Astronomy 309R – Spring 2009
Important ideas and concepts will
be on the test, but minor facts, such as the years of birth and death of a specific historical
person discussed on the textbook, will not be.
Please also read the slides of Lecture 1
containing an introduction to the subject of cosmology, Lecture 2 on the early history of
cosmology, Lecture 3, on momentum and gravity, and Lecture 4, on energy, stability, and
the virial theorem.
Particularly general and important concepts are underlined.
What is cosmology?
Provide and explain a descriptive definition cosmology. What are
some of the questions that modern cosmology aspires to answer?
What do we mean by “universe”?
Our notion of what the universe is like is constrained
by our technical ability to observe, measure, and quantify physical and celestial
phenomena, and by our intellectual intuitions and preferences.
Be prepared to give
examples of what “universe” meant to thinkers and scientists in the ancient
Mediterranean and in the 16
(Hint: define and explain the
geocentric and the heliocentric pictures of the universe.)
Identify the main themes in ancient Greek (or “Hellenistic”) cosmology.
Here is an
outline of the main themes that we discussed in class:
Starting with Pythagoras, the Greeks developed the notion that simple
mathematical relations describe the universe.
Plato thought that our perceptual experience is a shadow of reality.
reality is composed of abstract “forms” and it can be reached by an intelligent
mind, but is not directly accessible to the senses.
The Greeks debated what are the basic constituents of the universe: is it made of
water, earth, fire, air, ether, or atoms?
The Greeks contemplated how the basic
constituents mixed and interacted with each other.
These were philosophical
arguments, without quantitative (i.e., scientific) content.
The Greeks discussed the origin of motion.
An example is Aristotle’s idea that
motion requires force and resistance.
The Greeks developed mathematical models for the motion and arrangement of
celestial bodies (the moon, the sun, the planets, and the stars). Examples of these
models are the spheres of Eudoxus, and the epicycles of Ptolemy.
Because of the democracy and intellectual freedom in ancient Athens, ancient
Greek philosophers were able to hold different points of view and to disagree and
debate with each other.
What were the observations that ancient astronomers hoped to explain?
origin of the apparent retrograde motion of planets. (Some planets like Mars occasionally
reverse the direction of motion on the sky, when viewed against the background of
The planet appears to make a U-turn, backtrack for a while along its
original orbit, make another U-turn, and continue where it had been moving on the sky
prior to the first U-turn.
This apparent retrograde motion is a consequence of the fact that