Aristarchus of Samos was an ancient Greek astronomer and mathematician. Aristarchus is best known for two things: his belief that Earth orbits (revolves) around the Sun and his work attempting to determine the sizes and distances of the Sun and Moon relative to each other. Thus, he is famous for becoming the first ancient astronomer to say that the Sun, and not the Earth, was the center of our Universe.
Background
Aristarchus of Samos was born in about the year 310 BC, probably on the Greek island of Samos, the same island Pythagoras was born on 260 years earlier. We know very little about Aristarchus’s life, but we know enough to be astounded by his science.
Education
Aristarchus studied at Athens in the Lyceum under Straton of Lampsacus, who was the head of the Peripatetic school from 288/287 to 270/269 B. C.
Career
Though Aristarchus is known to have written on problems of vision, light, and color, his primary work was in astronomy, specifically on the interrelations of the sun, moon, and earth. With respect to their relative positions he pointed out that, mathematically, one can imagine the earth rotating about the sun as easily as the sun about the earth; all that is required is a vastly increased radius of the sphere of the fixed stars and the daily rotation of the earth about its own axis rather than the rotation of the sphere of the fixed stars. Though all serious astronomers in antiquity and the Middle Ages would have realized the mathematical equivalence of the geocentric and heliocentric hypotheses (and many do refer to it), arguments from physics compelled them to accept geocentricity, as Aristarchus himself does in his sole surviving book. Only with the abandonment of Aristotelian physics could the heliocentric hypothesis attain credibility.
Following many predecessors in the 6th to 4th century (Cleostratus, Meton, Eudoxus, and Callippus), Aristarchus tried to fix a "Great Year"—a period in which integer numbers of days, solar years, and the various kinds of months would occur exactly. His Great Year of 2, 434 solar years contains 45 exeligmi, and each exeligmus contains three periods in which the period-relation holds: 223 synodic months = 239 anomalistic months = 242 draconic months. Neither the exeligmus nor its third (both Babylonian period-relations) contains an integer number of years, though the exeligmus has an integer number of days. The 45 exeligmi of Aristarchus's Great Year are based on the following period-relations: 30, 105 synodic months = 32, 265 anomalistic months = 32, 670 draconic months = 32, 539 sidereal months = 889, 020 days = 2, 434 solar years.
In his treatise On the Sizes and Distances of the Sun and Moon, using Euclid's laws of proportions, Aristarchus seeks to define the limits of the ratios of the sizes and distances of the sun, moon, and earth to each other. He uses the situation of a lunar eclipse, assuming that the diameters of the sun and moon are each 2° and the diameter of the disk of the cone of the earth's shadow at the distance of the moon is 4°; thus he uses a diameter of both sun and moon that is about four times what it should be (in another lost work he gave a more correct value of 0:30°) and ignores the variation in the distance and apparent diameter of the moon.
He arrives at the conclusions that the distance of the sun from the earth is between 18 and 20 times that of the moon from the earth, that the diameter of the sun is between 19/3 and 43/6 times the diameter of the earth and the diameter of the earth between 108/43 and 60/19 times the diameter of the moon, and that the diameter of the moon is between 1/30 and 2/45 of the distance of the moon from the earth. Though these results are not correct, their limitations are largely imposed by the state of the mathematics available to Aristarchus, though the erroneous estimate of the moon's diameter contributes. The method was more fully developed and fruitfully applied by Hipparchus a century later.
Aristarchus is often called the "Copernicus of antiquity. " In a sense this is true, though the identification need not be taken as being in praise of either man. Both realized, as did many others, that a heliocentric system is equivalent to a geocentric system as far as the observed celestial phenomena are concerned; and both were willing, as others were not, to propound this mathematical hypothesis without reference to current theories of physics, and in particular to the laws of motion. Aristarchus wrote when Aristotelian physics and Platonic cosmology were both gaining acceptance and there was no one willing, or perhaps able, to construct an adequate alternative theory embodying his cosmology.
