29:52 Characteristics and Origins of the Solar System

            February 1, 2004                                                                                         

.           Addendum to Lecture 4

 

Note to students in EXW section:  Check the material on the CD which you should have received by now.  Introduction clip #3 talks about the orbit of the Moon.  Demonstration clip # 6 discusses what ellipses are.  Also,  Demonstration clip #3 illustrates precession. 

 

For all  students:  Get on WebCT for this course and look under “Course Content”.  Lecture 3 gives the introductory video clip about the orbit of the Moon; the corresponding demonstration discusses precession.  The video clip under Lecture 6 shows what kind of figure an ellipse is. 

 

Orbit of the Moon

 

A picture is worth a thousand words, and below is a diagram of the Moon’s orbit around the Earth.

 

 

Note here how the phase of the Moon (crescent, half, full, etc) depends on where it is in its orbit, relative to the Sun. 

 

The synchronous rotation  of the Moon means that we always see the same hemisphere. 

 

Kepler’s Laws of Planetary Motion

 

Let’s see some pictures to go along with the laws. 

 

  1. Kepler’s First Law,  Planets move on elliptical orbits, with the Sun at one focus.  The properties of ellipses are shown in Figure 4.13,  illustrating the main properties of major axis, semimajor axis, and eccentricity. 

 

 

For how to draw these figures, see Figure 4.14, or the video clips on the WebCT part of the course. 

 

  1. Kepler’s 2nd Law, the equal area law.  This says that as a planet moves around the Sun,  a line from the Sun to the planet sweeps out equal area in equal amounts of time.  It is illustrated in Figure 4.15. 

 

 

 

  1. Kepler’s 3rd Law is the harmonic, law, relating the semimajor axis to the orbital period. 

 

Kepler’s Laws, in somewhat modified form,  also describe the motion of stars in binary star systems, and the motion of the Sun about the Milky Way galaxy.