Lunar phases and eclipses

[lupinesque]

Barb wrote, on the main list:

> 2) This means that if the exact time that the earth stops
 obstructing the light of the sun from striking the moon--even a
 little--

Actually, as Tex said, what you are describing there is a full lunar 
eclipse, which arises from the earth blocking the sun's light from the 
moon.  This happens only rarely, when the moon is in the same plane as 
the earth and sun.  Let me explain . . . <clears throat, picks up 
chalk>

The phases of the moon are caused by how much of the side of the moon 
that we see is illuminated by the sun.  

We always see the same side of the moon regardless of where it is in 
its revolution around the earth.  Imagine the moon as the face of a 
person looking directly at you.  If the sun, moon, and earth form a 
90-degree angle, it is like that person's face being strongly 
illuminated by a light from his right side.  You will see the right 
side of his face clearly while the left side is in deep shadow--that's 
a half moon (astronomically speaking, a quarter moon--one in which we 
see half the moon).  When the person has his back to the sun, even if 
he is not blocking the sun's light, his face is cast in shadow; that's 
the new moon.

When the moon is on the far side of the earth from the sun, and is 
*not* blocked by the earth (because it is not in the same plane), we 
see a full moon.  When, as occasionally happens, the moon falls in the 
same plane as the earth and sun during this phase, we see a lunar 
eclipse.  Lunar eclipses therefore occur only during a full moon, but 
not during every full moon.

FormerEarthScienceTeacher!(andCurrentLOON!)Amy