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F - 162 : Aristarchus - the Greek Astronomer who inspired Copernicus

One of the most beautiful things when sailing at night is the night sky. There is no light pollution when you are in the middle of the sea and a few days away from land. The night sky presents itself in its full beauty, the Milky Way is easy to see so are the planets and the major stars and constellations. It was the same for the ancient Greek. Light pollution is a recent phenomenon, not older than 100 years when electric light was installed everywhere.

Moon and Jupiter at 10 0'clock on our Crossing from Sardinia to Sicily in July 2020


Humans must have been gazing in the night sky forever. The ancient civilizations of Mesopotamia and Egypt took it a step further and started to map the night sky and the movements of the planets. They interpreted them as signs and messages from the Gods. Their observation tables were very detailed. Many survived to this day. Greek merchant sailors who traded with the Levant and Egypt learned about the tables and brought some home. Sailing at night with sky maps made navigation easy. The tables also found their way to the small community of Greek astronomers and mathematicians. Not satisfied with the supra-natural explanation for the movements in the sky, they looked for the laws driving the planets to predict their future positions and the occurrence of solar and lunar eclipses. They also used the sky to measure time.

Aristarchus' Calculation of the Size of Sun, Earth and Moon


The Greek Philosopher and Astronomer Thales was the first to predict a solar eclipse in 585 BC. He also considered water to be the essential substance from which everything else was made. Pythagoras (571 – 497 BC), the philosopher and mathematician we found in Crotone last year (see my blog E - ) had a few decades later a different idea. In his opinion numbers were the essential substance from which everything derived. For him the harmony of music was mathematical. The measurable size of a string could produce a given sound. He further concluded that the harmony in the sky must have a mathematical base.


Pythagoras was a rather secretive man and did not write any books but his student and successor Philolaus of Croton made notes and published them later. Philolaus wrote that the universe was numbers based and maintained by a celestial fire around which the Sun, the Earth, the Moon and the planets rotated. This concept of rotating spheres was later taken up by Plato (428 – 348 BC) and Aristotle (384 – 322 BC). Both put the Earth at the center of the universe. The geocentric view of our universe was then further developed by Hipparchus of Nicaea (190 – 120 BC). It was in use for almost 2’000 years until challenged by Nicolaus Copernicus (1473 – 1543 AD).

Aerial View of the Island of Samos - We are looking North-East on this Photo


Back to our sailing this summer. Two weeks after leaving Istanbul and having visited Miletus, the home town of the Philosopher Thales, we will arrive on the island of Samos. There we are going to visit the amazing Eupalinos tunnel from 530 BC, a monument to the mathematical prowess of the island’s citizen. The hand-chiseled tunnel is longer than 1 kilometer (see my blog on aqueducts F - ). And of course the Pythagoras Museum. It was his home before he moved to Crotone in Magna Graecia.


Aristrcho - how People imagined him to look like -

there is no Bust or Statue of him surviving


A good 200 years after Pythagoras, another mathematician and astronomer was born here: Aristarchus of Samos (310 – 230 BC). He was the first person to propose a heliocentric universe with the sun at the center. We do not know much about Aristarchus. He moved early in his life to Alexandria where he studied and worked. Having access to the famous library with all its ancient texts and astronomical data was a gift from the Gods for Aristarchus. He could analyze ancient astronomy tables and complement them with his own observations. Sadly, none of his original works survived. We have to assume they burnt in 275 AD when Alexandria was fought over between the Romans and the Palmyra Empire. Luckily, many other mathematicians and philosopher refer to his work, mainly to reject his arguments. Our most prominent sources are Archimedes from Syracuse and Plutarch (45 – 120 AD) who even wrote that the astronomer Seleucus of Seleucia (190 – 150 BC) mathematically proved the validity of Aristarchus’ heliocentric model. But again, Seleucus’ work was lost.


Aristarchus' Heliocentric Model of the Universe


Aristarchus is most famous for his calculation of the size and distance of Sun and Moon. He measured the angel between Sun and Moon when the Moon was half illuminated. Having measured 87 degrees, he then calculated that the sun was 20 times more distant from the Earth than the moon. The angle is actually 89.7 degrees and the distance 400 times further. His work made him reconsider the geocentric model of the universe. Being familiar with Pythagoras’ and Philolaus’ thinking, his observations made more sense with a heliocentric universe. He also used these methods to calculate the size of moon and sun. He concluded that the Sun was 19 times larger than the Moon.

Aristarchus' Method to Calculate the Distance of Sun and Moon


One of Aristarchus contemporaries was Eratosthenes from Cyrene (270 – 194 BC), who accurately measured the circumference of the earth, the diameter of the sun and the distance to the sun to the earth. Without precise instruments, his results were not perfect. Eratosthenes calculated the sun to be 27 times bigger than Earth. The correct result is 109.

Could not find out whether these two mathematicians knew of each other but the combination of their findings may have toppled the geocentric model already in antiquity.


But we had to wait for Copernicus, Tycho Brahe and Keppler who established the laws of planetary motions which put the sun at the center. Aristarchus is today recognized as the astronomer who discovered the heliocentric nature of our solar system. That he achieved this with relatively primitive observation tools makes sheer logic of his conclusions even more impressive. Whilst his observation lacked precision, his approach and methods did not and still stand the test of time.


The Monument in Samos is for Pythagoras ...


Surprisingly, we find little evidence of Aristarchus on Samos. He is overshadowed by Pythagoras. But since the 17th century, a crater on the moon is named in his honor. It is not possible to look at Samos from everywhere in the world. But we can see Aristarchus’ crater every night on the moon.


... But Aristarchus has a Crater on the Moon, visible for all of us





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