PHYS 1040 (Anger)
Spring 2009
Homework 2
Chapter 4:
1. In what direction does a planet move relative to the stars when it is in
direct motion? When it is in retrograde motion? How do these compare
with the direction in which we see the Sun move relative to the stars?
Direct motion of a planet is eastward relative to the stars, while
retrograde motion is westward. The Sun moves eastward relative to
the stars.
2. What is the significance of Occam’s razor as a tool for analyzing theories?
Occam’s razor is a guide that suggests that the simplest explanation
of all the facts is the most likely one.
3. Is it ever possible to see Mercury at midnight? Explain your answer.
To be visible at midnight, a planet must be at least 90° from the Sun
as measured along the ecliptic. This is not possible for Mercury or
Venus.
4. List and explain Kepler’s three laws.
Kepler’s first law: The orbit of a planet about the Sun is an ellipse
with the Sun at one focus.
2nd law: A line joining a planet and the Sun sweeps out equal areas
in equal intervals of time. This is also called the law of equal areas.
3rd law: The square of the sidereal period of a planet is directly
proportional to the cube of the semimajor axis of the orbit. This law
becomes the equality
P
2=a
3
when
P
is expressed in years and
a
is
expressed in AU.
5. A line joining the Sun and an asteroid is found to sweep out an area of 6.3
AU2 during 2010. How much area is swept out during 2011? Over a
period of five years?
According to Kepler’s second law, equal areas are swept out in
equal times. In 2011 (as in 2010) 6.3 AU2 will be swept out. In five
years, the area would be 5x6.3 AU2=31.5 AU2.
