Phases of the Moon

Several Harvard graduates had the misconception that the phases of the moon are caused by the Earth's shadow. If this is what you may have thought let me give you some information that will allow you to say proudly, "I am smarter than a Harvard graduate!"

When the Earth's shadow is cast upon the moon it is called a Lunar Eclipse. In order for a lunar eclipse to occur the Earth, Sun and Moon must line up. The Sun, Moon and Earth have to line up in the same plane. Because the Moon's orbit is inclined 5 degrees to the Earth's, it spends most of its time below or above the plane of the Earth. Lunar eclipses do not occur that frequently. Lunar eclipses come in clusters. There can be two or three during a period of a year or a year and a half, followed by a lull of two or three years before another round begins. The phases of the moon are pretty regular and we get the length of our months as a result of this cycle. So we can see that many more moon cycles occur than do eclipses.

The phases of the moon are a result of the direction of the sun in relation to the view we (on the earth) have of the moon.

Consider an artist (Earth) sketching a ball (moon). If the light source (sun) is directly behind the artist (Earth), the entire surface of the ball (moon) will be drawn in light tones.




If the light source (sun) is to the right of the ball (moon) the right half of the ball (moon) will be shaded in light tones and the left half of the ball (moon) will be shaded in darker tones.


If the light (sun) is slightly to the right and behind the ball (moon) only a very small crescent of the ball (moon) will appear light and the rest will be shaded dark.
If the light (sun) is directly behind the ball (moon) the surface facing the artist (Earth) will be dark.

The same applies if the sun is to the left of the moon.

It takes the moon about thirty days to orbit the Earth (i.e. one month). As the moon orbits the earth the sun shines on the moon from different angles. The sun shines on the surface of the moon that faces it. On the earth we only see that portion shine that is in our direct view. Sometimes that is the entire lit surface (Full Moon) and sometimes only part of the lit surface (Half, crescent new, etc.)

I hope that clarifies your understanding of the phases of the moon.