The Motion and Phases of the Moon
ONLINE PHYSICAL SCIENCE II Name: .
The Motion and Phases of the Moon Lab Activity
” The Motion and Phases of the Moon”
Equipment
For this activity you will need:
- A smartphone with a compass application (or an actual compass)
- Tape and a Pencil
- A digital camera (a smartphone camera will work too)
Learning Outcomes
After this activity you will be able to identify:
- the phases of the moon.
- the relationship between the moon’s phase and its apparent distance from the sun in the sky.
- the relationship between the moon’s phase and its location in its orbit.
Introduction
The moon is the Earth’s only natural satellite, and one of only two celestial objects in our sky that presents a visible disk that can be seen with the naked eye. The moon does not generate visible light of its own – like Earth it reflects light from the sun. As a result, one half of the moon is always in daylight, and the other half is dark (night).
The moon’s “phase” references what fraction of the moon daylit (illuminated) side is visible from Earth, and depends on where the moon is in its orbit. When the moon is in the half of its orbit that is over the day side of the Earth, we see more of the moon’s night side than its day side (which we call a crescent moon – or the new moon if the moon is almost directly between us and the sun). In the half of its orbit that is over the night side of the Earth, we see more of the moon’s day side than its night side (which we call the gibbous moon – or the full moon if the moon is almost directly opposite the sun).
The Motion and Moon’s Phases
At one quarter of the way around its orbit (measured from the position of the new moon), it appears half lit from Earth and is called the first quarter moon. It also appears half lit when three quarters of the way around its orbit; this is called the third quarter (or sometimes last quarter) moon.
You will also notice the terms waxing and waning used to describe the moon’s phases. If the moon’s illuminated side is growing from day to day it is waxing. If the illuminated side is shrinking it is waning.
In this activity you will track the moon’s phase cycle, gathering data on how the phase relates to how far the moon is from the sun in the sky, and where the moon is in its orbit.
(This picture shows the relationship between the moon’s phase and its location in its orbit.)
Phases of the Moon
Getting Started
Before you can begin gathering your data, you will need to make sure you’ve gathered all of your supplies, done a few “startup” activities (to get your equipment ready), and practiced how you will take your measurements. A few notes:
- For this activity you will need to start on the day of the new moon; there is usually one new moon per month, so you’ll only have a limited number of possible starting dates over the semester. If you own a calendar that shows moon phases, you can use that to figure out when you’d like to start the activity. Otherwise, you can do a web search for web sites that list the dates for moon phases over the semester. Some examples are or .
- Be aware that this project requires you to take data at sunset every day for the next two weeks, so be sure you select a start date that will work for your schedule. (You might find picking a start date during Daylight Saving Time, if possible, works best since it is more likely you’ll be at home at sunset.) The measurements you make will take about 5-10 minutes per day.
Motion and Phases of the Moon
Each evening, you’ll record the heading to the setting sun, the moon, and you’ll sketch (or photograph) the moon’s illumination (and determine its phase). You will take your heading measurement with the compass application on your smart phone.
Take your smart phone (or whatever portable device your compass application is on), and tape a pencil (or something else that will act as a pointer) to the back of the device so that it points in whatever direction the smart phone is pointing. The idea is you want to be able to point the pencil and have the compass heading be whatever direction the pencil is pointing. Obviously you’re not expect to go around all day with a pencil taped to your phone – you can “set up” your pointer shortly before each time you need to make your measurement.
The Motion and Phases of the Moon How to Make Measurements (Procedures for the Activity)
For each measurement, you will point the compass at the sun, and record the compass heading, and then point it at the moon and record the compass heading (You’ll record both in Table 1). You will then subtract the moon heading from the sun’s heading. We will call this value the moon’s elongation (which is, basically, how far it is from the sun).
In Table 2 you will then make a sketch of the moon’s illumination, and based on your sketch, determine the phase the moon is in. (You do this simply by looking and the moon and sketching what you see.) Alternately, you can take a photograph of the moon, and enlarge and crop it, and insert it into the table (you may need to use a significant amount of digital zoom to make this work).
Finally for a few key dates (tagged with the word orbit in bold in both Tables 1 and 2) you will mark the moon’s general location in its orbit (Figure 1) on that date (be sure to write in the date next to wherever you sketch the moon along its orbit).
Data Collection and Analysis
Don’t forget to answer any questions in this section in addition to filling out the tables! To fill in your answers, click on the text box provided and type your answers in (or follow the instructions given in the question). To fill in cells in a table, click on the cell and type in the information. Save frequently! You will e-mail your completed activity to your instructor, along with any attachments specified by the activity. (Note that the table containing the sketches of the moon phases has special instructions.)
Table 1: The Moon’s Elongation
On the first day (or two) of this activity, the moon may be too close to the sun to see. If this occurs, write “moon not visible” under Moon’s Heading, and write the Elongation as “close to 0°.”
On the last day or two of this activity, the moon may not actually be up yet at sunset. If this occurs, write “moon not visible” under Moon’s Heading, and write the Elongation as “greater than 180°.”
| Date | Sun’s Heading | Moon’s Heading | Moon’s Elongation |
| Orbit | |||
| Orbit | |||
| Orbit | |||
| Orbit | |||
| Orbit |
Table 2: The Moon’s Phase
You’ll want to fill in the following table from left to right (so complete each row before moving down to the next row. Write in the date, sketch in the moon’s phase (carefully! Circles are provided for you), and write the name of the phase below the sketch of the moon. Once the table is filled out, take a picture of it. You will e-mail the picture of the filled out table to your instructor as an attachment to this activity. (The table will be left blank in the electronic copy of this file that you submit to your instructor when the activity is finished.)
| Date: . Orbit
| Date: . Orbit
| Date: . Orbit
| Date: . Orbit
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| Date: . Orbit
| Date: . Orbit
| Date: . Orbit
| Date: . Orbit
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| Date: . Orbit
| Date: . Orbit
| Date: . Orbit
| Date: . Orbit
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| Date: . Orbit
| Date: . Orbit
| Date: . Orbit
| Date: . Orbit
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Figure 1: The Moon’s Orbit
Following is a line sketch of the Earth, the moon’s orbit, and the Sun. On the moon’s orbit, mark the moon’s location on each of the dates tagged with the word Orbit. You will do this by dragging one of the gray circles lined up above the drawing of the orbit to the correct location on the orbit. Be sure to also label each mark with the corresponding date – do so by dragging the text boxes provided below next to the corresponding gray circle and entering in the date.
The Motion and Phases of the Moon Questions:
(Answer these with complete sentences)
- As the moon gets further from the sun in the sky, what happens to the percentage of its face that appears illuminated?
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- Calculate the average increase in the moon’s elongation each day (For example, if the elongations from table one were 5°, 21°, 35°, 53°, and 64°, then, each day the increase in the elongation was 16°, 14°, 18°, and 11°. So the average increase would be 14.8° per day). Explain all steps of your calculation (the example I gave is a good model to follow, although your numbers will be different, and you’ll have more of them).
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- Let’s assume the moon’s average elongation is how many degrees along its orbit the moon is moving per day (keep in mind that because there is 360° in a circle, that means when the moon has moved a total of 360° along its orbit, it has completed one orbit). Use your result to question #2 to calculate an estimate of how long it takes the moon to orbit the Earth once. Again, explain all steps of your calculation.
- Finally, calculate the percent error between your estimate and the moon’s true orbital period around Earth (which is 27.3 days). The formula for that will be:
Percent Error = (Your estimate) – 27.3 days × 100%
27.3 days
Percent Error = .
The Motion and Phases of the Moon