Lunar Phases - Misconceptions

We've all observed the Moon and noticed that it doesn't always look the same.  Sometimes it is in its full phase where we see all of it.  Sometimes it's in a quarter phase where we see half of it, and at other times we see more than half or less than half of the Moon.  These are referred to as the lunar phases.

 What causes the moon to appear differently?  The common misconception is shadows.  Take a waxing crescent, for example.  The misconception states that the reason we see only a sliver of the Moon is that Earth's shadow is blocking the rest.

Let me state this very clearly.  Lunar phases are NOT caused by Earth's shadow.  At no point during the regular lunar month does Earth cast its shadow onto the lunar surface.  So if phase are not caused by shadows, what is the cause?

The cause is geometry.  It's all about where the Moon is located in its orbit relative to the Earth and the Sun.  See the image below.  

The original uploader was Minesweeper at English Wikipedia. [CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/)]

Stated in another way, half of the Moon's surface is lit at all times.  This is the same as Earth.  On any given day at any given time, half of the Earth experiences day, while the other half experiences night.  Your location on Earth determines if you are experiencing day or night.  The same is true on the Moon.  Half of the Moon is experiencing day and the other half is experiencing night.  When we look at the Moon in the sky, sometimes we see the entire half of the Moon that is experiencing day and thus we see a Full Moon.  Sometimes we only see half of the half of the Moon that is day.  In this case we see the Moon has half lit or a first/third quarter.  At other times we only see a small sliver of the Moon, so we are only observing a tiny portion of the Moon that is lit.  

The length of the lunar month is about 29.5 days, or about one Earth month.  So over the course of this month you will see the month go through an entire cycle of phases from new to crescent to quarter to gibbous to full, back to gibbous, quarter, and crescent.  

If you didn't know before, now you know the truth behind lunar phases.  If you encounter someone stating that the Moon's phases are caused by shadows, you can confidently step in and correct them.  

When are shadows important?  Occasionally Earth does cast a shadow onto the Moon.  When this happens we observe a lunar eclipse (to be discussed in a future post).  Lunar eclipses are somewhat rare, so again, when you only see part of the Moon during a non-eclipse time, no shadows are involved.  

My daughter is a good spotter of the Moon.  It seems like she is always spotting it, day or night.  She's a bit young to discuss lunar phases, but she is old enough to understand what is a shadow and what isn't.  So depending on the age of your child, talk to them and ask them questions about lunar phases when they point out the Moon.  This will help prevent misconceptions from forming later in their education.