Moon’s Phases
Rotation is the movement of an object in a circular motion. Earth rotates, or spins,
around a line called an axis. If the axis of rotation is within the body, the body is said to
rotate upon itself, or spin.
axis
Revolution, or orbit, is the cyclical path taken by one object around another object. The
moon revolves, or orbits, around the Earth.
The moon’s revolution around the Earth takes 27.3 days, or a little less than a month.
The revolution time of 27 days explains why we have about one full moon a month.
Because the moon’s orbit is less than 30 days, sometimes there are two full moons in one
month. This is called a Blue Moon. A Blue Moon occurs about every 2.7 years.
Contrary to what many believe the moon does rotate. It just rotates very slowly. It
rotates at the exact same rate as it revolves around the earth which is why we always see
the same side of the moon. This means the moon rotation is the same as its revolution,
27.3 days. At one point in its history the moon rotated much faster but it was slowed
down by the earth gravity.
The moon itself does not have self illumination or “glow”. The moon appears illuminated
because of sunlight being reflected off of the moon’s surface. One half of the moon is
always facing the sun. This half, facing the sun, ALWAYS reflects the sun’s light and
appears illuminated. The half being illuminated just is not always the half of the moon
facing the earth.
The phases of the moon are caused by the relative positions of the sun and the moon in
relation to the earth. The new moon occurs when the moon is at its closest distance to the
sun. At this time the moon is passing between the sun and the earth. During a new moon
the side of the moon facing away from earth is the side being illuminated, so we cannot
see it. The full moon occurs when the moon is at its farthest distance from the sun.
During a full moon the side of the moon being illuminated by the sun is the side of the
moon facing earth, so the entire moon appears illuminated.
A common misconception is that the earth’s shadow on the moon causes the phases of
the moon. If that were true then you would not be able to see both the sun and the moon
at the same time, like in the morning and evening. Also, have you ever seen all of the
moon during a new moon or a quarter moon? All of the face of the moon is always
facing earth, but the half of the moon being illuminated changes. We cannot always see
the part of the moon that is illuminated.
(Image taken from http://www.astronomynotes.com/nakedeye/s13.htm#A3.2 reproduced
for educational purposes only. Copyright Nick Strobel's Astronomy Notes )
To see an animation showing the movement of the moon go to;
http://www.astro.wisc.edu/~dolan/java/MoonPhase.html
Also see:
http://aa.usno.navy.mil/faq/docs/moon_phases.html