Astronomy from Ptolemy to Galileo Essay Sample

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Introduction of TOPIC

It has been said that Astronomy has always been the first science to develop in all civilizations. This is not surprising seeing how the sun, moon and stars are such an ever present part of our daily lives. From the moment man first notices the presence of day and night, he is already making an astronomical observation. From the moment, man first asks why the sun rises at dawn and sets at dusk, he is already making conjectures about astronomy.

Given the majesty and grandness of the sky, it is not surprising that early man gave equally grand and majestic explanations for the sky and its motions. Drums would beat during eclipses in order to scare away the dragon that swallowed the Sun. They noticed stars that varied positions – planets – and gave these wanderers names reserved for the mightiest of Gods. The tapestry of the sky formed patterns, patterns which were the most fitting resting place for some of the greatest heroes of legends and epics.

While being a source of wonder, the heavens were also a source of mystery. Calendars were drawn up based on the motions of the stars. These same calendars helped civilizations grow and flourish as the stars told man when to plant his crops, or when the river is going to flood. Yet far from utility, some saw in the stars a sense of grandness. Seeing the great sun, moon, Jupiter, and his brothers revolve around the Earth – it was clear for the early observers that he was standing in the center of the Universe and that everything else literally revolved around him.

And such the first picture of our universe emerged – the geocentric model. In the geocentric model, the earth (geo) is at the center of the universe and all heavenly bodies revolve in orbits around it. Philosophers such as Hipparchus and Aristotle have left their mark on the geocentric theory. It was however a scholar at Alexandria named Claudius Ptolemy who established a major part of the science of geocentric theory.

An important distinction between Ptolemy’s work and of those before him was his dedication to observation and analysis. Ptolemy himself compiled a star chart detailing the positions and magnitudes of 1,022 stars. To aid him in his reckoning, he constructed what is arguably the first astronomical instrument – the astrolabe.

However observation was not enough to put Ptolemy in the history books. Ptolemy is best remembered for the geocentric system that bears his name. In the Ptolemaic system, the Moon, planets, and the Sun revolved around the Earth in concentric orbits. This explained the east to west motion of the heavenly bodies. Moreover, the planets each moved in a circle as they orbit the earth – the epicycle. The epicycle would cause the planets to move farther or closer to Earth depending on which phase of the epicycle the planet was on.

Using the concept of the epicycle, Ptolemy was able to account for the retrograde motion observed as some planets moved eastward occasionally. While we know now that Ptolemy’s was mistaken in his concept of geocentrism and epicycles. However, for his contemporaries, Ptolemy’s model held to the test. Using his model, they could make empirical predictions about the starts of retrograde motion, the reason the sun falls behind any star by four minutes each day, as well as the eastward drift of the moon. For those in Ptolemy’s footsteps, the Ptolemaic system works and is most certainly useful. The Ptolemaic system was so successful that it would hold its ground for 1400 years until the work of a man named Nicolas Copernicus.

Nicolas Copernicus was convinced that the universe was much simpler than the Ptolemaic model with all its epicycles upon epicycles. He proposed an alternate system, one where the sun and stars were fixed. A system where the earth moved around the sun at the center  – the heliocentric system.

Nicolas Copernicus did not invent the heliocentric system, Greek astronomers and philosophers before him had already conceived of a Solar System with the sun at the center. However it was Copernicus’ writings which rocked the scientific community, sending shock waves to be felt by scientists like Galileo years after Copernicus’ death. Copernicus himself was unable to witness the effects of his writings as his major work, De revolutionibus orbium coelestium, was published after his death.

In De revolutionibus, Copernicus put the sun at the center of the solar system with the planets (including the Earth) revolving around it

in concentric orbits. The moon in turn revolved around Earth. Proposing a system is not enough, like

Ptolemy before him, Copernicus’ system must also account for observed phenomena. Under the Copernican system, the westward motion of the Sun was due to the Earth’s motion, not the Sun’s. The Copernican system also allowed for an explanation of retrograde motion without having to resort to epicycles. The drift of the sun could be explained by the Earth;s movement relative to the fixed sun and stars. In fact, every observable phenomena that the Ptolemaic system could explain, the Copernican system could account for too.

Another factor was the church. The Ptolemaic system had been enforced church doctrine. To dismiss the Ptolemaic system was taken as an affront to the very essence of nature, of God’s design. In the Ptolemaic system, the Earth is at the center of the universe, at the spot of single importance, an object which the heavens revolved around. What the Copernican system gained in simplicity, it also gained in controversy. It would still take centuries before Nicolas Copernicus’ heliocentric system would finally take hold as the accepted model of the solar system. The Copernican model still had to wait for the work of Brahe, Kepler and Galileo to show conclusively its superiority over the Ptolemaic model.

