Well not exactly descending on Neptune but on Tuesday December 6th the first quarter Moon will be crossing the plane of the ecliptic moving south. This is known as the descending node, one of two intersections the Moon’s orbital path (dark green line) has with the ecliptic. Several hours after the node crossing the Moon will pass within less than 0.5o from Neptune – that separation depending on your longitude/time zone.
(The Moon and Neptune in the graphic have exaggerated sizes.)
On the day of the node crossing the 7-day old first quarter Moon rises around mid-day local time and is visible most of the night, setting a couple of hours after midnight.
Click here to go to the Qué tal in the Current Skies web site for monthly observing information, or here to go to bobs-spaces.