Video pre-questions (and sample answers) from:

The Birth of Astronomy

NOTE: It will not be our usual practice to post questions from class here on the website; it is your responsibility to write them down during class (as you do for lecture notes). This set is posted to give you a feeling for the level of depth and detail that you should be able to achieve in answering them.


What is the retrograde motion?

"Retrograde motion" the the illusion that a planet is moving backwards through the constellations (east to west as opposed to the normal west to east) for a few weeks.


What is the major difference between geocentric and heliocentric models of the universe?

In a geocentric universe model, the Earth is at the center of everything.  In a heliocentric model, the Earth goes around the Sun.


How did the Ptolemaic (geocentric) model account for retrograde motion?

It did so by use of smaller circles called "epicycles" arbitrarily superimposed on the circular paths that planets followed around the Earth in this model.


How did the Copernican (heliocentric) model do so?

It did so by analysis of the effects of a changing perspective from a moving Earth: outer planets will appear to go backward for a while as the Earth catches up with them and passes them in orbit; inner planets will do so as they pass Earth in orbit.


What characteristic of a planet's orbit around the Sun does each of Kepler's Laws of Planetary Motion describe?

Orbit shape, speed at each place in orbit, and orbit period, respectively.


Which of Galileo's observations with his telescope convinced him that the Earth could not be the center of the universe?

The phases of Venus.


Why did Galileo spend the last years of his life under house arrest?

He violated the Catholic Church's order that he stop advocating against Geocentrism.