Weather 101: What causes moon phases? - Tucson News Now

Moon phases are one of the most fun things for students to observe and understand.

Most of the misconception when discussing the moon-phase process is around one myth: the earth is casting a shadow on the moon, thus changing its appearance or "phase" throughout the month.  NOT!  That's the tricky part!

It's not about a shadow cast on the moon by the earth.  Instead, think about it this way: we are seeing the sun illuminating the moon.  The illuminated part is what changes.

Let's take the phases one-by-one.

In the following image, the cone shape shows our field of view from the earth to the face of the moon.

In the "new" moon phase, we are looking at the side of the moon that is NOT being illuminated by the sun.  See the moon in the image below?  The yellow arrows show the sun's rays illuminating the side of the moon that we DON'T see.  So, during the "new" moon, we see a shadowy, dark disk in the night sky.  The reason it isn't completely invisible to us is simply that the sun's light is reflecting off of the earth back to the moon. 

Here is an example of a "new" moon as it looks in the night sky.

Now, let's follow the moon as it revolves around the earth. Now, we are seeing the "crescent" moon. See the image below? It shows us that there is now a sliver of illuminated moon showing.  Just a sliver...  All but that sliver is NOT illuminated, meaning it's missing.

Here is what a "crescent" moon looks like in our night sky.

As the moon continues around the earth, as in the image below, 1/2 of the disc of the moon is now illuminated from our perspective (remember to look for our perspective by following the cone in the image below.)  I know this part may not make sense...  When the moon is at this position, its phase is called "quarter" moon.  It's a bit strange because 1/2 of the moon is actually illuminated from our perspective.  But, since it is now one-quarter (1/4) of the way through its journey around the earth, it's called the "quarter" moon.  Also, since this is the first of two "quarter" moons on the journey around the earth, it's often called the "first quarter" moon.

Here is an example of a "first quarter" moon in our night sky.

Now, in the image below, the moon is in a position where MOST of the moon from our perspective is illuminated.  It is missing only a small sliver.  This is called the "gibbous" moon. Gibbous is defined by Merriam-Webster as "seen with more than half but not all of the apparent disk illuminated."  That definition works well! Since this moon is increasing it's illuminated area nightly, it's called a "waxing gibbous" moon, where "waxing" means increasing.

Here is an example of the "waxing gibbous" moon in our night sky.

Now, the most exciting of moon phases happens...  In the image below, we are seeing the moon's disc completely illuminated.  This is the "full" moon.

Here is an example of a "full" moon in our night sky.

Following the moon around further, as in the image below, some of the "full" moon is now missing.  This is again a "gibbous" moon, or a moon not quite full but more than half full. Since the illuminated area of the moon is now decreasing each night, it's often referred to as a "waning gibbous" moon.

Here is an example of a "waning gibbous" moon in our night sky.

The next phase of the moon is the "quarter" moon again. This moon, as was discussed above, has 1/2 illuminated and 1/2 dark.  Since this is the second time we've had a quarter moon in its trek around the earth, this moon is often referred to as the "second quarter" moon. Remember, the name is referring to its position along its journey around the earth, NOT to how much is illuminated.  1/2 illuminated, "quarter" moon...

Here is an example of the "second quarter" moon in our night sky.

The final phase of the moon is another "crescent" moon. Only a tiny sliver of the moon is illuminated, while most of it is "missing".  See the image below.

Here is an example of  the second "crescent" moon of the month in our night sky.

For "eclipses", see the story in Weather 101.

Do you have ideas for Weather 101, or are you using it in the classroom?  We'd love to hear from you!