From the Greeks to Newton

Greek Astronomy

The earliest efforts to understand astronomical events were to make predictions and appease various gods. Little attention to an explanation of celestial events was made since such was considered too lofty for human understanding. The first attempts to explain astronomy were made by the Greeks, such as Aristotle, Aristarchus, Erasthothones, and Ptolemy. Rival solar system models of a geocentric and heliocentric nature were proposed and discarded, as well as an accurate understanding of the very shape, movement, and size of our planet. By clicking on the image of Aristotle to the left, you can get a nice look at his contributions to the early natural explanation of astronomical events. Aristotle (384-322 B.C.) was the first to propose that the earth is round. While he still maintained the view that the Earth occupies a central stage amidst the celestial objects, he did not believe the Earth to be flat. Two lines of evidence supported his claim. While witnessing a lunar eclipse, Aristotle noticed that the shadow on the moon was curved. Believing that the earth was blocking the light of the sun toward the moon, he concluded that the curved shadow represented a curved earth surface. He also cited the fact that Egyptians could not see all of the same stars as those living in Greece because they lived further south and were curved away from the most northern star groupings. While Aristotle was far more a philosopher, his ideas of a round earth still proved helpful in gaining a true picture of our world.


The Greeks were also the first to really organize a weekly calendar, giving names to the days of the week according to their views of the relative motions of the planets in the sky. To learn about the development of the days of the week, please visit the Calendar Development page in this course. If you want to know more about the "Days of Our Lives," then you will have to skip school to watch the soaps, or tape it and watch later :)

To the Greeks, philosophy and science ran hand in hand, and thus their view of the universe was a combination of the natural and perceived spiritual realms. The Greek cosmos is shown to the left, and demonstrates the idea of rotating spheres within which celestial bodies as well as the elements of earth, wind, water, and fire existed.




Eratosthenes (276-192 B.C.) applied his considerable mathematical skills to a study of Astronomy, and he was the first to set a circumference figure for the Earth. While in Syene, Egypt on a midsummer day, he noticed that the sun shone directly to the bottom of a well. During another midsummer day in Alexandria, which is practically north, the sun did not shine directly to the bottom of the well, but was off by about 7 degrees. Since this represents one fiftieth of a circle, then the distance between Syene and Alexandria should represent one fiftieth of the circumference of the earth. By using an astronomical technique in measuring angles between stars, Eratosthenes determined that the earth was 250,000 stadia around. This would make the measure about 46,000 km, which is too great, but still represents an interesting idea and intuition on his part.

Aristarchus (310-230 B.C.) who was a contemporary with Eratosthenes was the first to suggest that the earth revolved around the sun. While Aristotle and most others held to the traditional "Geocentric Model," Aristarchus went against all with his "Heliocentric Model." His mathematics were horribly off the mark, but his idea was true. Unfortunately, no one accepted his model for sun-centeredness.

Hipparchus (writing between 134 and 124 B.C.) was a brilliant Greek astronomer who made very accurate determinations of the length of the year, and he devised a simple system of classifying stars according to their brightness. His star-brightness method is still used today, where the more dim stars visible to the naked eye are 100 times more dim than the brightest stars. We will learn more about star magnitudes later. Most importantly, Hipparchus recognized the precession of the Earth ... the slow wobble while the earth rotates, that causes the north star to be different. Presently, Polaris is aligned with the northward point of the earth's axis. But since the earth slowly spins in a wobbled circle, the north pole of our axis will slowly make a circle against the stars. To Hipparchus, the celestial sphere is rotating. He also noted that this precession would cause the constellations to move slowly, the result of which is that some Zodiac constellations are not in the ecliptic, or plane of the sun's motion.

Please move next to a look at this history by clicking on Ptolemy, or return to the Introduction for this History Unit, the Syllabus, or the Home page.

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