### A tribute to Ancient Greek astronomers

## Monday, December 20, 2010

Looking through this book (Portuguese) on astronomy and astrophysics the other day I particularly enjoyed the section in the beginning on the astronomers of the ancient world, and thought a reminder here on their achievements would be worth writing, mostly a translation of the page or two in the book there but with a number of changes and additions. It's pretty amazing what one can do with just the naked eye and a lot of careful careful record keeping.

Interestingly, ancient astronomy is also quite a bit more fascinating than recent astronomy a few decades old. Astronomy and space exploration from the 1970s and 1980s was interesting at the time and we owe much of what we know of the universe thanks to that, but from a technological standpoint it's really just a less advanced version of the techniques we use now. Somewhat smaller telescopes, less impressive probes, simple landers instead of rovers, etc. On the other hand a lot of the techniques used in the ancient world have been forgotten and their rediscovery can be both helpful and inspirational, kind of like how Leonardo da Vinci's flying machine drawings are more interesting and unique now than a Boeing 707 (the Boeing 787, however, is very exciting). This post on the astrolabe is a good example of that.

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Thales statue at Union Station in Washington, D.C. |

**Thales**of Miletus (~624 - 546 B.C.) introduced to Greece the fundamentals of geometry and astronomy, brought from Egypt. He saw Earth as a flat disc in a vast expanse of water. Along with his disciple

**Anaximander**(~610 - 546 B.C.), also from Miletus, they were the first to propose celestial models based on the movement of celestial bodies and not as godly manifestations. Anaximander discovered the obliquity of the ecliptic, namely the inclination of the Earth's equator in relation to the annual apparent trajectury of the Sun in the sky.

Possible rendering of Anaximander's world map:

Pythagoras with bells! |

**Pythagoras**of Samos (~572 - 497 B.C.) thought the Earth, the Moon and other celestial objects to be spheres. He believed that the planets, Sun and Moon were transported by separate speheres that carried the stars. He emphasized the importance of mathematics in the description of cosmological models that could be compared with the observed movements of celestial bodies, in a regularity of "cosmic harmony". The Pythagoreans (his followers) were the first to call the universe the "cosmos", a word indicating a rational, symetrical and beautiful order.

**Philolaus**of Crete (~470 - 390 B.C.) introduced the idea of the movement of the Earth: he imagined that the Earth turned along its own axis and, along with the Sun, Moon and the planets, revolved around a "central fire" that would be the centre of the universe and the source of all light and energy.

**Eudoxius**of Cnidus (408 - 344 B.C.) was the first to propose the duration of the year as 365 days and 6 hours. He explained the observed movements of the Sun, Moon and planets through a complex and ingenious system of 27 concentric spheres that moved at different velocities around the Earth, fixed in the centre.

**Aristotle**of Stageira (384 - 322 B.C.) gathered and systematized the astronomical knowledge of his time, seeking rational explanations for all natural phenomena. He explained that the phases of the Moon depended on how much of the face of the Moon illuminated by the Sun was facing the Earth at any given time. He also explained eclipses: that an eclipse of the Sun happens when the Moon passes between the Earth and the Sun; an eclipse of the Moon happens when the Moon enters into the Earth's shadow. Aristotle argued for the sphericity of the Earth, given that the shadow of the Earth on the Moon during a lunar eclipse is always round. He believed in a geocentric model of the universe partially due to his observation of the stars: since the stars did not move amongst each other (at least, not to the naked eye) they must be fixed, unlike planets which wandered to and fro. To him the universe was composed of a number of concentric spheres that rotated around the Earth, and the stars were fixed on one of them. Had Aristotle lived long enough to observe the motion of stars amongst each other (that is, a lifespan thousands of years long instead of decades) he certainly would have come to a different conclusion about their fixed nature.

