A Brief History of Astronomy's Evolution

" You have to know the past to understand the present."

                                                                      -Astronomer and Author, Carl Sagan (1980)

All innovations and discoveries, no matter how small or large, are all built upon the work and knowledge of predecessors - even Einstein's famous general theory of relativity was only possible due to the foundation built by astronomers throughout the ages! That's why knowing how our understanding of the universe has changed through the ages is crucial for laying the groundwork for any future discoveries. Understanding the challenges and sacrifices faced in order to allow astronomy to become what we all know and love also allows people to gain a brand new admiration for this vast subject. So without further ado, let's get into seeing the history and evolution of astronomy!

Fig 1- Geometric Model

Born in 100AD, the Greek astronomer Ptolemy established the Ptolemaic geocentric system - a model of the universe with the earth at the centre while the sun, the moon, and each of the planets lay on their own transparent, moving sphere (Fig 1). This system also had an outer sphere- the prime moover which was believed to be the source of any motion noticeable in the sky. 

    For almost 15 centuries, this was the core of astronomical education until the 16th century, when a Polish astronomer, doctor, mathematician, fortification engineer, and priest Nikolai Copernicus proposed a mathematically simpler system with the sun in the middle- the Copernican heliocentric model. Being a Catholic priest, he understood that this would be a huge problem for the Catholic Church as it relegated the Earth to a mere planet so his research was not published until 1543, a few days before his passing. Despite this, in 1616, the Church forbade all Catholics from studying or teaching the new system.

    At the end of the 16th century, Danish astronomer Tycho Brahe proposed a compromised system with the Earth at the centre but all the other planets orbiting the Sun. This system preserved the mathematical simplicity from Copernicus's model while also not getting on the bad side of the Church.

    The shift of turning the Earth from the centre of the universe to one of the many ordinary celestial bodies continued as glassmaker Galileo Galilei built one of the first telescopes used to look up into the sky where he noticed several interesting and shocking things. One of the most important Galileo observed was that there were 4 moons orbiting around Jupiter! This was a big issue as it raised the question of how many more spheres would be needed to understand celestial motion thoroughly. In 1624, Galileo's good friend was elected as the new pope, causing Galileo to make a huge blunder, he published a book in defence of the Copernican system. He was later "strongly suspected in heresy" by the inquisition and lived the rest of his years under house arrest- a much better result compared to past astronomers who were executed by burning... 

    A year after the passing of Galileo, a great mathematician and physicist, Sir Isaac Newton, was born. Formulating a new method of mathematical computation, Newton created what has been described as the strongest analytical research tool that humanity has ever invented- Calculus. Using this, Newton calculated the orbits of all the planets based on very detailed observations conducted by astronomers of the past: Johan Kepler, Copernicus and Galileo, to name a few. His results were published alongside his general theory of motion in his famous book, Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy). Since the publication of this book, the Ptolemaic System lost all sense. 

    Newton's genius did not stop here! He went on to define one of the most important laws ever, two particles of mass M and m are related by the equation F(r)= G*m1*m2/r^2 . This is Newton's Law of gravitation, also more commonly known as Newton's fourth law. Today, we have experimentally calculated the value of G to be 6.67e-11. Based on this assumption, we can accurately calculate the orbits, the orbit shapes of planets in the solar system and the period of rotation of each planet. The calculations match with observation tables of astronomers like Johan Kepler. Unfortunately, one big question that Newton left open was - how does gravitational force affect bodies through empty space?

It wasn't until 1915 when an attempt to understand gravity was made by Albert Einstein in his theory of General Relativity. In this, gravity was described as a geometrical effect, where a mass causes a curvature in space. At a certain range, this is mathematically identical to Newton's fourth law but near bigger masses, Newton's theory can't be applied. One important calculation done using the general relativity equations was obtained by Russian mathematician Alexander Freidman, who applied Einstein's equation, taking the universe as endless space with homogeneously distributed matter, and found that the universe has to be either expanding, collapsing or fluctuating! This was a huge shock, even to Einstein himself. It wasn't until 1929 that astronomer Edwin Hubble proved that the universe is expanding. This led physicist George Gamow to propose a theory that we now know as the Big Bang theory. 

Of course, this isn't the end. Every day, physicists are learning and discovering more and more about this treasure box filled with mysteries, and there is already so much we know (and so much we don't), which unfortunately cannot all be covered in this brief blog. If you are interested in this topic and would like to learn more about the wonderful discoveries past astronomers have made, and what this means to use, I recommend checking out: https://www.britannica.com/science/astronomy/Enlightenment.

Hope you enjoyed reading about the past of astronomy and learnt something about the sacrifices made to allow astronomy to become what we all love today!

-Written by Pranav Garg

Fig.1, an image from Physicist Page on Facebook