Presentation: lunar motions and cycles

Mike Myers as Dr. Evil in Austin Power: The Spy Who Shagged Me (New Line Cinema, 1999) with a model of the moon and Earth. We'll come back to this clip later at the end of this presentation.

So far we've covered the motions and cycles of the stars and sun in the sky, here we'll discuss the motions and cycles of the moon (but not eclipses, which will be covered in the next presentation).

First, let's review the boring but necessary terminology from the online reading assignment, and then review with a picto-quiz.

Make sure you can distinguish between very similar terms:

• full moon and new moon (well, this should be self-explanatory).
• waxing phases and waning phases (waxing phases get more full over time, waning phases get less full over time).
• crescent phases and gibbous phases (crescent phases are less than half full, gibbous phases are more than half full)
• first quarter and third quarter moon (both of these are exactly "half-moons," but the first quarter moon is a waxing phase that occurs one-quarter of the way through its cycle of moon phases, while the third quarter moon is a waning phase that occurs three-quarters of the way through).

Let's do a picto-quiz--you'll be shown a picture or movie clip of a certain moon phase, and then be prompted with possible responses. At that point, if you know the correct answer, shout it out--because the loudest answer is the correct answer...

Which phase is this--full, or new? (Yes, this one is mean to be easy.)

Which phase is this? (Look carefully at the logo--Twilight: New Moon is not a new moon!) Crescent, quarter or gibbous? Waxing or waning?

Crescent, quarter or gibbous? Waxing or waning?

Crescent, quarter or gibbous? First or third?

Crescent, quarter or gibbous? Waxing or waning?

Crescent, quarter or gibbous? Waxing or waning?

Now let's watch a time-lapse movie of the moon going through its phases. Note that the terminator--the border between the lit and dark sides of the moon--always sweeps across the moon from right to left. When a given phase happens, you all say "now." Synchronize! (Hat tip to Edward E. Prather et al., Center for Astronomy Education, University of Arizona for this activity.) You know that person that keeps saying "meow" instead of "now?" Don't be that person.

Second, let's get to our Earth-moon diagram model of phases.

Consider the origin of the word "month." A "moonth" is the time for the moon go through its phases, which is approximately four weeks, or...a month.

The different phases are caused by the moon revolving around Earth, and by convention the "moonth" starts with new moon in this simplified diagram (not to scale), with the sun far off to the right.

This is another synchronizing activity, so when a given phase happens, you all say "now." Synchronize!

Simultaneously, while the moon revolves around Earth, Earth is also rotating on its axis. This simplified diagram will have 12 hours of day an 12 hours of night, and the observer on Earth (looking down on the north pole) by convention will start at 12 PM (noon). We'll only consider eight key times of the day (followed by 3 PM, 6 PM (sunset), 9 PM, 12 AM (midnight), 3 AM, 6 AM (sunrise), 9 AM).

One last synchronizing activity--when a given time of day occurs, you all say "now." Keep in mind that the rotation of Earth and the revolution of the moon occur at the same time, and this is what makes the moon rise and set at different times of day, depending on its phase. We'll practice using this Earth-moon diagram on an in-class activity to determine these patterns.

Back to Dr. Evil in our opening movie clip. What are some things that you notice?