Aristarchus

Aristarchus of Samos, who lived from about 310-230 B.C. is often referred to as the Copernicus of antiquity, as he laid the foundation for much scientific examination of the heavens. According to his contemporary, Archimedes, Aristarchus was the first to propose not only a heliocentric universe, but also one larger than any of the geocentric universes proposed by his predecessors. He was a brilliant but little-known natural philosopher, a master of geometry and a devout watcher of the skies.

Aristarchus hit the truth when he had the courage, against the mystical tastes of his time, to suggest that it was the Earth that moved around the Sun. There is a reference in the writings of Plutarch not only to Aristarchus's theory, but also to the way it was received by his contemporaries who in general held the geocentric view of the universe. Accepted opinion appeared to be:

“We must suppose the Earth to remain fixed, and the planets with the whole embracing heaven to move, and we reject with abhorrence the view of those who have brought to rest the things which move, and set in motion the things which by their nature and position are unmoved, such a supposition being contrary to the hypotheses of mathematics."

As we can imagine, this did not look good for Aristarchus, and was probably one of the main reasons the heliocentric hypothesis did not re-emerge until the middle of the 15th century with the Copernican revolution, 2,000 years later.

Though some of his reasoning was out of place in his time, Aristarchus nevertheless was able to adapt to the conventions of society and use the methods of known geometry to explain other phenomena. His treatise On the Sizes and Distances of the Sun and Moon , written from a geocentric point of view, was a breakthrough in finding distances to objects in the universe, and his methods were used by later astronomers and mathematicians through the time of Hipparchus and Ptolemy.

Aristarchus introduced six hypotheses, from which he determined first the relative distances of the sun and the moon, then their relative sizes:

1) The moon receives its light from the sun.

2) The earth is positioned as a point in the centre of the sphere in which the moon moves.

3) When the moon appears to us halved, the great circle, which divides the dark and bright portions of the moon, is in the direction of our eye.

4) When the moon appears to us halved, its [angular] distance from the sun is 87 degrees.

Although his geometry was perfect, Aristarchus's methods of measurement were extremely inaccurate, as the requisite instrumentation for giving precise angular measurements did not exist. However his basic value for the angle from Sun to Earth to Moon was off by only a few degrees (the actual value is 89 degrees, 50 minutes).

5) The breadth of the earth's shadow is that of two moons.

We now know that the width of the earth's shadow cone at the moon is actually three rather than two moon diameters.

6) The moon subtends one-fifteenth part of a sign of the Zodiac. (The 360 degrees of the celestial sphere are divided into twelve signs of the Zodiac each encompassing 30 degrees, so the moon, therefore, has an angular diameter of 2 degrees.)

Although he proved many propositions (eighteen to be exact), the three most well-known are the following:

1) The distance of the sun from the earth is greater than eighteen times, but less than twenty times, the distance of the moon from the earth.

2) The diameter of the sun has the same ratio (greater than eighteen but less than twenty) to the diameter of the moon.

3) The diameter of the sun has to the diameter of the earth a ratio greater than 19 to 3, but less than 43 to 6.

In order to determine the values for the sizes of the sun and moon, Aristarchus used two observations: first, the disc of the moon normally just covers the sun during a total solar eclipse, and second, that during a lunar eclipse the shadow of the earth appears to be twice as large as the moon at the moon's distance. Despite inaccuracies he discovered that the sun is many times larger than the earth and many times further away from the moon.

Using improved values, today we can show that the sun is about 400 times farther from the earth than the moon, and that the sun’s diameter is approximately 109 times greater than that of the earth.

Aristarchus contributed a great deal to both geometry and astronomy, and his methods, as adapted by Hipparchus and others, were used well into the 17th century. In 1681 Aristarchus was recognised as one of the greatest astronomers in history, when Giovanni Riccioli named the brightest crater on the moon, in the Ocean of Storms, after him. This crater was on the NASA list of possible landing sites for Apollo, and is particularly subject to the reddish glows and obscurations known as ‘trans-lunar phenomena’ which may suggest gaseous emissions from the moon’s crust.

Overall, Aristarchus was a pioneer both in his depiction of the universe and his geometric approach to the measurement of the heavenly bodies. He was a man of perception and genius who deserves at least the recognition awarded to Ptolemy, Brahe, Keplar, Newton and Herschel.


Colin Brett