Course Icon

Chapter 3: Motions of the Moon

Course Materials are always under revision! Weblecture content may change anytime prior to two weeks before scheduled chat session for content.

SO Icon


Scholars Online Astronomy - Chapter 3: The Motions of the Moon


Reading Preparation

Reading: Astronomy, Chapter 3: Eclipses and the Motion of the Moon

Study Notes: notes on your assigned reading from the text

Key Formulae to Know

  • The Saros Cycle

    Eclipses repeat when the nodes and phases of the moon line up. The type of eclispe (annular vs full) and eclipse length will depend on the Moon's position relative to perigee or apogee.

    The synodic month (phase to same phase) is 29.53 days (the lunar month).

    The line of nodes shifts all the way around the lunar orbit in 346.6 days (the eclipse year.)

    We need a situation where X lunar months = Y eclipse years. At that time, the eclipse cycle will restart. From observation, ancient astronomers determined the Saros cycle as 223 lunar months = 19 eclipse years = 6485.3 days, or 18 years, 11 days, and 8 hours.

    If an eclipse occured on August 11, 1999 over Europe, a similar eclipse should have occured 18 years, 11 days, and 8 hours later.
    1999 + 18 = 2017
    August 11 + 11 days = August 21 (adjusted for 2000 not being a leap year)
    8 hours means the eclipse will occur 1/3 of the way around Earth to the west, i.e., over the US.

Web Lecture

Read the following weblecture before chat: Lunar Eclipses

Study Activity

These tasks should be possible on any computer-based planetarium program. You will need to review the user guide for your program to identify exactly how to do each one.

Set your planetarium program to look down on the Earth-Moon system from above the Earth's North Pole. Allow the program to run so that the Moon orbits the Earth at least once and try to stop it when the Moon is at New, First Quarter, Full, and Third Quarter phases. What time of day is it visible in first quarter? in third quarter? Which direction must you look to see it?

Now set your location for your home, and run the planetarium program through the same period. How does the moon appear to move against the background sky over the course of a month? Where does it rise (or set)? Does this point change? How does rising or setting time correspond to the phases of the Moon?

Use the NASA eclipse site (below) to identify the date and time of a solar eclipse and a lunar eclipse. Set your planetarium program to observe the Moon from two hours before he start of the eclipse until two hours after it is over. Which direction does the Moon move across the face of the sun in a solar eclipse? Which direction does it move through the Earth's shadow in a lunar eclipse?

Optional websites: Visit NASA's Eclipse site.

  • Will there be any solar eclipses during our course this year (September to June)? Where will the eclipse(s) [if any] be visible? HINT: look at the map of the moon's shadow. Which direction does the shadow move (based on the times of totality)? Will it be a total eclipse, or annular?
  • Check out the time for the total lunar eclipse on 10 December 2011 for your location. Will you be able to observe a complete eclipse? The beginning or end of the eclipse?
  • If the ecipse will not be visible from your location, check out the possibility of following the eclipse online.

Chat Preparation Activities

Chapter Quiz

Lab Work

Read through the lab for this week; bring questions to chat on any aspect of the lab, whether you intend not perform it or not. If you decide to perform the lab, be sure to submit your report by the posted due date.