Phases of the Moon: Try the animation, then read below.

 
Animation Controls:

 

Use the buttons above to control the animation.


This animation shows the Moon (gray) and the Earth (green) moving around the Sun (yellow) for a period of one year. The entire animation takes about five minutes but you can learn about the phases of the moon by watching the first minute or so.

The Moon and the Earth are drawn unrealistically large and the distances from the Sun to the Earth and Moon are drawn unrealistically small to make the shaded portions of the Earth and Moon large enough to see while allowing the animation to appear in one picture. The lighter green color shows the portion of the Earth that is experiencing daytime. The darker green color shows nighttime. The same is true of the shades of gray on the moon.

The north pole is shown as a black dot on the surface of the Earth. In reality, the north pole would not point "straight up" in a direction perpendicular to the plane in which the Earth rotates. The north pole is actually tipped. This will be shown in a later animation and for the time being we will pretend that it points in a direction perpendicular to the rotation around the sun.

The motion of a city, indicated as a red dot on the surface of the Earth, shows the rotation of the Earth and the passing of the days. The rotation of the Moon is not shown. The "Time" in the upper left shows the number of "days" that have passed since the start of the animation. 

Notice the positions of the dark sides of the Earth and Moon. The dark side of the Earth is the shadow of the Earth on itself. Sunlight falls on the light side of the Earth, and so it doesn't reach the dark side. In the same way, the dark side of the Moon is the shadow of the Moon on itself. 

The shadows always face away from the Sun. The shaded part of the moon indicates the region of the Moon that is in darkness. From the Earth we can only see the part of the moon that is brightly lit (facing the sun). The dark side of the moon is almost invisible in our sky. If we could see the moon from a faraway star, as we see it in this animation, half would always be lit and half would always be dark. Although half of the moon is always bright and the other half is always dark, we cannot always see the same amount of the bright side of the moon from our vantage point here on Earth. This is what causes the moon to sometimes appear as a semicircle and sometimes as a crescent. We are only seeing the part of the moon that is brightly lit, and only that portion of the brightly lit part that is visible from Earth.

Stop the animation at different times and think about how the moon would appear to an observer on the Earth.  It is often useful to recreate the arrangements of Sun, Earth, and Moon on the screen with objects around you. You can play the part of the Earth. A friend or the computer screen can be the Sun (you can use a bright light as the Sun, but only if it is a couple of meters away and at the same level as your "Earth" and "Moon"). A ball can play the part of the Moon. Ideally, you should try doing this with some friends and take turns playing the part of the Earth.

Click here to move on to the next animation.


Credits:
Web Page by Keith Clay
Animation by Keith Clay