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

Weather 101: What causes moon phases?

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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. 

Moon Phase 1

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

New Moon

 

 

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.

Moon Phase 2

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

Waxing Crescent

 

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.

Moon Phase 3

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

First Quarter Moon

 

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.

Moon Phase 4

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

 

Waxing Gibbous Moon

 

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.

Moon Phase 5

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

Full Moon

 

 

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.

Moon Phase 6

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

 

Waning Gibbous Moon

 

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...

Moon Phase 7

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

Second Quarter Moon

 

 

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.

Moon Phase 8

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

Second Crescent Moon of the Month

 

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!

 

 

 

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