Look back at the journal entries you have been making for the phases of the Moon. You should notice that the different phases are visible in the sky at different times. The reason for this is easy to understand. For example:
- A new moon occurs when the Moon is located in the same direction in the sky as the Sun. Therefore, the new moon rises and sets with the Sun.
- A full moon occurs when the Moon is opposite the Sun in our sky. Therefore, the full moon rises around sunset, reaches its highest point in our sky around midnight, and sets around dawn.
The following video explains this idea in more detail, and describes a procedure you can use to determine the rise, highest point, and set times for any phase of the Moon. Be sure to notice that these are the times you have already seen listed in Figure 2.22.
Video
Moon Phases Part 2
Determining the times of day at which we see different phases of the Moon in our sky.
Group Activity
When We See Different Phases
Follow the procedure shown in the video to determine the rise, highest point, and set times for a first-quarter moon, full moon, and third-quarter moon. For a bigger challenge, also do a waxing crescent moon and waning gibbous moon. Then answer the following questions:
- Do we ever see a full moon during the daytime?
- Suppose you go outside in the morning and you see a gibbous moon. Does this mean that the Moon will be full within the next few days, or that it was already full within the past few days? Explain how you know.
Teacher Notes. Doing this activity should help students understand how to determine the times at which we see different phases. It can be challenging for some students, but they should still be able to answer the two questions:
- The full moon is opposite the Sun in the sky, so in general we see the full moon only at night; depending on the time of year, we might sometimes see a full moon rising near the eastern horizon before the Sun is fully set (or setting near the western horizon shortly after the Sun has risen), but we will never see a full moon in the midday sky (except near the poles, where the Sun remains above the horizon most or all day long in the summer).
- A morning moon must be waning, which means we had full moon within the past few days.
Note: For students who are still having difficulty, you should at least have them memorize the fact that we see waxing phases in the evening and waning phases in the morning.
I was wondering…
If a “moonth” is 29 ½ days, why do our calendar months vary in length?
Part of the answer is the ½ day, which means that to keep a calendar lined up with the “moonthly” cycle of phases, we’d need to have some months with 29 days and some months with 30 days. This is indeed the case for what we call lunar calendars, such as the calendars used in the Islamic and Jewish religions, the traditional Chinese calendar, and many other traditional calendars. However, notice that 12 “moonths” is only about 354 days (because 12 x 29½ = 354), or about 11 days short of the 365 days (or 366 days in leap year) of our solar calendar, which is based on the time it takes Earth to orbit the Sun. This means that if you want to have 12 months in a solar year, then your average month needs to be about a day longer than the “moonth.” There are many ways this can be done, and our modern calendar in which most months have either 30 and 31 days — except for February’s 28 or 29— arose for various historical reasons. You might also notice that the fact that 12 “moonths” is shorter than a solar year means that lunar calendars will tend to get out of sync with solar calendars by about 11 days per year; that is why, for example, the start of the Islamic month of Ramadan moves up about 11 days each year. Some cultures sought to keep their lunar calendars at least partly synchronized with the Sun; that is why, for example, the Jewish calendar adds a 13th month in selected years.