If you watch the moon every night, you see its shape
appear to change. Does the moon really change shape? Of course not, but
its appearance from Earth certainly
changes. How does this work? The answer lies within the part of the moon that
receives sunlight, and the part of the moon that does not receive sunlight.
Letís look at a diagram of the Earth moon system to figure out how this works.
Sunlight approaches the Earth-moon system from the right on this diagram
(Click on the picture to start the program).
Your job is to determine which half of the moon is
receives sunlight, and which half of the Earth receives sunlight.
Move the mouse on each part of the diagram to arrange
the light/dark shades over each moon position, click to set the shadow on the
moon. (The program will tell you when you have them all correct).
Sunlight will shine on the side of the Earth (or moon) that faces the sun. We
call this day on the side of the Earth that faces the sun, and
night on the side of the Earth that does not face the sun.
Keep in mind that sunlight will illuminate the side of an object that faces it!
Click on the arrow in the corner to go to Part 2.
OK, so what does this have to do with moon phases?
Well, the moon "shines" by reflected sunlight. As the moon orbits Earth, sunlight "shines" on
different parts of the moon on the side of the moon that we can see.
We always see the same side of the moon. (Why? Because the moon only spins
once per lunar month on its own axis as it orbits the Earth, so we always
see the same side.)
The key to understanding moon phases is: although the sun always shines on the same side of the moon,
the moon is not always in the same place while orbiting Earth with respect to the
sun. That sounds confusing, so let's work through a little diagram so you can visually see
this and figure it out.
Click Activity 2 above to go to the next part of the activity.
The diagram still shows the shadows
that you correctly identified before, but in this diagram, you will need to determine what
the moon looks like to humans on Earth. It's all a matter of perspective.
The green dashed line divides the moon into two halves -
the half we can see from Earth, and the half we don't see.
Look at the moon when it is at position (a). It looks like half of the moon is light
and half is dark.
Find and click the picture that shows what the moon looks like in position (a).
If you pick the correct picture and information box will pop up telling
you about the phase. You can close the box and move on to the next phase.
Your job is to continue through all the phases, through
(h), to determine if you can figure out which moon phase matches with the
position on this diagram.
After you get match the picture for all of the phases, Try answering these
questions to see if you really understand the facts about the Earth-moon
This part puts everything in action and lets you observe
moon moving around the earth in it's orbit.
You can click on the moon and drag it with the mouse.
Click the "Start Simulation" button and the moon will move in it's orbit and the earth will spin on it's axis. (The
timing of the simulation is scaled accuratly, The earth makes a complete
rotation 29.5 times in the time it takes the moon to go through a complete cycle).
You can also click on the number buttons to jump to a certain
day in the moon's phase cycle.
Or click on the phase buttons 'First Quarter', 'New Moon', etc.