Expert: Tom Whiting - 11/30/2007

Question
1.DOES THE MOON ORBIT DIRECTLY OVER THE EARTH'S EQUATOR OR IS THE PLANE OF THE ORBIT TILTED?

2.IS THE MOON CONSIDERED TO BE ROTATING 1 REVOLUTION PER ORBIT, OR LIKE A TRAIN ON A CIRCULAR TRACK WITH ONE SIDE ALWAYS FACING THE CENTER?

Answer
Hi David,
No, the moon's orbit is not directly over the Earth's equator
and neither is any other body in the plane of the Solar System
(called the ecliptic, and that's what COUNTS  here)....
because the Earth's axis itself is tilted 23.5 degrees to that plane, so we, the Earth, are not even aligned with the plane of the Solar System.  Now the moon's orbit is tilted 5 more degrees from the ecliptic plane too, so the total tilt of the moon from our equator can be as much as 23.5 + 5 or 23.5 -5 degrees.
So we don't use the Earth's equator as the 'standard'...
we use the plane of the ecliptic, or plane of the entire Solar
System as our reference plane.  (It's defined as the plane passing through the center of the Earth and the center of
the sun.)
That 5 degree tilt of the moon's orbit from the ecliptic  is why we don't get a lunar eclipse at each full moon or a solar eclipse at each new moon.  Most of the time the moon passes either over or under our Earth's shadow at full moon, and/or above or below the disk of the sun, at new moon.

2.  The moon's orbit, like every other body, is an ellipse, in
fact the moon can get, at times as close as 221,000 miles
from the Earth (perigee) or as far as 252,000 miles at apogee.
Yes, the moon is locked in synchronous rotation, that is
it rotates once on it's axis per every one revolution around
the Earth.  But due to the elliptical orbit, sometimes the rotation gets ahead of the revolution, and sometimes behind the revolution...just a little bit.  Thus the moon exhibits what astronomers call librations and perterbations, that is,  it tilts forward a bit, and falls behind a bit in its rotation cycle, THUS over a period of a month, we actually get to see a little bit around the edges....we actually get to see 59% of the lunar surface,
(not at the same time, of course) so there is only 41% of the lunar surface that we can never see from the surface of the Earth.
So no, it's not like a train on a circular track either, because
the lunar orbit is not circular, it's elliptical and as I said, the rotation gets ahead of or behind, the revolution around the Earth, as explained above.

BTW, be careful of your terminology in astronomy...
rotation is ALWAYS spin on an axis, whereas revolution is
always the orbit of one body around another body.  NEVER get them mixed up when discussing astronomy, or even worse, NEVER interchange the two terms, using one for the other!  In astronomy, they are, and have to be, very specific definitions.
Hope all this helps,
Clear Skies,
Tom Whiting
Erie, PA