**Aristarchus**of Samos (310 - 230 B.C.) was the first to propose a heliocentric (Sun-centred) model consistent with the Solar System, predating Copernicus by almost 2000 years. He arranged the planets in the order of distance from the Sun, in the same order we have them arranged today. He developed a method to determine the relative distances of the Sun, Moon and the Earth, and measured the relative size of the Earth, Sun and Moon. He calculated that the Sun was about 30 times larger than the Moon (the actual diameter of the Sun is 400 times greater), concluding that the Sun could not be in orbit around the Earth because a body that large could not orbit a body as small as the Earth.

**Eratosthenes**of Cyrene (276 - 194 B.C.), librarian and director of the Library of Alexandria from 240 to 194 B.C., was the first to measure the diameter of the Earth. He noted that in the Egyptian city of Syene (now Aswan), on the first day of spring at noon the sunlight would reach the base of a large pit, meaning that the Sun was perpendicular to the Earth at that location. In Alexandria to the north though this did not occur. Measuring the shadow with a vertical rod, Erastothenes observed that in Alexandria at the same day and hour, the Sun was about seven degrees further to the south. The rest of this is best explained by quoting Wikipedia:

Eratosthenes calculated the circumference of the earth without leaving Egypt...He also knew, from measurement, that in his hometown of Alexandria, the angle of elevation of the sun would be 1/50 of a full circle (7°12') south of the zenith at the same time. Assuming that Alexandria was due north of Syene he concluded that the meridian arc distance from Alexandria to Syene must be 1/50 of the total circumference of the earth. His estimated distance between the cities was 5000 stadia (about 800 km) by estimating the time that he had taken to travel from Syene to Alexandria by camel. He rounded the result to a final value of 700 stadia per degree, which implies a circumference of 252,000 stadia. The exact size of the stadion he used is frequently argued. The common Attic stadion was about 185 m, which would imply a circumference of 46,620 km, i.e. 16.3% too large. However, if we assume that Eratosthenes used the "Egyptian stadion" of about 157.5 m, his measurement turns out to be 39,690 km, an error of less than 1%.

**Hipparchus**of Nicaea (190 - 120 B.C.), considered to be the greatest astronomer of the pre-Christian era, constructed an observatory on the isle of Rhodes, where he carried out observations from 160 to 127 B.C. From this he compiled a catalogue showing the position and magnitude of 850 stars. Magnitude, which specifies the brightness of a star (and any other object), was divided into six categories - category 1 being the brightest, and 6 the hardest to see with the naked eye. Hipparchus correctly deduced the direction of the celestial poles, and even the precession of the poles, a process taking 26000 years by which the rotational axis of the Earth wobbles about like that on a spinning top, eventually reaching the original point. To find out this precession, he compared the position of various stars with those catalogued by Timocharis of Alexandria and Aristillus of Alexandria some 150 years earlier (around 283 to 260 B.C.). These were members of the Alexandrian School in the 3rd century B.C., the first to measure the distance of the stars from fixed points in the sky (ecliptic coordinates). Hipparchus also calculated the Moon was a distance of 59 radii from the Earth, which was only off by a single radius (the actual number is 60), and the duration of a year with a margin of error of just 6 minutes.

**Ptolemy**(85 A.D. - 165 A.D.) was the last major astronomer of the ancient world. A Roman citizen, it is unknown whether he was Greek or Egyptian. He compiled a series of thirteen volumes on astronomy known as the Almagest, the greatest source of information on astronomy in Greece. Ptolemy's greatest contribution was a geometric representation of the Solar System with circles, epicycles and equants, which allowed one to predict the movement of the planets with considerable precision, and was used until the Renaissance in the 16th century. Despite the destruction of the Library of Alexandria, a copy of the Almagest was found in Persia (Iran) in 765 A.D. and translated into Arabic. The Spanish Gerard de Cremona (1114 - 1187 A.D.) translated a copy of the Almagest into Latin left behind by the Arabs in Toledo, Spain.