|| Back to Lectures || A. Introduction) || B. Measurements || C. Motion of sky || D. Motion of Sun || E. Moon || F.History || G. Motion || H. Newton's laws || I. Gravity || J. Work-Energy || K. Electricity-Waves || L. Gas laws || The End ||

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Astronomy References 1) Encyclopedia 2) Glossary 3) Glossary 4) Astronomy 5) Physical Science

Illustrations(1) History of Astronomy (2) Nicolaus Copernicus (3) Retrograde motion (4) Kepler's Third law (5) Johannes Kepler (6) Kepler's three laws (7) Galileo's Pendulum Experiments (8) Galileo's Acceleration Experiments (9) History of Astronomy

Further reading in ancient history of astronomy:

* The Babylonians accumulated records of astronomical observations for many centuries. The records enabled them to see repeated patterns in the motions of the celestial objects. They used the patterns to predict the positions of the Moon and planets.

* The idea that the celestial bodies were spheres and that they moved on perfectly circular paths originated with Pythagoras and his students.

* Aristotle taught that all celestial motion must be circular and that celestial bodies were spheres. He argued that the Earth must be motionless at the center of the universe.

* Aristarchus showed that it is possible to use geometry to find the distance of the Moon and the relative distances of the Moon and Sun. By doing so he showed that the universe is enormous compared with the size of the Earth.

* The small angle equation describes the relationship of the angular diameter of an object to its linear diameter and its distance. If any two of these variables are known, the third can be found.

* Aristarchus proposed that the Sun, not the Earth, is the center of the universe, but this idea was not accepted by most other Greek astronomers.

* Eratosthenes found the difference in the altitude of the noonday Sun at Syene and Alexandria. He showed that this is the same as the difference in latitude of the two cities. This difference allowed him to find the ratio of the circumference of the Earth to the distance between Syene and Alexandria

* Hipparchus discovered Precession when he compared his measurements of stellar positions with those of earlier Greek astronomers. The celestial coordinates of stars change with time because of precession, the slow circular shifting of the celestial poles with respect to the stars.

* Greek astronomy culminated with the epicyclic system of Ptolemy. In Ptolemy's model the retrograde motion of a planet was produced by the combination of two circular motions.

* Epicycle . One of the circles upon which a planet moved according to the Ptolemaic (geocentric) model of the solar system. The center of the epicycle moved on a larger circle, called the deferent

* Deferent .One of the circles on which a planet moved according to the Ptolemaic model of the solar system

* A planet moved in a circle on an Epicycle, which itself moved on a Deferent. The system could predict accurately the positions of the planets and was in use for nearly 1500 years.

* The civilization of Greece and Rome began to decline shortly after the time of Ptolemy. Fortunately, the discoveries of the ancient astronomers were preserved by the Arabic astronomers,who found them and translated them into Arabic.

* Astronomical knowledge was gradually reacquired in Western Europe. By the fifteenth century, the level of knowledge matched or exceeded that at the time of Ptolemy.

* The geocentric model of Ptolemy was almost universally accepted as the correct description of the solar system.

* In the early sixteenth century, Copernicus proposed that the Sun rather than the Earth is the center of the solar system.

* In the heliocentric model, the daily and annual patterns of celestial motion are explained by the rotation and revolution of the Earth.

* Retrograde motion of the planets occurs whenever the Earth passes or is passed by another planet.

* In the model of Copernicus the orbital distances of the planets can be found through observations and geometry. In contrast, the geocentric model makes no specific predictions about the relative distances of the planets.

* Through his care in building and using astronomical instruments, Tycho Brahe was able to make observations of unparalleled accuracy.

* His regular observations of the Sun, Moon, and planets covered many years. His data replaced the ancient observations that earlier theorists had been using for centuries.

* Tycho was unable to detect stellar parallax and thus rejected the model of Copernicus. Tycho proposed a model in which the Earth was orbited by the Sun and Moon but all of the other planets moved about the Sun.

* Using Tycho's data, Kepler was able to discover the laws of planetary motion.

* His first law says that the planets move on elliptical paths with the Sun at one focus.

* The second law says that a planet moves so that a line drawn between the planet and the Sun sweeps out equal areas in equal amounts of time. This means that the product of speed and distance from the Sun remains constant as a planet moves about the Sun. The planet moves fastest when it is nearest the Sun.

* Kepler's third law says that the square of the sidereal period of a planet is proportional to the cube of its average distance from the Sun. The third law implies that there is a common principle that governs the orbital motions of the planets.

* Galileo's telescopic observations provided strong support for the heliocentric model. In particular, his observations that Venus shows all the phases from new to full could not be explained by Ptolemy's model of the solar system.

* Galileo summarized his arguments for the heliocentric model in his book, Dialogue Concerning the Two Chief World Systems. The book put Galileo in conflict with church authorities and resulted in his persecution.