Ancient Greek Astronomy and Astronomers
Ancient people all over the world were fascinated by stars. There were astronomers in ancient India, Mesopotamia, Egypt, China, and elsewhere. It is possible that we know more about astronomy in ancient Greeks than other places because we have more of their historic records. Thus we discuss their achievements here. Ancient Greek astronomers relied on observation and mathematical calculation to determine the operation of the universe and Earth’s place within it. Below is a brief description of four well-known Greek astronomers and a link to two books on this topic.
(1) Eudoxus of Cnidus (408-355 BCE)
Eudoxus was born in Cnidus (on Resadiye peninsula, on the Black Sea, now in Turkey). He studied mathematics with Archytus, who was a follower of Pythagorean school of mathematics (Pythagoras’ theorem is studied in many high school classes today). Eudoxus had built an observatory in Cnidus and from there he observed the star Canopus. He developed a theory to explain the retrograde motion of planets – retrograde was a puzzle to ancient people because planets sometimes seem to move backward among the stars.
(2) Aristarchus of Samos (310 - 230 BCE)
Aristarchus developed the heliocentrism theory of astronomy, i.e., the earth orbit around the sun. This is more than a thousand years before Nicolaus Copernicus. He also used geometry to estimate distance and dimension of celestial objects and concluded that:
(i) The Sun was about 19 times as distant from the Earth as the Moon, and also has a diameter about 19 times that of the moon;
(ii) The sun was at a distance 180 earth diameters from the earth.
Below is a 10th century CE Greek copy of Aristarchus' calculations of the relative sizes of the Sun, Moon and the Earth:
These estimates were badly wrong. However the fault in Aristarchus's estimates did not lie in his geometric reasoning, which was entirely correct, but in his complete lack of accurate instruments.
(3) Eratosthenes of Cyrene (276 - 194 BCE)
Eratosthenes was born in Cyrene in North Africa (now Shahhat, Libya), which belonged to the Ptolemaic Kingdom (ruled by the Greeks) at that time. Through long period of observations, he compiled a star catalog containing 675 stars.
His most famous contribution to astronomy was his measurement of the circumference of the earth. His initial idea was based on his observation that the sun is directly overhead at noon in Cyrene in southern Egypt (now Aswan, the site of the famous dam) on the first day of summer. Postulating that the earth was a sphere, he correctly reasoned that if he could determine the altitude of the noon sun at some other location on the first day of summer, and if he knew the distance between these two locations, he could compute the circumference of the earth. The other location he selected was Alexandria. The angle of the shadow cast at Alexandria was 7 degrees off the vertical. Eratosthenes reasoned that since a full circle is 360 degrees, the distance between Cyrene and Alexandria is a fraction (7/360) of the circumference of the earth. The distance from Alexandria to Cyrene was known very accurately. With these numbers, the circumference of the earth could then be calculated. His answer was accurate to roughly 1 per cent of the actual number - a remarkable achievement.
Below is a diagram of his theory:
Eratosthenes also measured the distance of earth to the sun, giving the result as roughly 120,000,000 km, and the moon's distance as roughly 120,000 km (the correct values are 149,000,000 km and 400,000 km, respectively). He computed these distances using data obtained during lunar eclipses. Eratosthenes also measured the tilt of the Earth's axis with great accuracy, obtaining the value 230 51’ 15" (the correct answer is roughly 230 30’).
The amazing accuracy of these results, obtained using naked eye observations, tells us that Eratosthenes must have been a superb observer with very accurate equipment at his disposal. Unfortunately we have little idea of what instruments he had. Below is a 17th century painting of Eratosthenes teaching in Alexandria:
(4) Hipparchus of Rhodes (190 - 120 BCE)
Little is known of Hipparchus's life. He was was born in Nicaea in Bithynia (now this town is called Iznik, on the eastern shore of Lake Iznik in north-western Turkey). His best-known discovery is known today as the “precession of the equinoxes.” This is a celestial event that has a cycle of about 26,000 years, much longer than that of recorded human history. To make the discovery, his had to do calculations using his own years of observations together with old data recorded by the Babylonians.
Below are two books:
(a) The Copernicus of Antiquity (Aristarchus of Samos)
Interesting notes from the book:
(i) On page 54: Eratosthenes had estimated the diameter of the earth to be about 7,850 miles, which is very close to the current estimate of about 7,917.5 miles.
(ii) On page 55: Hipparchus had estimated the distance of the moon from the earth to be about 67 times the radius of the earth. The current estimate is about 60. However, his estimate of the distance of earth to the sun was 2,490 times the radius of the earth, which is about 10 times off.
(iii) On page 56: Posidonius had estimated the distance of earth to the sun as 13,090 times the radius of the earth, which is about half as the current estimate of 23,250 times the radius of the earth.
(iv) Also on page 56: Copernicus, born about 1,500 years after the Greeks, had estimated the distance of earth to the sun as 1,500 times the radius of the earth, which was less accurate than the Greeks.
(b) History of the Planetary Systems from Thales to Kepler (file size: about 25 MB)
The first half of this book contains the history of ancient Greek astronomy. The author, John Louis Emil Dreyer (1852 – 1926) was a Danish/British astronomer. He won the Gold Medal of the Royal Astronomical Society in 1916 and served as the society's president from 1923 until 1925. Dreyer compiled the New General Catalogue of Nebulae and Clusters of Stars. A crater on the far side of the Moon is named after him.