[Note to Teachers and Students: This Gizmotm supplements the Moon Phases Gizmo. We recommend doing the Moon Phases Gizmo first.]

Tinted orange by the setting Sun, the September Full Moon rises low in the horizon, marking the beginning of the fall harvest. One night in 14th century China, the Moon signaled something else, revolution! Organized by secret messages hidden inside traditional mooncakes, peasants rose up on the night of the Harvest Moon. Their rebellion soon led to the overthrow of the Mongolian overlords who had conquered China a century before.

Like those Chinese peasants, you have probably noticed the bright Full Moon rising as the Sun sets, or perhaps you have seen the slender Crescent Moon low in the evening sky. And while you have probably not taken this as a signal to start a revolution, you may have wondered about why the Moon appears in different places at different times.

In this Gizmotm, you will see the connections between the phase of the Moon and the timing of moonrise and moonset.

Measuring Time

In this activity, you will see how our units of time are related to the motions of the Earth and the Moon.

  1. In the Gizmo, set the Speed slider midway between Slow and Fast and click Play (play button). Observe the orbit of the Moon and the rotation of the Earth on the SIMULATION pane.
    1. Notice the position of the Sun. What fractions of the Moon and Earth are always lit up by the Sun?
    2. In what direction does the Earth rotate, clockwise or counterclockwise?
    3. In what direction does the Moon orbit the Earth?
    4. Set the Speed slider to Slow. Observe the person on Earth and the time of day. What time of day is it for each of the following positions? (Assume the Sun is at right in each diagram.)

      Four images of the Earth with a human figure.
  2. Click Reset (reset button), set the Speed slider to Fast, and click Play. Observe the Moon until it returns to its original position, then click Pause (pause button).
    1. How many days does it take for the Moon to orbit the Earth?
    2. What commonly used unit of time is also about that long?

Moonrise and Moonset

In this activity, you will find and record the times of moonrise and moonset for a variety of phases.

  1. With the simulation paused, click Show horizon on the SIMULATION pane. For an observer on Earth, the horizon is where the Earth appears to meet the sky in the distance. In the Gizmo, the horizon is shown as a white line, separating what is visible to the observer (above the line) from what cannot be seen (below the line).
    1. Click Reset. Using your mouse, click on and drag the Moon about one quarter of the way around Earth, to the very top of the SIMULATION pane. Set the Speed to Slow and click Play. Click Pause when the person on Earth is directly beneath the Moon. Where should the person look to see the Moon? Will the Moon be near the horizon or high in the sky?
    2. Click Play, then Pause when one end of the horizon line is pointing directly at the Moon. In what direction is the Moon now? (Notice the E and the W on the horizon line, indicating east and west for the observer.) Do you think the Moon is rising or setting? How can you tell?
    3. Click Play, then Pause when the person on Earth is facing away from the Moon. Can the person see the Moon at this time? Why or why not?
    4. Click Play, then Pause when the other end of the horizon line is pointing towards the Moon. In what direction is the Moon now? Is the Moon rising or setting at this time?
  2. Now you are ready to record the times of moonrise and moonset. Moonrise can be defined as the moment the Moon rises above the eastern horizon into view, and moonset as the time the Moon sinks below the western horizon out of sight.
    1. In your notes, create a table like the one below. Click Reset to bring the Moon back to the New Moon phase, and set the Speed to Slow. Click Play, then Pause when the eastern end of the horizon line is pointed towards the Moon. Record the time in your chart under "Moonrise."

      Table of Moonrise and Moonset.
    2. Click Play,and then click Pause when the Moon is directly over the person on Earth. Record the time under "Overhead."
    3. Click Play, and then click Pause when the Moon crosses the western horizon. Record the time under "Moonset."
    4. Repeat these steps to collect data for the remaining phases shown in the table. Use the VIEW OF MOON FROM EARTH pane to find the correct phases.
    5. Look at the moonrise data on your table. As the Moon orbits the Earth, does it rise earlier or later each day? Why do you think this is?
    6. Compare the times of moonrise, "overhead," and moonset for a single phase. From the time the Moon rises, how many hours does it take to be overhead? How many hours until it sets?
    7. If the Moon rises at noon, what phase are you in? If the moon sets at sunrise, what phase is it? Explain, and include a sketch if you like.
    8. You can use your table to estimate the time of moonrise for any phase of the Moon. Try it tonight! (Note: The actual times of moonrise and moonset depend on the latitude and the time of year, so the predicted values will not be exact.)
  3. The lunar cycle starts with the New Moon, in which the Moon rises at about 6:00 AM. Each subsequent day, the Moon rises at a little bit later until it is a New Moon again. As you have seen, that cycle takes about 29.5 days. You can use a little bit of math to determine how much the moonrise time changes each day. (Note: This method gives you an average value. The time difference in moonrise from one day to the next does vary depending on the time of year.)
    1. Each day of the lunar cycle, the Moon rises a little later than it did the previous day. Therefore, the 24-hour day can be divided up into 29.5 different moonrise intervals. To find the size of each interval, divide 24 by 29.5.
    2. You now have a time interval in hours. To convert to minutes, multiply your answer by 60. This is the time difference in minutes between moonrises on consecutive days.
    3. Test your answer using the Gizmo. Click Play and wait until the eastern end of the horizon line points to the Moon, then click Pause. Record the time of moonrise. Then, click Play and wait until the eastern end of the horizon line again points to the Moon the very next day. Record the time of the second moonrise. Compare your two moonrise times. Is the time difference similar to the difference you calculated in step b? What are the sources of error?