Learning Objective: In this activity students use the Sky Tonight software to "watch" the Moon for a whole month and observe all phases of the Moon's cycle. They measure how long it takes for the Moon to orbit the Earth and see how the Moon's phase changes.

Specific Science Content Standards

• Changes in Earth and sky (K-4)
• Earth in the solar system

Space Update: The Sky Tonight

Introducing the activity:
Ask students what the Moon looks like in the sky. Some will describe a full moon; others will describe a crescent or perhaps a half moon. Tell students that they are going to observe the Moon using the Sky Tonight software to discover how the shape of the Moon's sunlit part changes during the month. We call these changes the "phases" of the Moon. (Of course the Moon itself does not actually change shape). When the Moon is in the opposite direction from the Sun, the Moon is "full" - fully lit. At other times the Moon is a crescent (less than half is lit) or gibbous (more than half is lit). When the Moon is a narrow crescent, the students can often see the dark part of the Moon faintly lit. This is called "Earthshine", light reflected off the Earth. This proves that the Moon is still a sphere, just with different lighting conditions.

Extending the activity:
After students have determined how the Moon's phase and location changes throughout the month, it is important for them to know why the Moon's appearance changes. At this time you should do a demonstration with a flashlight and white ball to show that the phases are caused by the relationship between the Sun, Moon, and Earth. As the Moon circles the Earth, the Sun illuminates different portions of its Earth-facing surface. This is also a good time to illustrate using three children that the Moon must rotate on its axis once per month in order to always keep the same face to the Earth. Is there a "dark side of the Moon"? Is it always the same half or does the dark half change from day to day?