**The great contribution of the Greeks to astronomy was Euclidean geometry**

National Observatory and the National Archaeological Museum of Athens contain the famous and mysterious Antikythera Mechanism.

In the first millennium BC many Greeks came to Babylon to study what the Chaldeans taught.

In addition to assimilating Babylonian science, they brought this knowledge to the West.

The Greeks reported the Babylonian sexagesimal system, which divides the circumference into 360 degrees, one hour in 60 minutes, one minute in 60 seconds.

In addition, they adopted the lunar calendar used by the Chaldeans, made up of 28-day lunar months.

To maintain agreement with the solar year, they established cycles in which some years had 12 months and other 13 lunar months.

**Leading Greek Astronomers of Antiquity**

The ancient Greeks were the first to try to understand how and why the stars moved in the sky, trying to explain natural phenomena without resorting to supernatural causes.

Anaxagoras (born 499 BC) was accused of impiety and punished for claiming that the Sun is not a god, but an incandescent rock much larger than Athens.

He also proposed that the light of the Moon is due to the reflection of sunlight that reaches the Earth. This idea allowed him to be the first to correctly explain the phases of the Moon.

**Anaximander of Miletus** (610-546 BC) was a great observer and thinker. He suggested that men lived in a cylinder curved to the north and south.

This idea was very natural when the known world extended much more in the north-south direction than in the east-west direction.

The observation of how the ships disappeared in the horizons, suggested some form of curvature in the Earth.

His most important contribution was the philosophical conception about the nature of matter, which he thought is immutable; and that all things in the Universe are formed from it.

**Hecateus of Miletus** (550 – 476 B.C) was the first known geographer. He was interested in finding out the size of the Earth, which in his time was conceived as a circular disk.

According to his calculations, this circular disk had a diameter of 8,000 km at most. This figure, in a time when the known world was extremely small, was a big gamble.

**Aristarchus of Samos** (310-230 BC) his observations and those of his predecessors led him to the conclusion that the Earth revolves around the Sun and that it rotates on its own axis.

Aristarchus’ hypotheses were that the Sun and stars are stationary; instead, the Earth is dragged in a circular path around the Sun, which would be located in the center of this orbit.

He affirmed that the sphere where the fixed stars are and which has its center in the Sun, is of a very great extent.

Aristarchus wrote a book on the dimensions and distances of the Sun and the Moon and was the first in history to propose a method for making such measurements.

To measure the size of the Moon relative to the Earth, Aristarchus followed Aristotle’s idea that the circular shadow seen on the Moon during a lunar eclipse is due to the spherical shape of the Earth. Following Aristarchus’ method, the radius of the Moon is found to be 1,738 km.

Once he determined the size of the Moon, it was very easy for him to establish that the distance from the Earth to the Moon should be 384,000 km. Truly amazing! A genius.

**Euclid the great Greek geometry**

Euclid (around 325-265 BC) is considered the greatest of the Greek mathematicians.

His 13 books on geometry are known, which are among the most influential documents in history. His invaluable contribution to astronomy is spherical geometry.

His developments in geometry were fundamental to the study of astronomy for all time.

In his work, this giant of thought, compiles much of the mathematical knowledge of his time, in the system known as “Euclid’s Postulates”, which in a simple and logical way gave rise to Euclidean Geometry of total validity until today. through the centuries.

His book “Elements” is the second most published book in history, more than 1,000 editions, second only to the Bible. One of the last, in a Spanish edition, is the one published in two volumes by Editorial Gredos, in 2007.

**Eratostenes the friend of Archimedes**

**Eratosthenes** (276 BC-194 BC) was born in Cyrene, a Greek city located in present-day Libya.

He was a great friend of Archimedes. He studied in Alexandria and, for some time, in Athens. He possessed a wide variety of knowledge and great aptitudes for study and science.

He excelled as an astronomer, historian, mathematician, geographer, and writer. He was known as the second Plato.

Eratosthenes used a gnomon, built by him, to measure the passing of the hours and also to measure the diameter of the Earth, with an extraordinary approximation to current data.

