hi everyone,
a subtle apparent "nodding" of the Moon on it's axis (actually due to the Earth-Moon geometry during the month) gives rise to an effect called libration.
it leads to lunar features near the limb being favourably placed for viewing on occasion (see Keith's note on Mare Orientale earlier in the forum threads).
the effect can be noticed by the nak'd eye observer by keeping a close watch on the positioning of Mare Crisium, a circular sea marking the right eye of the "Man in the Moon". The effect of libration can sometimes make the Sea appear well in from the edge while on other occasions it is an elongated dark streak close to the limb.
December 18th this month is a good time to see Mare Crisium shifted far from the Moon's edge due to a favourable libration. Check the First Quarter Moon again in January and the Sea won't be as well placed -- a nice discovery not requiring binoculars or telescopes to see.
more on this in the Nov/Dec Arcturus magazine (download in the FILES section and the ARCTURUS subdirectory)
[
www.irishastronomy.org/user_resources/fi...63243-arc_nov_04.pdf
~Al]
all the best,
John
  