Thanks so much for all of the nice comments everyone. I really appreciate it!
Michael, yes the libration of November 2020 that you captured was also highly favorable, only slightly less so than November 2021 by about 0.5 deg combined effect of latitude and longitude for the Orientale Basin region.
I first became interested in these extreme librations by chance when I happened upon an extreme combination of southern and eastern librations in 2018 (which you can read about here). I was wondering how often this occurred, and learned many interesting things. Whereas libration in latitude corresponds to the position of the Moon relative to the orbital nodes (where the lunar orbit crosses the plane of the ecliptic), libration in longitude corresponds to the position of the Moon relative to orbital apogee and perigee. Therefore, libration in latitude cycles with the draconic month (27.21 days) whereas libration in longitude cycles with the anomalistic month (27.55d). This slight discrepancy results in the independent cycling of libration in latitude and longitude. Further complicating matters is the fact that whereas libration in latitude always cycles between +/- 6.8 deg (ignoring diurnal effects), libration in longitude varies from month to month, sometimes reaching as high as +/- 8 deg but other times as low as +/- 5 deg maximum in a given month. This variation results from a dependence on the magnitude of the apogee and perigee for the month. Maximum librations in longitude occur each month about a week after apogee and perigee, and the strength of the effect is maximized when the apogee and perigee distances are themselves are maximized or minimized, which depends upon the Earth-Moon-Sun positions. This arrangement has the interesting consequence of guaranteeing that extreme values in longitudinal librations (up to almost 8 deg) never occur near the Full Moon or the New Moon, and instead favor the Quarter phases. Note that my 2018 images favoring the eastern limb occurred at First Quarter, and the images in this thread favoring the western limb occurred at Last Quarter.
How often any given libration occurs can be calculated using LTVT (Lunar Terminator Visualization Tool). The frequency depends upon the stringency of your test. For my value of 6 year cycles I quoted at the top of the page, I used a strict value of >9.4 deg combined effect in order to determine how often the exact values (or better) would occur as compared to November 27, 2021. For the search, I used a point within the crater Hohmann Q, which I could detect albedo evidence of near the far western floor of Mare Orientale in my image. As you can see from the chart below, this region would encounter a >9.4 deg libration during exactly 12 occurrences within 50 years from 2000-2050, beginning in 2003 and occurring in isolated groups of several days and lunations separated by 6 year intervals. The exact parameters will depend upon your search criteria, but needless to say, the events are rare. However, you don't need anywhere near this magnitude of libration to see Mare Orientale, but these extreme values are required in order to see to the furthest possible extent. Although not indicated in the chart below, there is a very favorable libration that occurs later this month around December 25, 2021, and is only about 0.5deg less favorable than my images here. If you miss it, you will have to wait until June 2026 for the region to reach >8 deg combined libration again, and June 2027 to reach >9 deg, although there will be a handful of other slightly less favorable opportunities available (but not many!).
Table of libration extremes (>9.4 deg combined effect) for Mare Orientale generated with LTVT, date range 2000-2049
Edited by Tom Glenn, 02 December 2021 - 12:45 AM.
