Expert: Tom Whiting - 2/10/2009

Question
I'm being really dense about understanding some of the phases of the moon.  I get the full moon and the new moon.  I just can't understand the crescent moon or the quarter moon.  I can't picture it.  My friend says the crescent moon is shaped that way because the Earth is casting a shadow on it.  That can't be right I think.  Can you help?

Answer
Hi June,
The moon phase we see on the Earth is strictly caused by the angle in the sky between the sun, Earth, and moon. So your friend is wrong; as the Earth's shadow is ALWAYS opposite the sun in the sky, and only comes into play at full moon causing... eclipses of moon.

When the angle between the sun, moon, and Earth is zero degrees
(straight line-- Sun, Moon, Earth arrangement, then we "see" a
new moon, which we can't see. At that time the far side of the
moon is illuminated but it faces away from us.
Two weeks later, the angle becomes 180 degrees, as the moon
travels halfway around the Earth, so the straight line is
Sun, Earth, Moon, and we see a full moon rising at sunset, and
the sunlit illuminated side is now facing the Earth. (Definition
of a full moon is that the moon is opposite the sun in our sky.)
Ok, the problem you are having is that region between new and full.

So find a golf ball or tennis ball and stand about 20 feet from
a small bright light source, which represents the sun.  Your head/ eye is the Earth, and hold the tennis ball out at arm's length toward the light source. This is new moon.
Move 90 degrees to your left keeping the tennis ball straight out in front of your eyes. Notice the right hand side of the tennis ball is being illuminated by the distant light, half of the back side which you can't see, and half of the front side (facing you) which you CAN see....{this is the first quarter moon, which is due south in our sky at sunset. Notice with the sun on the western horizon, a line out to the first quarter moon makes a 90 degree angle, just like
you are duplicating with the tennis ball.}
By placing the tennis ball halfway between new and first quarter, (at about 45 degrees of the orbit) you are seeing the 3.5 day old crescent moon. Notice with the tennis ball, you are only seeing a small sliver of the 1/2 lit side, the rest of the sun's light is illuminating the back side of the tennis ball which you can't see.
(This is duplicating our western evening sky crescent moon).
So as the moon orbits the Earth once per month, we see ALL the
phases from New to first quarter to full to last quarter, and back
to new, each taking about 7.4 days from say first quarter to full,
and 7.4 days from full to last quarter, and so on.

So that's how it works month after month.  Realize there is no such
thing as a permanent DARK side of the moon, rather it's a permanent
"FAR" side and permanent Earth-facing side. All parts of the moon receive sunlight as it progresses on it's monthly (actually 29.5 days)orbit around the Earth.  I hope now you can visualize the phases
and the phase angle.  So the Earth's shadow has nothing to do with
the phase angle, as it's called.  Also realize that like the Earth,
one half of the moon is ALWAYS illuminated by the sun....it's just
that we can't always SEE the entire illuminated half side, which is
continuously changing....in fact, the only time we see the ENTIRE
illuminated side is.....full moon.

BTW, Mercury and Venus being inside our Earth's orbit, exhibit those
same exact phases, in fact Venus in a telescope right now
is coming down to a crescent phase as the phase angle between the
sun and Venus decreases over the nights.  About a month ago, we
were observing a "half" Venus, and prior to that a gibbous Venus.
(The phase of the moon between First Quarter and Full is called the
gibbous phase).
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA

FOLLOW UP:
Your welcome, and thanks for the very nice comments. In fact,
feel free to contact me directly at
bwhiting@velocity.net if you have any more basic astronomy questions.
Clear skies,
Tom Whiting
Erie, PA