A Brief History of Astronomical Thought From Aristotle to Hawkings From Stonehenge to the Hubble Space Telescope

Ancient Astronomy Religion and Myth

Astronomy is the oldest of the sciences. When Stoneage humans turned to an agrarian way of life and began to settle into communities, their interest must naturally have turned to the "heavens": 1.The seasons became important; during different times of the year, different stellar patterns appear in the sky. In the spring, Virgo and her accompanying constellations signal the time to prepare the earth, to plant crops, and to be wary of floods. In the fall, Orion rises to indicate time to harvest and to prepare for winter. 2.The approximate equivalence of the human menstrual cycle and the 30 day orbital period of the Moon which produces lunar phases led to the belief that the heavens, and the Moon in particular, were related to fertility.

3.To early humans facing an uncertain and changeable future, the constancy of the heavens must have suggested perfection and certainly led to deification in many cultures. 4.We may expect that eclipses would have been especially frightening to early humans. After predicting the seasons, eclipse prediction may have been one of the earliest astronomical activities. Ancient Astronomy can be characterized as: More Religious than Scientific More Utilitarian than Explanatory or Theoretical Stonehenge, constructed between BCE on England's Salisbury Plain, may be one of the earliest monuments to the beginnings of Astronomy.

Classical Greek Astronomy The Universe is a rational place following universal, natural laws. Plato to Ptolemy

Plato B.C.E. Plato taught that there are absolute truths and mathematics is the key. Plato had Four Basic Beliefs: 1.There is certainty. 2.Mathematics gives us the power of perception. 3.Though the physical applications of mathematics may change, the thoughts themselves are eternal and are in another realm of existence. 4.Mathematics is thought and, therefore, it is eternal and can be known by anyone.

Western scientific history begins with the ancient Greek civilization about 600 BCE. Pythagoras of Samos (~ B.C.E.) Pythagoras’ developments in astronomy built upon those of Anaximander from whom, apparently, came the idea of perfect circular motion. The Pythagoreans believed that the planets were attached to crystalline spheres, one for each planet, and a separate sphere for the stars. These spheres were centered on the Earth, which was itself in motion. Pythagoras is also credited with recognizing that the "morning star" and "evening star" are both the planet Venus.

Aristotle ( B.C.E.) Aristotle was a student of Plato, founding his own school of Natural Philosophy, the Lyceum, in Athens about 335 BCE. Aristotle's philosophy involved the qualitative study of all natural phenomena, pursued without the aid of mathematics which was deemed to be too "perfect" for application on an imperfect terrestrial sphere.

In Aristotelian cosmology, the "imperfect" Earth was situated at the center of the Universe (Solar System). It was composed of the four elements: earth, air, water, and fire, each of which sought its natural place in the Universe. Aristotle adopted Pythagoras' model of concentric spheres for the planets, but deduced that the Earth must not only be the center of the Universe but must be immobile. Aristotle's Natural Philosophy was embodied in the writings of St. Thomas Aquinas and became the foundation of Church doctrine and University instruction in medieval times. Why did Aristotle believe the Earth was stationary?

At the time of Aristotle it was believed that “man” was a special creation and therefore man’s home, the Earth, must be at the center of the Universe. Since the Earth was already in its proper location there was need for the Earth to move. Aristotle also could make observations that, to him, proved the Earth did not move. Did the Earth spin causing the surface of the Earth to move? Did the Earth orbit around the Sun?

Parallax Suppose we want to measure the distance across a river. If the distance from “A” to “B” is known and the parallax angle can be estimated the distance “d” can be determined. d=? This is how with two eyes we can judge the distance to an object.

Another aspect of parallax is the apparent shift in position of an object due to change in the observing position. Object appears in front of blue background Object appears in front of green background

Aristotle perceived that if the Earth traveled around the Sun the observed position of a star should shift when viewed at different times of the year…stellar parallax.

Nearby Star 6 months later Background Stars When Aristotle attempted to observe the stellar parallax he could see none and so concluded that the Earth did not travel around the Sun.

Geocentric (Aristotelian) Model 1.The Earth is still; motionless. 2.Earth is at center of Universe. 3.Celestial bodies move in perfect circles at uniform speeds. 4.Stars were set in a rotating sphere that turned E to W once a day. 5.Planets, moon, sun also set in separate spheres that moved slower.

Aristarchus of Samos (310 BC B.C.E.) Aristarchus was both a mathematician and astronomer and he is most celebrated as the first to propose a sun-centered universe. He is also famed for his pioneering attempt to determine the sizes and distances of the sun and moon. From his measurements and calculations Aristarchus found that the Sun was many times larger than the Earth. It seemed more reasonable that a tiny Earth would orbit around a large Sun rather than a large Sun orbit around a tiny Earth. His sun-centered model failed to change the minds of the Aristotelians. The earth-centered model persisted.

Hipparchus of Rhodes ( B.C.E.) Hipparchus compiled a star catalogue containing about 850 stars. His star catalogue, probably completed in 129 BC, has been claimed to have been used by Ptolemy as the basis of his own star catalogue. The Hipparchus star catalog indicated the brightness of stars and planets and observed that the brightness of planets vary from time to time.

Claudius Ptolemy ( C.E.) Ptolemy combined the best features of the geocentric models that used epicycles with the most accurate observations of the planet positions to create a model that would last for nearly 1500 years. These refinements were incompatible with Aristotle's model and the Pythagorean paradigm---a planet on an epicycle would crash into its crystalline sphere and the motion is not truly centered on the Earth. Ptolemy’s model was seen as only an accurate calculator to predict the planet motions but the reality is Aristotle's model.

The Ptolemaic system consisted of multiple circular paths to explain such things as a planet’s varying brightness and retrograde motion.

Ptolemy published his work in a large 13-volume series called the “Almagest”, and his model and the geocentric view of the cosmos was generally accepted for the next 1500 years. Medieval Astronomy Fall of the Roman Empire to the Renaissance During the Middle Ages the Islamic civilization had flourished in the Arabic countries. They had preserved and translated the Greek writings and adopted the Greek ideals of logic and rational inquiry. Islamic astronomers were careful observers of the sky and created accurate star catalogs and tables of planet motions. Advances in the explanations of the motions of the stars and planets were made by astronomers in Europe starting in the 16th century. Aristotle's Natural Philosophy was embodied in the writings of St. Thomas Aquinas ( AD) and became the foundation of Church doctrine and University instruction in medieval times.