Copernicus was followed by many who questioned and eventually, with the help of new instruments and improved observational methods, disproved Aristotelian physics. The failure of Aristarchus and the success of Copernicus had less to do with their individual merits than with the intellectual milieu in which their views were expounded. In any case, Aristarchus's attempt to measure solar and lunar distance had a far greater influence on his successors than did his heliocentric theory.
Aristarchus died in 230 BC.
Views
Ancient authorities are unanimous in attributing the heliocentric theory to Aristarchus. Archimedes, who lived shortly afterward, says that he published his views in a book or treatise in which the premises that he developed led to the conclusion that the universe is many times greater than the current conception of it. Archimedes, near the opening of The Sand-Reckoner, gives a summary statement of Aristarchus’ argument:
His hypotheses arc that the fixed stars and the sun are stationary, that the earth is borne in a circular orbit about the sun, which lies in the middle of its orbit, and that the sphere of the fixed stars, having the same center as the sun, is so great in extent that the circle on which he supposes the earth to be borne has such a proportion to the distance of the fixed stars as the center of the sphere bears to its surface.
Plutarch (ca. A.D. 100) gives a similar brief account of Aristarchus’ hypothesis, stating specifically that the earth revolves along the ecliptic and that it is at the same time rotating on its axis.
After reporting Aristarchus’ views, Archimedes criticizes him for setting up a mathematically impossible proportion, pointing out that the center of the sphere has no magnitude and therefore cannot bear any ratio to the surface of the sphere. Archimedes intrudes the observation that the “universe,” as it is commonly conceived of by astronomers, is a sphere whose radius extends from the center of the sun to the center of the earth. Accordingly, as a mathematician he imputes to the mathematician Aristarchus a proportion that he feels is implicit in his statement, namely, that the ratio that the earth bears to the universe, as it is commonly conceived, is equal to the ratio that the sphere in which the earth revolves, in Aristarchus’ scheme, bears to the sphere of the fixed stars.
The only extant work of Aristarchus is called On the Sizes and Distances of the Sun and the Moon and it contains no hint of the heliocentric model. In fact, it adheres to the geocentric view. There are several possibilities for this. It could well be that, for the purpose of the works, it makes no difference which theory is adopted and, therefore, Aristarchus decided that presenting a view in contradiction to the general consensus would have been unwise. Another option would be that he may have arrived at the heliocentric view after writing this work. Some historians who have studied this matter in detail, such as Sir Thomas Heat, believe the latter. In this work, by means of careful geometrical analysis based on the size of the Earth’s shadow on the moon during a lunar eclipse, Aristarchus concluded that the Sun must be much larger than the Earth. It is possible that the idea that tiny objects ought to orbit large ones and not the other way around, motivated his revolutionary ideas. Aristarchus also suspected that the stars we see in the night sky are actually nothing more than distant suns.
Aristarchus's interest wasn't limited to our own planet. He suspected that, beyond the solar system, the stars were similar to the Sun. This idea, along with his work on the heliocentric model putting the Earth in rotation around the Sun, held for many centuries. Eventually, the ideas of later astronomer Claudius Ptolemy — that the cosmos essentially orbits Earth (also known as geocentrism) — came into vogue, and held sway until Nicolaus Copernicus brought back the heliocentric theory in his writings centuries later.
Quotations:
Earth was spinning on its own axis, taking one day to complete one revolution. The stars and the sun don't move, and that the earth revolves about the sun and that the path of the orbit is circular.
Two equal spheres are comprehended by one and the same cylinder, and two unequal spheres by one and the same cone which has its vertex in the direction of the lesser sphere; and the straight line drawn through the centres of the spheres is at right angles to each of the circles in which the surface of the cylinder, or of the cone, touches the spheres.
The circle in the moon which divides the dark and the bright portions is least when the cone comprehending both the sun and the moon has its vertex at our eye.
The moon moves (in an orbit) lower than (that of) the sun, and, when it is halved, is distant less than a quadrant from the sun.
Personality
Quotes from others about the person
It is said that Nicolaus Copernicus credited Aristarchus in his treatise, De revolutionibus caelestibus. In it he wrote, "Philolaus believed in the mobility of the earth, and some even say that Aristarchus of Samos was of that opinion."