PTYS/ASTR 206 – Section 1 – Spring 2009 Homework #1 (Page 1/3)
NAME:_________________________________________________________________
Due Date: start of class 1/27/2009
• 5 pts taken off if turned in late, but before 5PM of the due date
• The assignment can be turned in for half credit before the start of class on Jan 29. The
assignment will not be graded if turned in later than this (too late!)
Turn in your own work. It is not acceptable to turn in work that is identical to that of another
student. You may work together to understand the material, but the work you turn in must be in
your own words.
There are 2 parts, plus an extra-credit part. Please put your answers to all questions in the space
provided (you may use the back of the page if necessary).
PART 1 (32 pts total): Conceptual. Please provide a concise short answer (not more than a few
sentences) for each of the following.
# 1. (8 pts) (Chapters 2) Suppose that you study the position of a particular planet (Saturn, for example)
relative to the stars nearby it each night for over a year. Briefly describe what you will see.
The planet will move eastward most of the year, but will move westward during retrograde motion. It
will also get bigger, smaller, brighter and dimmer, depending on how close it is to Earth. The closer it
is, the bigger and brighter it is likely to be (except for the inner planets, but Saturn will be brighter when
it is closer).
# 2. (8 pts) (Chapter 4 #21) Why does Venus have its largest angular diameter when it is new and its
smallest angular diameter when it is full? In what way does this rule out the Earth-centered model of the
solar system?
Venus is closer to us when it is in the new phase meaning that it is between us and the Sun (so
that we cannot see the illuminated side) – at this point Venus is at its closest to Earth. When
Venus is on the opposite side of the Sun, we see its fully illuminated side, hence it is full, and it
is also at its farthest point from the Sun meaning that it is also the smallest. This rules out the
Ptolemaic, Earth-centered model, because the full phase, where Venus is on the opposite side of
the Sun, was not possible in this model.
# 3. (8 pts) (Chapter4: #29 of the textbook) Why was the discovery of Neptune an important
confirmation of Newton’s law of universal gravitation?
Its existence and position in the sky was predicted using Newton’s law of gravitation using the forces
from all of the known planets. It was then discovered rather easily, once the position had been
determined. This confirmed the Newton’s theory.
