Expert: Tom Whiting - 1/20/2007

Question
When looking at the moon, sometimes it appears to be lit from slightly to the rear, and sometimes from slightly to the front. The angle from which the moon is lit (relative to a plane which touches the earth at the point of observation), appears to show that the moon is lit from 'below' or lit from above this 'plane'. I have observed the moon looking as though it is lit from above, and from the rear in the morning. If it is being lit from 'above', how can this correlate with the fact that the sun is not 'above' the horizon (for my observation point)i.e. not yet up?

Answer
Hi Ian,
The moon's orbit is tilted 5 degrees to the ecliptic plane, so at times ....for half of it's monthly orbit....it's above the ecliptic (the plane of the earth-sun line) and half the time it's below it.  This is why it looks the way it does....illuminated sometimes from slightly below, and then sometimes slightly above...the plane.

{And this is also why there is not an eclipse of the moon at
every full moon (most of the times the moon passes either above our Earth's shadow, or below it)......and why there is no eclipse of the sun at every new moon.  Most of the time
the new moon is passing either above, or below the sun.}
Thus, we only can get an eclipse when the moon is either
passing up through  (ascending node) or down through
(decending node) the ecliptic...AT roughly the SAME TIME
the moon is either full or new, phases.  eg..next eclipse for USA is a total of the moon coming up on March 3, 2007.
(And another Aug 28 and still another on Feb 20, 2008.)

Notice....neither OUR location on the earth....or our earthly horizons, OR the position of the sun relative to our Earthly horizon.....have nothing to do with how the moon is illuminated by the sun.  Every observer, everywhere on the
earth....sees roughly the "same"illuminated  moon....but not at the same time of course.  For about half a day, the moon is above any individual observer's horizon....anywhere....and for about half the day, it isn't (above your horizon)....just like the sun, or any equatorially placed star.  (But of course, this does
not apply to the extreme polar regions...but that's another
story.)

Hope all this helps,
Clear Skies,
Tom Whiting
Erie, PA