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The Moon is a familiar sight in the night sky. Did you know it can be seen during the day? Do you know all the names for the phases, or what they mean? Can you describe the motion of the Moon throughout its cycle? After doing this activity you will be able to do so.

At right are depicted eight different phases of the Moon. The images, as read from left to right and top to bottom, show the cycle of the phases of the Moon from one New Moon to the next. For this activity you will work in the classroom and the computer lab to meet the objectives. You will use classroom materials, the NAAP Lunar Phase Simulator (http://astro.unl.edu/naap/lps/lps.html), and SkyView Café (http://www.skyviewcafe.com).

Motion of the Moon

The Moon is a familiar sight in our sky. It orbits the Earth once every 27.3 days. Interestingly, the rotation period of the Moon is also 27.3 days.

Phases of the Moon

The Moon changes its appearance from our point of view on a monthly cycle. The differing appearance of the Moon is described in terms of the phase. The phase of the Moon describes how much of the face of the Moon is lit up from our point of view here on Earth. The phases of the Moon have names that you will need to learn to keep straight. They are as shown in the table below:

New	Waxing Crescent	First Quarter	Waxing Gibbous	Full	Waning Gibbous	Third Quarter	Waning Crescent	New

In the illustrations above the gray portions represent parts of the Moon that are in shadow. The white portions represent the parts of the Moon that are lit by the Sun. The New Moon is the traditional beginning of the cycle and is the moment when the Moon and Sun are lined up with the Earth. There is no illumination of the Moon visible to us on Earth at New Moon. The Moon waxes from New to Full; that is, the lit part of the Moon gets bigger and bigger. The next phase describes a whole series of days in which the Moon appears as a larger-and-larger crescent: Waxing Crescent. A crescent Moon is one in which less than 50% of it is lit. Next, the Moon reaches the First Quarter when it is 90° from the New Moon’s position and exactly one-half of the Moon’s face is lit up. Between the First Quarter and the Full Moon the phase is called Waxing Gibbous. It is waxing because the lit part is still growing and it is gibbous because gibbous means rounded. A gibbous Moon is one in which more than 50% is lit up. At the Full Moon the whole face of the Moon (100%) is illuminated and the Moon is 180° from its position at New Moon. After the Full Moon the Moon goes through its waning phases. To wane means to decrease. First, there is a Waning Gibbous phase which is the mirror image of the Waxing Gibbous phase. Next there is the Third Quarter in which the Moon is again exactly half illuminated but now on the left instead of the right. Finally, the Moon is a Waning Crescent that gets smaller and smaller until the Moon is New again.

The cycle of the phases of the Moon is a bit longer than the period of the Moon’s orbit of the Earth. It lasts about 29.5 days. This happens because as the Earth orbits the Sun the Sun is changing its apparent position in the sky. The Sun moves about 360°/12 or 30° through the sky per month. The Moon has to catch up this distance to become a New Moon again.

1. In the diagram below fill in the shadow on the Moon at each of the eight positions. You are looking at the Earth-Moon system from a point above the Earth’s North Pole. Do not shade each Moon-position according to what we see from Earth. Remember, the only thing that determines what part of the Moon is lit is the Sun!

2. In the following diagrams draw the Moon in the correct position relative to the Sun, starting with number 1 from the diagram in question one in this packet. Shade the shadowed part of the Moon. Then draw a line between the center of the Earth and the center of the Moon. Next, draw a line perpendicular to the line between the Earth and the Moon that also passes through the center of the Moon. Use these diagrams to figure out what the Moon looks like from the point of view of the Earth and shade the circle provided appropriately. Finally, give the phase you drew a name. Number one has been done for you.

3. What does the word crescent mean with regard to the Moon?
4. What does the word gibbous mean?
5. What does the word waxing mean?
6. What does the word waning mean?

7. If the angle of the Moon to the Sun is 0° at the New Moon then what is the angle of the Moon at the First Quarter?
8. What is the angle of the Moon to the Sun when the Moon is Full?
9. What is the angle of the Moon to the Sun when the Moon is at Third Quarter?
10. What is the range of angles that correspond to the Waxing Crescent phases?
11. What is the range of angles that correspond to the Waxing Gibbous phases?
12. What is the range of angles that correspond to the Waning Gibbous phases?
13. What is the range of angles that correspond to the Waning Crescent phases?
14. How long does it take for the Moon to orbit the Earth?
15. How long does it take for the Moon to come around to New Moon again?
16. How long does it take the Moon to rotate once on its axis?
17. How much of the Moon is lit by the Sun at all times?