He was the first to measure the size of the Earth. He did it with astonishing accuracy: 12,800 km. Nobody believed. Actually, even he himself did not believe this result.

Ptolemy III called him to educate his children and to take charge of the Library of Alexandria, since he held it until the end of his days.

Note: The magnitudes of the size of the Earth postulated by Eratosthenes seemed so grossly exaggerated to his contemporaries that by other measurements they established that the diameter was only 9,100 km and that the circumference would be 29,000 km.

These figures were considered good until the end of the 15th century. Hence, Christopher Columbus had hoped to reach India in a not too long time.

**Hipparchus of Nicaea**

Hipparchus of Nicaea (190-120 BC) was born 4 years before the death of the great sage Eratosthenes.

In time, Hipparchus succeeded him as Director of the famous Library of Alexandria.

Among the contributions of Hipparchus to astronomical science we can highlight:

- the invention of trigonometry;
- the invention of the concepts of geographic longitude and latitude;
- the first ordered catalog of 1,080 stars classified according to the magnitude of their brightness;
- the implementation of the division of the day in 24 hours;
- the most accurate measure of the Earth-Moon distance.

**Philolaus of Tarentum**

The Greek Philolaus (born in Tarentum and also lived in the 5th century BC) was one of the main students of the Pythagorean school.

As far as we know, he was the first person to hypothesize that the Earth is spherical in shape.

According to Nicolaus Copernicus, Philolaus had also come to the conclusion that the Earth moves. He also explained correctly, that lunar eclipses are due to the passage of the Moon through the shadow of the Earth.

**Oenopides **

Oenopides (born in Chio, Greece, 450 BC) discovered that the ecliptic makes an angle of 24º with respect to the celestial equator.

He calculated the 59-year period it takes for the Sun and Moon to return to their original positions relative to Earth.

**Pythagoras**

Pythagoras was born. in Samos, near Miletus

He lived in the 5th century BC. C. he is one of the most important mathematicians of all time.

Pythagoras taught that the planets move in independent and inclined orbits with respect to the celestial equator.

**Thales of Miletus**

Among the Greeks who studied in Babylon was Thales of Miletus (born in 640 BC), who also studied in Egypt where he learned geometry.

Thales was a great observer. He stated that the Universe is governed by immutable laws that have nothing to do with gods or demons, but can be known through reason.

His line of thought was the first precedent for modern scientific reasoning and method.

He believed that the Sun and the stars were made of fire, and that the Moon has no light of its own.

Thales learned in Babylon and Egypt that lunar eclipses were the result of the interposition of the Moon between the Earth and the Sun.

Shortly before the great eclipse of 585 BC occurred. C., which was total and of a relatively long duration – 5 minutes or more – over a long extension of Asia Minor, Thales had been talking about eclipses with the merchants of Miletus.

This strengthened the reputation that he was able to predict these phenomena.

As he lived during the initial days of alphabetic writing, his writings are not known.

**Claudius Ptolemy**

Claudius Ptolemy (100 – 170 AD) is considered Greek and Egyptian because, although his birth and education were Greek, much of his life and his work as an astronomer and astrologer was developed in Egypt.

Ptolemy is the Greek astronomer with the greatest impact in the world because, based on the data obtained by his predecessors.

He built a geometric model to try to represent the movement of the stars that was unanimously accepted until the arrival of Copernicus, Kepler and Galileo.

His model of his Universe followed the teachings of Plato and Aristolles that the Earth was immobile and occupied the center of the Universe.

In this model, the Sun, Moon, planets, and stars revolved around the motionless Earth.

However, his geocentric theory clearly departed from Aristotelian physics by postulating that the orbits of the planets are eccentric and not circular and perfect as Plato and Aristotle said.

One of the main objectives of him was to be able to know the future positions of the planets and the stars, in order to make astrological predictions.

He is the author of the astronomical treatise known as Almagest, which contains a catalog of stars that Ptolemy took from a lost work of Hipparchus of Nicea (190 – 120 BC).