Tycho Brahe had been one of the most prolific observers in the history of Astronomy. In his own island dedicated to astronomical observation, Tycho Brahe plotted the positions of eight hundred stars over a span of 21 years. His precision was astounding considering that it would still be decades before Galileo first pointed a telescope to the heavens. To aid his studies, he constructed gigantic instruments for reckoning stellar position. Using only his eyes, Brahe could plot stellar position to within one-one hundred and twentieth of a degree. He can even correct for the distortion of the atmosphere.

Tycho Brahe’s precision was aimed toward discovering parallax – the apparent shift of stars in the night sky. If the earth really was moving around the Sun, then the stars would seem to change position over time. The same effect could be noticed by holding your finger in front of your face and viewing it with only your left and right eye alternately. If stellar parallax were detected, it would be a huge confirmation for Copernicus’ moving Earth model.

While Brahe failed to detect parallax, his detailed observations had been the basis of the work of Johannes Kepler. Brahe hired Kepler as his assistant one year before his death. Kepler’s mathematical tenacity was a match for the observational abilities of Brahe. A believer in the Copernican system, Kepler tried to use Brahe’s detailed observations to verify the Copernican model. However, Kepler wasn’t able to reconcile his calculations with Brahe’s observations.

Instead of forcing his calculations to meld with Brahe’s data, Kepler tried to look at Brahe’s data before making his calculations. Looking at the data, Kepler saw that the path of the planets did not follow circular paths but rather elliptical orbits. An ellipse is one of the four conic sections formed by the intersection of a plane at an angle with a cone. An ellipse’s shape is defined by two focus points, with a circle being a special case of the ellipse wherein the two foci are located in the same point. Over a span of 18 years, using this new found discovery, Kepler formulated his three laws of planetary motion.

With Kepler and Brahe’s data, the heliocentric model was able to get the mathematical precision that lacked the geocentric model. Kepler’s ellipses were the only way to reconcile the detailed observations by Brahe with the available mathematical tools. Kepler and Brahe’s work served as the first test where geocentrism lost to the heliocentric model.

While Kepler gave mathematical verification, Galileo’s telescopes gave observational confirmation. Galileo’s view through the telescope brought worlds of discovery. In seeing an imperfect Moon, Saturn and Sun, the view of the consummate celestial heavens was put to the test. The notions of perfection did not hold as craters were found on the Lunar surface, as spots were discovered on the surface of the sun.

Moreover, Galileo found phases on Venus and moons revolving around Jupiter. Phases on Venus could only be explained by a heliocentric model. While Ptolemy could account for these phases, it would involve more epicycles and more complexity in stark contrast to the simple explanation that Copernicus provides. Additionally, seeing bodies revolve around another body challenged the idea that everything revolves around the Earth. While Galileo did not deliver a death blow to Ptolemy, it was becoming more and more apparent that the heliocentric model was a more elegant, a more compact explanation.

While tracing the evolution of solar system models is a good practice in history, it is also a good exercise in understanding Science. Galileo, Copernicus, Kepler, Brahe and Ptolemy had no way of knowing for sure the organization of the solar system. They could not directly see how the planets move in relation to one another. They could only infer how the universe works from limited earthbound observations. Limitations imposed not only by our diminutive stance in the solar system but also by technology. Tycho Brahe failed to detect parallax simply because he did not have the equipment to do so. It would take until 1830 for the first measurement of stellar parallax.

Science works because it is always challenging itself. It is not a central dogma but rather a continuing search for an explanation of the Universe. As we can see in the progression from Ptolemy to Galileo, a new model was not simply substituted for another due to simplicity or elegance. New models and theories must be more correct in order to replace older theories and models. Their correctness lies in their ability to make predictions or explain phenomena which the previous model cannot account for. The story of Ptolemy and Galileo has been repeated over and over again in other fields, from Lamarck to Darwin, to the evolution of Gravity from Newton to Einstein.

We may never be able to know what is the truest form of the universe as we are limited individuals living a limited time in a limited society. In the absence of omniscience, we have our observations, we have our tests, we have our criteria of falsifiability, we have our notions of elegance and simplicity over complexity and crudeness. We do not have the eyes of God but we have the Science of Man. And as we expand our limitations, as we break barriers, as we see things previously beyond perception, Science will update itself and we will get a better glimpse to the majesty of the universe.


Perlman, J.S. (1995). Science Without Limits: Toward a Theory of Interaction Between Nature and Knowledge. Amherst, NY: Prometheus Books.

Burke, J. (1995). Connections. New York, NY:Simon and Schuster.

Kartunnen, H., Kroger, P., Oja, H., Poutanen, M., and K.J. Donner. (2003). Fundamental Astronomy. New York, NY: Springer Verlag

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