In the political cartoon Non Sequitur cartoonist Wiley Miller provides a humorous yet pessimistic view of people.  In one of his cartoons Miller portrays an astronomy professor sitting at the front of his introductory astronomy class at the beginning of the semester.  The professor asks if anyone has any questions before the course begins.  One student asks, “What is the difference between astronomy and astrology.”  The professor replies, “Lots and lots of math.”  In the final frame the classroom is empty.

This is typical of the public’s view of astronomy and astrology, that they are two names for one concept.  While this is not true today it was true in the past, especially in ancient Greece and Rome.  In the past astronomy and astrology were one art in which the motions of the planets, sun, moon, and stars were observed and humans tried to find patterns in these motions so that their influence could be understood.  But the question of how astronomy and astrology diverged into two concepts has its roots in that time.  Astronomical measurements of celestial objects were recorded alongside ancient horoscopes.  So although they were more interested in predicting the future, these astrologers laid the groundwork for modern astrophysics.

The earliest accounts of philosophical writings are scarce and incomplete; however, the early Ionian philosopher Thales of Miletus left us with some of the earliest cosmological ideas.  From his own writings, and an account from Herodotus, we know that Thales had some astronomical knowledge, most likely obtained during a stay in Egypt (Dreyer 12).  Some of his ideas that we know to be correct today include his understanding of solar eclipses.  Dreyer explains that Thales knew a solar eclipse occurred when the Moon passed between the sun and the Earth (Dreyer 13) and that he knew the Sun was larger that the Earth (Dreyer 14).  However correct Thales may have been he still did not have a complete understanding of the solar system.  For example, he believed that the moon was a self-luminous object, like the sun; however he left no explanation for the phases of the moon.

Others tried to explain astronomical phenomena by attributing it to divinity.  This was the seed of the differentiation of astronomy and astrology.  Manilius, a stoic, calls astrology divine knowledge passed down to humanity by Hermes (Luck 325).  As a stoic, Manilius believed that the gods had a direct influence on human affairs, and in his Astronomica 1.474-531 he tries to refute the Epicurean argument that the gods do not (Luck 328).  Even Ptolemy, who proposed the most scientifically accurate model of the solar system until that time said, “…in studying the convoluted orbits of the stars, my feet do not touch the earth, and seated at the table of Zeus himself, I am nurtured with celestial ambrosia” (Brecher 61).

A direct result of the divinity of the planets was the belief that they emitted “elements” and these are what affect humans (Burton 94).  These “elements” were the same elements-earth, air, fire, and water-natural philosophers believed made up everything we see.  Because of proximity and size the Sun and Moon have the greatest effect on Earth, such a large affect that it is visible in the rhythms of nature.  Ptolemy writes, “the rivers increase and diminish their streams with her light, and seas turn their own tides with her rising and setting, and plants and animals in whole or in some part wax and wane with her” (Burton 94).  The planets, which are smaller and more distant, naturally have less effect, but it is “measurable.”

This measurability was the core of predicting the future and horoscopes.  Every celestial object has an effect that, combined with the other effects from other objects, can dramatically change nature.  Manilius explains some of the details of the nativity cast at birth.  The zodiac sign a person is born under is the primary factor determining their fate.  There are also effects from the “temples” and “parts” of the sky where other objects are at birth.  Other factors include the angle which planets, the moon, and the sun make with each other (Luck 337-338).

Even in ancient times it is possible to see the schism in astrology/astronomy.  Until the Renaissance, the two were essentially one art.  Only with the advent of precise measurement tools and the creation of calculus and numerical methods was astronomy able to become a true science.

Before the Renaissance, astronomy was based entirely on plane geometry which cannot adequately describe the complex observable motions of the planets and sun in a geocentric model, like the one proposed by Ptolemy.  For example, when the position of Mars in the night sky is recorded for several months it progresses linearly across the sky as expected.  However, its motion stops and it begins moving in the opposite direction for a few days, then stops again and resumes in its initial direction.  This is called retrograde motion (Kutner 430).

Retrograde motion was one of the most perplexing phenomena for the Ancient Greeks.  One philosopher, Apollonius, introduced the concept of epicycles to account for retrograde motion (Dryer 145).  Epicycles are small circles that planets move in while progressing in the orbit around Earth.  Only after acceptance of Copernicus’s Heliocentric model of the Solar System in the sixteenth century were astronomers able to fully explain retrograde motion.

