May 20, 2008
Farside 50 - Almost
||D'Alembert & Slipher
||Van de Graaff
||Fabry & Catena Sumner
||Orientale Ash Ring
||Joliot & Catena Dziewulski
||Schrödinger & Valley
Recently I was asked by one of the folks who work with the Japan Aerospace Exploration Agency (JAXA) Lunar 100, a listing of 100 interesting observing objects on the nearside. The Lunar 100 has been used to select targets for the marvelous HDTV camera. A number of objects came to mind immediately for the farside list: Orientale Basin, Giordano Bruno brilliantly-rayed crater, impact-melt splashing King crater, Icarus with the too high central peak and others. I sat down with the Bussey and Spudis The Clementine Atlas of the Moon and Byrne's The Far Side of the Moon and spent a couple of pleasant hours selecting 48 candidates. Actually, 47, for Shackelton, the polar crater perhaps with ice on its floor, is at 89.9°, not quite on the farside. (Of course, coordinates at the poles may may be off by more than 0.1° so I am leaving it in the list for now.) I need two (or three) additional features so as to not end up with the Farside 48! Can anyone suggest some candidates that are relatively large and unusually interesting?
Yesterday's LPOD: Edge World
Tomorrow's LPOD: Apennine Snows
(1) Chuck - I don't know about "unknown" (#21) and the remaining two slots (#49 & 50), but I think you can safely remove the asterisk from Shackleton. It's pretty clear that it's not properly located in the IAU Planetary Gazetteer database. The rim diameter is between 20.5 and 21.1 km, and my best guess for the position of its center is at 89.69°S/130.8°E (based on the nine or so clear Clementine photos that form the basis of the current 2005 Unified Lunar Control Network for this area). That puts the center 9.6 km from the pole and 6.3 km onto the farside. I wouldn't trust the "ULCN 2005" very far, but Lunar Orbiter frames registered to the older 1994 ULCN at lower latitudes place Shackleton at about the same position, as (I believe) do the Earth-based radar maps; so I don't think the ULCN 2005 position could be that far off (at lower latitudes most positions seem reliable to 1 km or so). The mean limb (the border between the near and far sides) is thought to run pretty much through the center of de Gerlache, and the bulk of Shackleton clearly lies on the farside of that line (even though the incorrect Planetary Gazetteer "definition" -- 89.9°S/0°E would place its center on the nearside).
P.S.: what are the "Eternity Mountains"?
-- Jim Mosher
(2) Jim--I'm curious. My understanding was that Shackleton is the location for the Moon's southern axis, which is located inside the crater but not dead center. Is this correct?
(3) Bill – Yes, the Moon has a very definite spin axis about which it revolves every 29 days or so; and although Shackleton is just a random impact unrelated to that, it happens, by pure chance, to fall nearby.
Most lunar coordinate systems (such as the ULCN 2005) attempt to determine latitudes relative to the spin axis (a very few use the very slightly different so-called "principal axes of inertia"). So, assuming those coordinate determinations are accurate, if you find the point where the latitude goes to -90°, you've found the south pole. The images on the South Pole page give a couple of reasonable guesses as to where the spin axis penetrates the lunar surface features. They are individual Clementine frames registered to the ULCN 2005 control points that fall on them. The intersection of the meridians of longitude (which should be at -90° latitude, and therefore at the spin axis) appears to fall right at the transition from light to dark, which places it on the rim crest of Shackleton. These frames were chosen because they seem typical, but control points falling in other Clementine images place the pole up to about 0.03° of latitude, or 1 km, away from this point -- a relatively small distance compared to the 21-km diameter of Shackleton. However, the entire ULCN 2005 coordinate system could easily be off by another 1-2 km or even more.
So in answer to your question, the exact location of the spin axis is unknown. It is thought to lie (by chance) within a kilometer or so of the Shackleton rimcrest. It could easily be a few kilometers inside or outside that. It's probably not safe to assert the spin axis is inside the rim; but, as you say, it's extremely unlikely to be dead center in Shackleton.
Should you be especially curious, there's a similar page for the Moon's North Pole, and also one for the Mean Earth Point (which serves as the origin of longitude for most lunar coordinate systems).
P.S.: the USGS's newly-released Warped Clementine Basemap, which is supposed to shift that particular mosaic of Clementine images into accordance with the ULCN 2005, places the Moon's south pole about 1 km inside Shackleton's rimcrest (i.e., slightly into the shadowed bowl, but still far from the center). It's my feeling that the individual frames used to create the ULCN 2005 (as shown on the South Pole page) are more likely to express its intentions than the less perfect and somewhat arbitrary shifted mosaic, but again the entire system could easily be in error by this amount or more.
(4) I'm still a newbie at this, but I would like to make a nomination for your list, Chuck. My nomenclature is surely out of date, but there is a feature that seems interesting to me at about 130W and 30N. It is unlabeled on my old globe, but between Bobone and Kovalevskaya. the floor shape and height on the north side seem unusual, but yet again, this is simply an old globe, so newer data may rule it out or explain it, anyway.
(5) Jeff, I'm at page 105 (LAC 52) of my Clementine Atlas (Bussey/ Spudis), and I see here a half-a-crater between Kovalevskaya and Bobone, which is Kovalevskaya Q (the northeastern part of Kovalevskaya Q is gone; overlapped by Kovalevskaya itself).
-- Danny C.
(6) Chuck, another interesting target could be the swirl-field near Lobachevsky, and Lobachevsky itself (the rare dark streak on Lobachevsky's inner slope, which was discovered by the orbiting CMP of Apollo 16)(Ken Mattingly). Lobachevsky's rare dark streak is also depicted as Figure 110 in the AOTM's Chapter 5, Craters (part 2 of 6).
-- Danny C.
(7) Jim--Thanks very much for the information about Shackleton and the spin axis, etc. I can use this in my PowerPoint presentations about the Moon.
(8) Jim - I added links to the two mystery features you asked about - F21 and F32 (F for farside) - see note 3 for F32.
Jeff and Danny - Thanks for your recommendations. I don't find much of interest at 130W, 30N - maybe on a Lunar Orbiter view..
Danny - the Lobachesky dark streak is very Mars-like isn't it. I considered it but thought it was too small for a R object - b ut maybe not!
Thanks for these suggestions - I await more!
(9) Chuck, another target... the swirl-field at and near Gerasimovich.