Evolution of Cosmological Theories: Aristotle, Plato, Galileo, and Kepler, Essays (high school) of Earth science

Download Evolution of Cosmological Theories: Aristotle, Plato, Galileo, and Kepler and more Essays (high school) Earth science in PDF only on Docsity! ARISTOTLE STATIONARY EARTH Sphere of the Prime Aristotle's Universe Aristotle, who lived from 384 to 322 BC, believed the Earth was round. He thought Earth was the center of the universe and that the Sun, Moon, planets, and all the fixed stars revolved around it. Aristotle's ideas were widely accepted by the Greeks of his time. The exception, a century later, was Aristarchus, one of the earliest believers in a heliocentric or sun-centered universe. In particular, he believed in four elements: earth, air, fire, and water He believed that the earth is a sphere. It is relatively small compared to the stars, and in contrast to the celestial bodies, always at rest. For one of his proofs of this latter point, he referred to an empirically testable fact: if the earth were in motion, an observer on it would see the fixed stars as moving, just as he now observes the planets as moving, that is from a stationary earth. However, since this is not the case, the earth must be at rest. To prove that the earth is a sphere, he produced the argument that all earthly substances move towards the center, and thus would eventually have to form a sphere. He also used evidence based on observation. If the earth were not spherical, lunar eclipses would not show segments with a curved outline. Furthermore, when one travels northward or southward, one does not see the same stars at night, nor do they occupy the same positions in the sky. Aristotle believed the Earth was unique and that mankind was alone in the universe. His hypothesis behind this was that if there were more than one world and the universe had more than one object at the centre, then elements like earth would have more than one natural place to fall to. The idea is that if multiple centres existed, the planets would be unable to revolve around Earth and have trouble understanding how to move. Remember, his idea of Earth, was that it was also the centre of motion. So by these standards, it makes sense for him to believe the Earth was unique PLATO SPHERICAL EARTH In Plato’s astronomical scheme, for instance, a spherical earth lies at the center of a greater sphere of the heavens, on whose inner surface the stars are embedded like bright nails. As the outer sphere turns, the stars are carried around the earth in a daily rotation. The sun, moon, and the other planets revolve at different speeds around what is known as the ecliptic. If the earth’s equator is extended outward to the sphere of the fixed stars, it forms a plane called the celestial equator. The ecliptic is an imaginary circular plane tilted about twenty-three degrees to the celestial equator. The path of the sun along the ecliptic intersects the celestial equator at the fall and spring equinoxes, the two days each year when day and night have equal lengths. The concept of a spherical Earth displaced earlier beliefs in a flat Earth: In early Mesopotamian thought, the world was portrayed as a flat disk floating in the ocean, and this forms the premise for early Greek maps like those of Anaximander and Hecataeus of Miletus. The Earth is massive enough that the pull of gravity maintains its roughly spherical shape. Most of its deviation from spherical stems from the EXODUS CONCENTRIC SPHERES inner sphe I eae - planet Eudoxus was a Greek philosopher, astronomer, and mathematician who accepted Plato's notion of the rotation of the planets around the Earth on crystalline spheres, but noticed discrepancies with observations. He tried to adjust Plato's model by postulating that each crystalline sphere had its poles set to the next sphere. His model contained no mechanical explanation; it was simply a mathematical description but it formed the basis for the theory of concentric spheres to account for the motion of the planets. Eudoxus calculated the length of the year as 365 days and 6 hours. Unfortunately none of Eudoxus' writings have survived to the present-day, and we have only the accounts of others to go on. Aristotle, a contemporary of Eudoxus devotes a passage in his Metaphysics to the Eudoxan spheres. Eudoxus supposed that the motion of the sun or of the moon involves, in either case, three spheres, of which the first is the sphere of the fixed stars, and the second moves in the circle which runs along the middle of the zodiac, and the third in the circle which is inclined across the breadth of the zodiac; but the circle in which the moon moves is inclined at a greater angle than that in which the sun moves. And the motion of the planets involves, in each case, four spheres, and of these also the first and second are the same as the first two mentioned above (for the sphere of the fixed stars is that which moves all the other spheres, and that which is placed beneath this and has its movement in the circle which bisects the zodiac is common to all), but the poles of the third sphere of each planet are in the circle which bisects the zodiac, and the motion of the fourth sphere is in the circle which is inclined at an angle to the equator of the third sphere; and the poles of the third sphere are different for each of the other planets, but those of Venus and Mercury are the same. CLAUDIUS PTOLEMY GEOCENTRIC THEORY © 2012 Encyclopzedia Britannica, Inc. Ptolemaic system, also called geocentric system or geocentric model, mathematical model of the universe formulated by the Alexandrian astronomer and mathematician Ptolemy about 150 CE and recorded by him in his Almagest and Planetary Hypotheses. The Ptolemaic system is a geocentric cosmology; that is, it starts by assuming that Earth is stationary and at the centre of the universe. The “natural” expectation for ancient societies was that the heavenly bodies (Sun, Moon, planets, and stars) must travel in uniform motion along the most “perfect” path possible, a circle. However, the paths of the Sun, Moon, and planets as observed from Earth are not circular. Ptolemy’s model explained this “imperfection” by postulating that the apparently irregular movements were a combination of several regular circular motions seen in perspective from a stationary Earth. The principles of this model were known to earlier Greek scientists, including the mathematician Hipparchus (c. 150 BCE), but they culminated in an accurate predictive model with Ptolemy. The resulting Ptolemaic system persisted, with minor adjustments, until Earth was displaced from the centre of the universe in the 16th and 17th centuries by the Copernican system and by Kepler’s laws of planetary motion. In Ptolemy's geocentric model of the universe, the Sun, the Moon, and each planet orbit a stationary Earth. For the Greeks, heavenly bodies must move in the most perfect possible fashion—hence, in perfect circles. In order to retain such motion and still explain the erratic apparent paths of the bodies, Ptolemy shifted the centre of each body's orbit (deferent) from Earth—accounting for the body's apogee and perigee—and added a second orbital motion (epicycle) to explain retrograde motion. The equant is the point from which each body sweeps out equal angles along the deferent in equal times. The centre of the deferent is midway between the equant and Earth. NICOLAUS COPERNICUS HELIOCENTRIC THEORY Nicolaus Copernicus was an astronomer who proposed a heliocentric system, that the planets orbit around the Sun; that Earth is a planet which, besides orbiting the Sun annually, also turns once daily on its: own axis; and that very slow changes in the direction of this axis account for the precession of the equinoxes. Sometime between 1508 and 1514, Nicolaus Copernicus wrote a short astronomical treatise commonly called the Commentariolus, or “Little Commentary,” which laid the basis for his heliocentric (sun-centered) system. The work was not published in his lifetime. In the treatise, he correctly postulated the order of the known planets, including Earth, from the sun, and estimated their orbital periods relatively accurately.