Satellite feature(glossary entry; aka Lettered crater)
One of 18 different categories of lunar features recognized in the current system of IAU nomenclature. The IAU defines a satellite feature as "a feature that shares the name of an associated feature".
- On the Moon, this term is used exclusively in connection with what were formerly called "lettered craters" -- that is, craters which share the name of a "parent" feature (usually a crater) and are distinguished from it by the use of a letter, or very occasionally, two letters, as a suffix. For example, Plato A is a satellite feature of Plato.
- The parent crater is sometimes called the "patronymic" crater, but this is not an official term.
- Satellite features are not necessarily small. Indeed, as with human children, some lettered craters are larger and more prominent than their "parents".
- The list of non-crater parents is small. The following are the only non-crater features with satellite craters named after them:
- In the original IAU nomenclature of Blagg and Müller, capital Roman letters were used for naming all depressions below the mean surface, other than rilles, that lacked primary names. This included not only what would today be regarded as distinct impact craters, but also "valleys" and "confluent craters". Blagg and Müller also used double Roman letters for a few small craters, regarded as "satellites" of a lettered crater. They suggested others could expand this system to describe smaller features in an area of interest by placing the second letter in lower case to signify it was not an official designation.
- Based on the list attached to the Army Map Service's LTM chart, the craters with the most lettered features assigned to them in Blagg and Müller appear to have been Abulfeda, Maginus and Plato, for each which all letters other than "I" and "V" (and "Y" in the case of Maginus) were used. Plato appears unique in also having a full complement of Greek-lettered peaks, running the gamut from "Alpha" to "Omega", as well as four Roman-numeralled rilles
- Blagg and Müller seem never to have used the letter "I", although they did occasionally use "V" in connection with other craters.
- In the System of Lunar Craters, Roman letters continued to be used for minor valleys as well as for certain "spots" of uncertain nature. The System also greatly expanded the use of double letters.
- In 1973 (IAU Transactions XVB, Item 5), the IAU declared its intention of gradually replacing the lettered crater designations with individual names; and from 1973 until 2006 lists published by the IAU (such as the IAU Planetary Gazetteer, appearing in 1983) included only those lettered craters that had been renamed. This has sometimes been taken to mean that the IAU "dropped" the lettered craters (e.g., Whitaker, NASA RP-1097, p. 3); but the resolution passed by the General Assembly does not actually say that; and in fact it suggests that for some indefinite period of time the new names, as they are assigned, be followed by the old ones in parenthesis. This recommendation is repeated in IAU Transactions XVB (1976, Item 1(b) of the Second Meeting), where NASA is encouraged to retain the (subset of) lettered craters appearing in the Second Edition of its LMP series (a recommendation that does not seem to have been followed).
- After about 1980 very little progress was made on the plan to rename satellite features, and beginning in 2006 (IAU Transactions XXVI) the 7000 or so lettered craters that had not been renamed were formally added (for the first time) to the lunar listings in the IAU Planetary Gazetteer. However, since the former IAU-endorsed list in the System of Lunar Craters was no longer conveniently available in digital form, the restored names and positions followed a 1982 list prepared for NASA by Andersson and Whitaker (NASA RP-1097, which had been electronically transcribed by Jonathan McDowell in 2004). Hence the designations after 2006 are not necessarily identical to those used prior to 1973. In particular, the Andersson and Whitaker list includes 1667 craters on the Moon's farside for which lettered designations were assigned by Whitaker at NASA's request. Prior to 2006, these farside names had not been approved or endorsed by the IAU. Since Andersson and Whitaker was not accompanied by any maps, the intended identity of a number of the satellite features designated in it has proved ambiguous, and was resolved only by consultation with Whitaker during the preparation of the USGS/IAU Digital Atlas maps. In addition, Andersson and Whitaker chose to drop (or in a very few cases rename) nearly all the double-letter designations that had been established in the System of Lunar Craters, as well as those letters (some dating back to Blagg and Müller) they regarded as designating non-crater features (such as "valleys"). - Jim Mosher
- One reason that giving names to lettered craters slowed greatly was that is was a misguided effort, led by an IAU Lunar Nomenclature Committee that didn't know what it was doing. The committee's leader, Menzel, died in 1976, and about that time a new NASA lunar nomenclature committee was established to correct the large number of bad decisions. As an example, giving a name to a lettered crater in the midst of other lettered craters often let to confusion about the patronymic for nearby lettered features. For details read pages 179-184 of Whitaker's Mapping and Naming the Moon. - tychocrater Jun 14, 2008
- Note: Almost all of the lettered features are shown on the System of Lunar Craters charts, but there have been numerous changes in the limb regions. - tychocrater Jun 14, 2008
- The USGS/IAU Digital Atlas maps are the only ones (aside from "live" software-produced ones like those from LTVT) that correctly depict the complete set of currently approved lunar names, including all satellite features. Antonin Rükl's celebrated Atlas of the Moon, first appearing in 1991, for example, depicts many, but by no means all, of the IAU-approved letter designations. The same can be said for the Clementine Atlas, which appeared in 2004 and lists farside satellite feature names that had not yet been sanctioned by the IAU. - Jim Mosher
- Andersson, Leif E., and Ewen A. Whitaker. 1982. NASA catalogue of lunar nomenclature. NASA reference publication, 1097. Washington, D.C.: National Aeronautics and Space Administration, Scientific and Technical Information Branch.
- see also: Whitaker (1999), pp. 180 and 184 where the statement that lettered crater designations were dropped by the IAU is corrected.