            Perhaps the greatest ancient astronomer was Claudius Ptolemæus of Alexandria.  A Roman who lived under the reign of Hadrian, his theories of the solar system were successful until the sixteenth century when Copernicus proposed his heliocentric model that fully explained phenomena like retrograde motion.

            Although their knowledge of the layout of the solar system was not correct, by today’s standards, ancient astronomers were able to obtain much empirical data about it.  Ancient astronomers understood the concept of parallax (a method for determining astronomical distances using the motion of objects).  Specifically, this was used to estimate the size of and distance to the sun and moon.  This procedure is detailed in Ptolemy’s Almagest, chapters 11 through 19 (Neugebauer 100).  In spite of the error of a geocentric model of the solar system Neugebauer says, “Ptolemy’s values for the mean distance to the moon and for the radius of the epicycle […] are of the correct order of magnitude” (Neugebauer 106).

            The question of eclipses was another problem that philosophers had to explain in their descriptions of the solar system.  The earliest explanation, given by Thales, was the most correct one.  He said that solar eclipses occurred when the moon passed between the sun and the Earth (Dreyer 13).  Xenophanes, founder of the renowned philosophical school the Eleatic, believed that the sun and moon were spheres of compressed gas and extinguished themselves every day and month, respectively.  This accounted for the phases of the moon (Dreyer 19).  Other exotic theories were created to account for astronomical phenomena, but none withstood the test of time.

            Although no theory could adequately describe eclipses, astronomers were able to predict them from naked-eye observations (Brecher 89).  Thales of Miletus, as stated earlier, was famous for his prediction of a solar eclipse (Dreyer 12).  These astronomical successes were likely an impetus for astrologers to attempt to predict other events.  If someone was able to predict an event as spectacular as a solar eclipse others may be able to predict more common events.  Thus astrology fed off of the advances of astronomy.

            Ptolemy was the last great Greek astronomer.  Shortly after his death Christianity began to have a major impact on society.  It suppressed astronomy and astrology alike and no advances were made in the Middle Ages (Dreyer 206).  The oppression of the Church was so bad that there appeared to be a regression of the knowledge obtained by the Greeks.  The Bible says in Isaiah 11:22 “it is He…that streatcheth out the heavens as a curtain and spreadeth them out as a tent to dwell in” implying that the Earth is flat and the heavens are held above like a tent (Dreyer 211).  This was taken as written by the church and was the basis for the claim that the earth was flat.  It was not until the Renaissance that inquiry into the nature of the heavens resumed.  The discoveries of Copernicus, Kepler, and Newton took the study of astronomy and made it into a science.

            Mankind has always had an interest in nature and the mechanics of why things are the way they are.  Astronomy began as an inquiry into the motion of the moon, sun, planets, and stars. The application of mathematics gave astronomers the tools necessary to describe and extrapolate the motion of celestial objects.  Soon it was synonymous with astrology as people tried to predict the future from what they could observe in the heavens.  The early years of astronomical research were replete with bizarre theories with little proof.  And although there were many outrageous ideas we would ridicule today, some natural philosophers were able to deduce relatively accurate descriptions of the universe.  In ancient Greece most astronomers knew that the Earth was spherical and that the sun and stars were not born anew every day and night.

Bibliography

Brecher, Kenneth, and Michael Feirtag, eds. Astronomy of the Ancients. Cambridge: The MIT Press, 1979.

Burton, Dan and David Grandy. Magic, Mystery, and Science. Bloomington: Indiana University Press, 2004.

Dreyer, J.L. E. A History of Astronomy. 2^nd ed. Cambridge: Cambridge University Press, 1953.

Kutner, Mark L. Astronomy: A Physical Perspective. 2^nd ed. New York City: Cambridge University Press, 2003.

Luck, Georg. Arcana Mundi. Baltimore: Johns Hopkins University Press, 1985.

Miller, Wiley. Non Sequitur. Cartoon. 01 Apr. 2004. <http://www.rpi.edu/dept/phys/Courses/Astro_F96/astroMath.gif >.

Neugebauer, O. A History of Ancient Mathematical Astronomy. New York City: Springer-Verlag, 1975.