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Sun always illuminates one half of Moon. However, as Moon orbits Earth, we can only see a portion of the illuminated side of Moon. The portion that we see depends on the location of Moon relative to Sun.
View this APOD (Astronomy Picture of the Day) by Antonio Cidadao to see an animated sequence of images of Moon during one lunar month, or lunation. A copy of the picture is shown below. But be sure to follow the link to the web site so that you can read more about it.
Study the simulation shown below. (Written by Mario Belloni and Todd Timberlake.) There is a lot of information in this simulation that will help you understand why we see phases of Moon.
You can perhaps understand this better if you demonstrate it, as described in the lab activity below.
In this lab, you will view the phases of Moon using a ball and lamp.
You should know the names of various phases of Moon (see textbook) and be able to identify the phase of Moon based on the orientation of Earth, Sun, and Moon. Also, based on the orientation of Sun, Earth, and Moon at any point, you should know approximately what time it is if Moon were highest in the sky (i.e. you are facing Moon in the animation).
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