LUNAR PHASES

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.

Let’s 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 system.

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The phases of the moon are produced by:
A) the spin of the Earth
B) varying amounts of sunlight reaching the side of the moon that faces Earth
C) varying amounts of sunlight reaching the side of the moon that faces the sun
D) the orbit of the Earth around the moon
At full moon:
A) the side of the moon facing the Earth receives no sunlight.
B) the side of the moon facing the Earth receives full sunlight.
C) the moon is between the Earth and the sun
D) none of these
At new moon:
A) the side of the moon facing the Earth receives no sunlight.
B) the side of the moon facing the Earth receives full sunlight.
C) the moon is between the Earth and the sun
D) A and C
E) none of these
At first and third quarter moon phases:
A) the moon is basically at a 90 degree angle from the sun.
B) half of the side of the moon we can see "shines"
C) A and B
D) none of these
In the first half of the lunar cycle, the "growing" part of the moon appears on the ______ side. After full moon when the moon is waning, the illuminated part of the moon appears on the ______ side.
A) right, left
B) left, right
C) right, right
D) left, left

Let’s summarize:

At new moon, the moon is nearly in line with the sun. The illuminated part of the moon faces away from Earth, so the moon appears invisible in the night sky.
A few days after new moon, the waxing crescent moon appears above the western horizon soon after sunset. Each evening it becomes larger and higher above the horizon.
About a week after new moon, the moon reaches first quarter and appears high in the southern sky at sunset. The first quarter moon does not set until about midnight, so to see it you must watch late-night talk shows and go outside after they are finished.
In the days following first quarter, the waxing gibbous moon continues to grow. It appears to move further east, and it sets later in the evening.
About two weeks after the new moon, the full moon arrives, rising in the east and setting in the west. The full moon is visible all night long, and it sets in the west around sunrise.
The waning phases of the moon are not as visible because the moon is not visible in the early evening when most people are out and about. As the moon wanes through gibbous, it rises later and later. By the time third quarter rolls around, moonrise doesn't occur until around midnight. If you want to see a waning crescent moon, you must get up early before sunrise and look for the moon in the eastern horizon.
The lunar cycle (moon phases) repeats with a period of 29.5 days. This is called a synodic period. Synodic comes from the Greek words meaning journey together, referring to the sun and moons positions with respect to each other.

Here's a question for you! Can you see the moon sometimes in the daytime? You'll never see a full moon in the daytime sky because it rises at sunset and sets and sunrise, but you can sometimes see other phases of the moon in the daytime. It looks a little pale and washed out, but it's there.

Why doesn't the moon appear as bright in the daytime as it does at night?
A) The moon is receiving less sunlight.
B) The sunlight in the atmosphere makes the moon appear less bright.
C) The moon does not reflect light in the daytime.

Here's another question for you! Let's see if your brain is really on today!
If you can see a full moon at your position on the Earth, what type of moon will you see if you go to exactly the opposite side of the world on the same day (24 hours)?
A) A new moon.
B) A full moon.
C) No moon at all.
D) None of these.
If you see two full moons in the same month of a calendar year, the second full moon is called a:
A) Moon River
B) Blue Moon
C) Moonraker

One last point: Remember that everyone on the planet will see the same phase of the moon at the same time. It is the Earth that rotates, giving us night and day.
Further thought question:
We know that tidal effect occurs primarily because of the moon orbiting the earth. The Earth's rotation is what causes the effect of two high and two low tides per day and each location. Do you think there would be a correlation between the time of moonrise and the time of high tide? Why or why not?

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.