The goal of this activity is to model data with a trigonometric function in a context that is not directly about circular motion. The periodic behavior of the amount of the moon visible is governed by the moon’s orbit around Earth and Earth’s orbit around the sun. These orbits can be modeled by ellipses rather than circles, though they are both pretty close to being circular. In this activity, students analyze the data from the Warm-up, making predictions about the amplitude, midline, period, and horizontal translation for a trigonometric model (MP4).

Next students check how well their model fits the actual data and use technology to modify the parameters in their model as needed. Finally, students use their models to make predictions about the moon for later dates in 2018. This prediction work is a very important part of the task, not only because making predictions is one of the main reasons for creating models but also because this can bring out deficiencies that are otherwise not apparent. For example, a 30-day period fits the January data well, but it predicts a full moon on December 28, 2018 when only 62% of the moon is visible. A 29-day period does not fit the January data as well and predicts a full moon on December 16, which is too soon. A 29.5 day period turns out to fit the overall data better than 29 or 30 days, even though it does not seem to fit the January data as well as 30. To the nearest hundredth, the orbit of the moon takes 29.53 days.

For reference as students work on the problems, the next two full moons in 2018 in the mountain time zone (the source of the data) are March 2, 2018 and March 31, 2018. There are a variety of online sites where more data about the moon on specific dates can be retrieved in order for students to check their birthday predictions.

If students do not have access to Desmos, they can experiment with the different parameters in their trigonometric model using other graphing technology. Some students may also use technology to propose a trigonometric model for the data.

If students struggle with the horizontal translation in their function that models the amount of the moon that is visible, consider saying:

• “Tell me more about the amplitude, midline, and period for your model.”

• “Try writing and graphing your model without the horizontal translation. How does the maximum of your model compare to where you want the maximum to occur?"

Highlight these features of the moon data:

• It ranges from 0 (none of the moon visible) to 1 (full moon). This means that the midline is 0.5 since it is the average of the minimum and maximum values. This also gives an amplitude of 0.5.
• The amount of time between full moons (the period) is about 4 weeks. There are 29 days between the two full moons but there are several days when the moon is “almost full” so it is hard to tell what the exact period is.

Ask students how they used the midline, amplitude, and estimated period in their equation. For the equation , the added constant is the midline while the coefficient of 0.5 is the amplitude. This means that the graph will be horizontally centered at 0.5 (half of the moon visible) with a maximum of (all of the moon visible) and a minimum of (none of the moon visible). The denominator of 29 in the expression   gives the function a period of 29. The term is a horizontal translation to the right by 1 so that the maximum (full moon) is predicted for January 2, one day after January 1.

Discuss how well the models worked for predicting the fraction of the moon visible for dates further out in the year. The models should work well for January and the next couple months but the further out you go, the period begins to be off (for a choice of 29 or 30 days). This makes sense because a small error in the period becomes more pronounced with each cycle through. Ask students what they might do to better estimate the period of the moon’s orbit around Earth. Looking at the table shows that the time between full moons consistently alternates between 29 and 30 days, making 29.5 days an appropriate estimate for the period.