* The civilization of Greece and Rome began
to decline shortlyafter the time of Ptolemy. Fortunately, the
discoveries of theancient 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.
Aphelion. Astronomical unit (AU). Eccentricity. Ellipse. Foci. Greatest elongation. Inferior planet. Kepler's laws of planetary motion. Major axis. Perihelion. Semi-major axis. Sidereal period. Stellar parallax. Superior planet
1. Which of the following was a consequence
of Tycho's failure to detect parallaxes of stars? it convinced
him that the earth was stationary
2. Which of the following is one of Kepler's Laws of Planetary
Motion planets move on elliptical orbits with the Sun at one focus
3. Suppose a planet has an elliptical orbit. The speed of the
planet is 20 kms when it is at its average distance from the sun.
Which of the following is most likely to be the planet's speed
when it is nearest the sun? 25 kms
4. A hypothetical planet orbits the sun a distance of 3 AU. What
is its orbital period? 5.2 years
5. It has been suggested that there are very remote planets in
the solar system, with periods of about ten million years. Approximately
how far from the Sun would such a planet be? 50000 astronomical
units
6. In which of the following models of the solar system is it
possible for stars to show parallaxes? Copernicus' model
7. When does retrograde motion of Jupiter occur in the heliocentric
model of the solar system? when the earth passes Jupiter
8. A shift in the direction of an object caused by a change in
the position of an observer is called parallax
9. A planet has a synodic period of 3 years. What is the sidereal
(orbital) period of the planet? 1.5 years
10. What was the significance of Galileo's discovery that Venus
shows all phases from new to full? in the geocentric model, only
some phases are possible