better explanation for Earth centric universe.

Ptolemy added an sub circle 'epicycles' in main orbit circle of planet, when they moves in these epicycles they show retrograde motion

Astronomers were seeking new models to reduce complexities in calculations. Nicholas Copernicus (1473-1543) came with a simple model. In this model he put Sun at the center of the solar system and try to explain retrograde motion. It was simple model to explain retrograde motion. All planets are rotating around Sun in their orbits. Planets orbits are different in size with respect to their position from sun. Also they move with different speed too. Mercury nearest to Sun moves rapidly. Saturn farthest from Sun moves much slowly. Their different speed in orbit explains retrograde motion. According to Copernicus model it was easy to understand why planets shows retrograde motion. Actually retrograde motion is not actual motion, it’s more like an illusion. When Earth move ahead in its orbit compare to outer slower planet they appear to move background on the background of stars.

Copernicus model of universe, he put Sun at the center of universe and make all planets including earth go around sun 

We can take an example to understand this, suppose two cars are running in their circular race track. One car is behind other car, driver in this car can see that car is moving ahead of her car in same direction. But when she pick up speed her car comes along side with other car, at this time she see the car is moving with her car with same speed. At this point it will appear steady for a while. Now she over speed other car, at this time car driver will see other car is slowly lagging behind of her car. It will appear to move against the moving direction. But when she go more ahead in her race track she will observe that the car is now moving in same direction as her car. Same thing happen with Earth and outer slower planets. Earth overtake them in their orbits and we see retrograde motion.

Copernicus suggested that when planets moves in circular path around sun they pass each other in their orbits and we can see retrograde motion

Copernicus theory gave simple explanation for retrograde motion. During middle ages it was difficult to put any option in front geocentric model. Because, it got divine place in religion. Talking against this model means talking against religion. But, there are certain enough evidences that Copernicus knew that planet Mercury and Venus are not far from Sun and it was obvious that they could not orbit around Earth but Sun. Planet like Mars sometimes become very bright when they are at the same side of earth in its orbit and sometimes it becomes very faint when it reaches far side of Sun away from earth. Copernicus believed heliocentric model can make many observation relevant and explainable. But, shaking beliefs of religion was very difficult at that time. Many peoples still have faith on geocentric model.

Tycho Brahe (1546-1601) the most proficient astronomer of his time. He measured accurately the position of stars and planets every night. From his observation he was agreed with the Copernicus view that all planets moves around Sun. But he was not agreement with Copernicus on one point that earth also moves around Sun. He also believed the divine position of earth in the universe is the center of it. He came with one very different model of universe. He showed all planets moves around the Sun and with all these planets Sun moves around Earth. It was like patch between earth centric and heliocentric models. In fact in 1616 when church banned on books with heliocentric views, church favored Tycho Brahe model. In 1601 when Tycho died he gave his observation data to Kepler and asked him to derive some proofs from his observation.

Tycho agreed with Copernicus that planet moves around sun but he believed that sun with all planets moves around stationary earth

Kepler (1571-1630) went through Tycho’s data and came with very different opinion than Tycho. It took him several years to fit Mars orbit in Tycho’s data. His analysis was involve idea of moving Earth around sun. He remove Earth’s motion from calculation and the remained was motion of Mars with elliptical orbit around Sun. He believed that Tychos data will not lie. So, he went further to calculate orbit of other planets and he found they follow the elliptical orbit too. So, he came up with heliocentric model. But, in this heliocentric model orbits of planets were not perfectly circle like in Copernicus model. These orbits were elliptical. It was very surprising conclusion.

Kepler showed that planets  with earth moves around sun in their elongated circle called as elliptical orbit

How planets will orbit in elliptical orbit and not in perfectly circular orbit. Aristotelian supporters thought it is distorted model. Actually Ptolemy and Tycho already distorted perfection of Aristotelian model. Aristotle gave picture of perfectly spherical crystals. Ptolemy put epicycles in perfect crystals. In Tycho model planets crystals crosses crystal of sun. But, Aristotelian supporter thought Ptolemy and Tycho’s model were good in terms of perfect model of universe.

Kepler was religious man and believed that Universe was an ordered place created by God. But he also believed that mathematics and geometry could be used to uncover this well-organized design. He came with the three laws of planetary motion. First law says that planets moves in elliptical orbits around the Sun. The second law is mathematical description of how planet speed up when they are near to sun and how it slow down when it is far away from sun. Third law gives the relation of speed of planet and its orbit size, as planet orbit size increases it move more slowly. Later, these laws puzzled Halley and he took them to Newton to know why these laws work. Newton then told how force of gravity work on planets and how they moves.

Kelper’s laws of planetary motion published during 1609 to 1619. During that time work of Galileo Galilei (1564-1642) changed the course of astronomy. Galileo gave some strong arguments in favor of heliocentric model. Galileo was not astronomer (or astrologer) like other. He was mathematician who turned his telescope towards sky and observed celestial objects. When he saw moon through his telescope, it was not perfectly smooth heavenly sphere like Aristotle described there were lots of craters on it. Then he saw planets like Mars and Jupiter like discs and not like stars. He saw four bright specks of light around Jupiter. On following nights he saw they were changing their position around Jupiter disc. He concluded those must be moons of Jupiter. Aristotelian supporter said there must be only one center of universe is earth around which all things revolve. Copernicus created two centers in his model one was Sun center of all planets and other one was Earth center of moon. Then Galileo saw third center, at Jupiter.

Galileo's observation of Jupiter and its moons through his telescope and found that not everything rotate around earth

Galileo observed phases of Venus. He tried to fit his observation of Venus phases in every model. Existing Venus phases were hard to fit with Ptolemy model. The best explanation for the Venus phases came with Copernicus heliocentric model. Galileo were agreement with the Copernicus perfectly circular orbits and not with Kepler’s elliptical orbits of planets. But, planet position data well fitted with elliptical orbits. After mathematical work of Newton it was known to everybody that its Earth which rotate around Sun in elliptical orbits just like other planets.

Galileo's observation of Venus phases best fitted with the sun centered model of Copernicus

Making out exact model of solar system was not easy task for people. It took many centuries to know exact position of earth. It was also required lots of other knowledge too for coming this conclusion. It would not be good if we say that Aristotle or Ptolemy model was wrong. They have given best possible explanation for the belief of at that time. But, their models lack of observational support. There were other models of universe too available at that time. Science does not work isolated from society. Accepting facts also depends also how much society is open for new knowledge. In India also Aryabhatta gave model of sun centered universe but scholars at that time questioned on this. Moving Earth model required lots of evidences not only observation based but also conceptual based like laws of motion, force and frame of references. This conceptual understanding evolve much later, when physics of force and motion developed. If we wanted to understand this then we need to get some ideas from all these sides. If we wanted to develop understanding in students then we need to give exposure to students with visual observation of celestial bodies and then discuss the concept like force and laws of motion.

Our Solar system

Resources used
Books
The children of Prometheus by James MacLachlan 
Story of Astronomy by Peter Aughton
The Unknown Universe by Stuart Clark

Images are taken from various internet sources