NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Lu Abel
Date: 2019 Dec 12, 21:46 +0000
The time of Full Moon is probably not defined by separation in hour angle (GHA or otherwise). One certainly could define the events of the Moon this way, but most astronomical events are instead defined by separation in ecliptic longitude. Full Moon occurs when the Moon and Sun are separated by 180°00' in geocentric ecliptic longitude. This occurred close to 12:12 EST (correct, as claimed by the memes). This is fundamentally an old astrological definition of an astronomical event. We also still define the seasons in astrological terms.
We could equally well define Full Moon by the time of maximum "lunar distance", that is, the instant of maximum angular separation between the Moon and Sun. This is close to, but not identical to, 180° separation in ecliptic longitude. I haven't double-checked this, but I estimate that maximum angular distance occurred around 18 minutes later, close to 12:30 EST. The center-to-center lunar distance (also known as the "elongation") was then just about 178°20.1'. Since angular separation directly determines the Moon's phase (the percentage of the Moon's face that is illuminated), this is perhaps a more "natural" definition of Full Moon.
If you want to experiment with elongations or separations in ecliptic longitude, this is possible in Stellarium, but there's a catch: Stellarium has no geocentric viewing option (at least I'm not aware of one). The position of the Moon is always adjusted by parallax (normal in astronomy software, a little inconvenient for us navigators). So if you want the coordinates and elongations right, you have to place your observation point such that the Moon is exactly at the zenith. For any UT, adjust your latitude to match the Moon's Dec, then tick the longitude up or down until the Moon is within a minute of arc of the zenith. The coordinates and also the elongation are then a close match for the geocentric values.
Notice how all of this depends critically on narrow definitions. The resulting exact times have no real astronomical or observational significance.
Frank Reed
